|Chapter 39 Neurons and Nervous Systems
This chapter presents a detailed study of the nervous system. After a description of the evolution of the nervous system, the chapter focuses exclusively on the human system. The structure and function of neurons are described in detail. The various areas of the brain and their functions, and nervous system physiology are described. The biochemistry of impulse transmission is discussed. A Science Focus box discusses “Five Drugs of Abuse.”
39.1 Evolution of the Nervous System
A. Invertebrate Nervous Organization
1. Comparative study shows the evolutionary steps leading to the centralized nervous system of vertebrates.
2. Even primitive sponges, with only a cellular level of organization, respond by closing the osculum.
3. Hydra (cnidarians) possess two cell layers separated by mesoglea.
a. The hydra can contract, extend, and move tentacles to capture prey and even turn somersaults.
b. A simple nerve net extends throughout the hydra body within the mesoglea.
c. The hydra nerve net is composed of neurons in contact with one another and with contractile epitheliomuscular cells.
d. The more complex cnidaria (sea anemones and jellyfish) may have two nerve nets.
1) A fast‑acting nerve net enables major responses, particularly in times of danger.
2) Another nerve net coordinates slower and more delicate movements.
4. The planarian nervous system is bilaterally symmetrical.
It has two lateral nerve cords that allow rapid transfer of information from anterior to posterior.
The nervous system of planaria is called a ladderlike nervous system.
The nervous system of planaria exhibits cephalization; at their anterior end, planaria have a simple brain composed of a cluster of neurons or ganglia.
Cerebral ganglia receive input from photoreceptors in eyespots and sensory cells in auricles.
The transverse nerve fibers between the sides of the ladderlike nerve cords keep the movement on both sides of a planarian body coordinated.
Bilateral symmetry plus cephalization are important trends in nervous system development.
The organization of the planarian nervous system foreshadows both the central and peripheral system of vertebrates.
5. The annelids, arthropods, and molluscs are complex animals with true nervous systems.
a. The nerve cord has a ganglion in each segment of the body that controls muscles of that segment.
b. The brain still receives sensory information and controls the activity of the ganglia so the entire animal is coordinated.
c. The presence of a brain and other ganglia indicate an increased number of neurons among invertebrates.
B. Vertebrate Nervous Organization
1. Vertebrate nervous systems exhibit cephalization and bilateral symmetry.
a. The vertebrate nervous system is composed of both central and peripheral nervous systems.
1) The central nervous system develops a brain and spinal cord from the embryonic dorsal nerve cord.
2) The peripheral nervous system consists of paired cranial and spinal nerves.
b. Paired eyes, ears, and olfactory structures gather information from the environment.
c. A vast increase in number of neurons accompanied evolution of the vertebrate nervous system; an insect may have one million neurons while vertebrates may contain a thousand to a billion times more.
2. The Vertebrate Brain
a. The vertebrate brain is at the anterior end of the dorsal tubular nerve cord.
b. The vertebrate brain is customarily divided into the hindbrain, midbrain, and forebrain.
1) A well‑developed hindbrain regulates organs below a level of consciousness; in humans it regulates lung and heart function even when sleeping; also, it coordinates motor activity.
2) The optic lobes are part of a midbrain which was originally a center for coordinating reflex responses to visual input.
3) The forebrain receives sensory input from the other two sections and regulates their output.
4) The cerebrum is highly developed in mammals and is associated with conscious control; the outer layer, called the cerebral cortex, is large and complex.
C. The Human Nervous System
Three specific functions of the nervous system are to:
a. receive sensory input,
b. perform integration, and
c. generate motor output to muscles and glands.
The central nervous system (CNS) consists of the brain (in the skull) and the spinal cord (in the vertebral column).
The peripheral nervous system (PNS) lies outside the CNS and contains the cranial and spinal nerves.
The PNS is divided into the somatic and autonomic systems.
a. The somatic system controls the skeletal muscles.
b. The autonomic system controls the smooth muscles, cardiac muscles, and glands.
The CNS and PNS of the human nervous system are connected and work together to perform the functions of a nervous system.
39.2 Nervous Tissue
Nervous tissue is made up of neurons (nerve cells) and neuroglia (which support and nourishe the neurons).
A. Neurons and Neuroglia
Neurons vary in appearance, depending on their function and location, but they all have three parts.
a. The cell body contains the nucleus and other organelles.
b. Dendrites receive information and conduct impulses toward the cell body.
c. A Single axon conducts impulses away from the cell body to stimulate or inhibit a neuron, muscle, or gland.
A long axon is called a nerve fiber.
The long axons are covered by a white myelin sheath.
Types of Neurons
Motor (efferent) neurons have many dendrites and a single axon; they conduct impulses from the CNS to muscles or glands.
Sensory (afferent) neurons are unipolar; they conduct impulses from the periphery toward the CNS.
The process that extends from the cell body divides into two processes, one going to the CNS and one to periphery.
Interneurons (association neurons) are multipolar
They have highly-branched dendrites within the CNS.
Interneurons convey messages between the various parts of the CNS.
They form complex brain pathways accounting for thinking, memory, language, etc.
B. Transmission of the Nerve Impulses
1. In 1786, Luigi Galvani discovered that a nerve can be stimulated by an electric current.
2. An impulse is too slow to be due to simply an electric current in an axon.
3. Julius Bernstein (early 1900s) proposed that the nerve impulse is the movement of unequally distributed ions on either side of an axonal membrane, the plasma membrane of an axon.
4. A. L. Hodgkin and A. F. Huxley later confirmed this theory.
a. They and other researchers inserted a tiny electrode into the giant axon of a squid.
b. The electrode was attached to a voltmeter and an oscilloscope to trace a change in voltage over time.
c. The voltage measured the difference in the electrical potential between the inside and outside of the membrane.
d. An oscilloscope indicated any changes in polarity.
5. Resting Potential
When an axon is not conducting an impulse, an oscilloscope records a membrane potential equal to negative 70 mV, indicating that the inside of the neuron is more negative than the outside.
This is the resting potential because the axon is not conducting an impulse.
This polarity is due to the difference in electrical charge on either side of the axomembrane.
The inside of the plasma membrane is more negatively charged than the outside.
Although there is a higher concentration of K+ ions inside the axon, there is a much higher concentration of Na+ ions outside the axon.
The plasma membrane is more permeable to K+ ions, so this gradient is less and the K+ ion potential is less.
The sodium‑potassium pump maintains this unequal distribution of Na+ and K+ ions.
The sodium‑potassium (Na+‑K+) pump is an active transport system that moves Na+ ions out and K+ ions into the axon.
The pump is always working because the membrane is permeable to these ions and they tend to diffuse toward the lesser concentration.
Since the plasma membrane is more permeable to potassium ions than to sodium ions, there are always more positive ions outside; this accounts for some polarity.
The large negatively charged proteins in the cytoplasm of the axon also contribute to the resting potential of – 70 mV.
6. Action Potential
When an axon conducts a nerve impulse, the rapid change in the membrane potential is the action potential.
Protein‑lined channels in the axomembrane open to allow either sodium or potassium ions to pass; these are sodium and potassium gated ion channels.
The action potential is generated only after the occurrence of a threshold value.
The oscilloscope goes from –70 mV to +40 mV in a depolarization phase, indicating the cytoplasm is now more positive than the tissue fluid.
The trace returns to –70 mV again in the repolarization phase, indicating the inside of the axon is negative again.
7. Propagation of Action Potentials
If an axon is unmyelinated, an action potential stimulates an adjacent axomembrane to produce an action potential.
In myelinated fibers, the action potential at one neurofibril node causes action potential at the next node.
The myelinated sheath has neurofibril nodes, gaps where one neurolemmocyte ends and the next begins.
The action potential “leaps” from one neurofibril node to another—this is called saltatory conduction.
Saltatory conduction may reach rates of over 100 meters/second, compared to 1 meter/second without it.
As each impulse passes, the membrane undergoes a short refractory period before it can open the sodium gates again.
The conduction of a nerve impulse is an all-or-nothing event.
This ensures a one-way direction to the impulse; during a refractory period, sodium gates cannot open.
C. Transmission Across a Synapse
1. The minute space between the axon bulb and the cell body of the next neuron is the synapse.
2. A synapse consists of a presynaptic membrane, a synaptic cleft, and the postsynaptic membrane.
a. Synaptic vesicles store neurotransmitters that diffuse across the synapse.
b. When the action potential arrives at the presynaptic axon bulb, synaptic vesicles merge with the presynaptic membrane.
c. When vesicles merge with the membrane, neurotransmitters are discharged into the synaptic cleft.
d. The neurotransmitter molecules diffuse across the synaptic cleft to the postsynaptic membrane where they bind with specific receptors.
e. The type of neurotransmitter and/or receptor determines if the response is excitation or inhibition.
f. Excitatory neurotransmitters use gated ion channels and are fast acting.
g. Other neurotransmitters affect the metabolism of the postsynaptic cells and are slower.
3. Neurotransmitters and Neuromodulators
At least 100 different neurotransmitters have been identified.
Acetylcholine (ACh) and norepinephrine (NE), dopamine, and serotonin are present in both the CNS and the PNS.
ACh can have either an excitatory or an inhibitory effect, depending on the tissue.
NE is important to dreaming, waking, and mood.
Dopamine is involved in emotions, learning, and attention.
Serotonin is involved in thermoregulation, emotions, and perception.
Once a neurotransmitter is released into a synaptic cleft, it initiates a response and is then removed from the cleft.
In some synapses, the postsynaptic membrane contains enzymes that rapidly inactivate the neurotransmitter.
Acetylcholinesterase (AChe) breaks down acetylcholine.
In other synapses, the presynaptic membrane reabsorbs the neurotransmitter for repackaging in synaptic vesicles or for molecular breakdown.
The short existence of neurotransmitters in a synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes.
Many drugs that affect the nervous system act by interfering with or potentiating the action of neurotransmitters.
Neuromodulators are molecules that block the release of a neurotransmitter or modify a neuron’s response to one.
Substance P is released by sensory neurons when pain is present; endorphins block the release of substance P and therefore act as natural painkillers.
D. Synaptic Integration
A neuron has many dendrites and may have one to ten thousand synapses with other neurons.
A neuron receives many excitatory and inhibitory signals.
Excitatory neurotransmitters produce a potential change (signal) that drives the neuron closer to an action potential; inhibitory signals produce a signal that drives the neuron further from an action potential.
Thus excitatory signals have a depolarizing effect and inhibitory signals have a hyperpolarizing effect.
Integration is the summing up of excitatory and inhibitory signals.
If a neuron receives many excitatory signals, or at a rapid rate from one synapse, the axon will probably transmit a nerve impulse.
If both positive and inhibitory signals are received, the summing may prohibit the axon from firing.
39.3 Central Nervous System: Brain and Spinal Cord
1. The central nervous system (spinal cord and brain) is where sensory impulses are received and motor control is initiated.
2. Both the brain and the spinal cord are protected by bone.
3. Both are wrapped in three connective tissue coverings called meninges; meningitis is a disease caused by many different bacteria or viruses that invade the meninges.
4. The spaces between the meninges are filled with cerebrospinal fluid to cushion and protect the CNS.
5. The cerebrospinal fluid is contained in the central canal of the spinal cord and within the ventricles of the brain.
6. The ventricles are interconnecting spaces that produce and serve as reservoirs for the cerebrospinal fluid.
A. The Spinal Cord
1. The spinal cord has two main functions.
a. It is the center for many reflex actions.
b. It provides the means of communication between the brain and the spinal nerves.
2. The spinal cord is composed of white and gray matter.
a. Gray Matter
1) The unmyelinated cell bodies and short fibers give gray matter its color.
2) In a cross section, the gray area looks like a butterfly or the letter H.
3) It contains portions of sensory neurons and motor neurons; short interneurons connect them.
b. White Matter
Myelinated long fibers of interneurons run together in tracts and give the white matter its color.
Tracts conduct impulses between the brain and the spinal nerves; ascending tracts are dorsal and descending tracts from the brain are ventral.
Tracts cross over near the brain; therefore the left side of the brain controls the right side of the body.
If a spinal cord injury occurs in the cervical region, the condition of quadriplegia (paralysis of all four limbs) results.
If the injury is in the thoracic region, the lower limbs may be paralyzed (paraplegia).
B. The Brain
The brain has four ventricles: two lateral ventricles and a third and fourth ventricle.
The cerebrum is associated with the two lateral ventricles, the diencephalon with the third, and the brain stem and cerebellum with the fourth.
The cerebrum, also called the telencephalon, is the largest part of the brain in humans.
It is the last center receiving sensory input and carrying out integration to command motor responses.
The cerebrum carries out higher thought processes for learning and memory, language and speech.
The right and left cerebral hemispheres (the two halves of the cerebrum) are connected by a bridge of nerve fibers, the corpus callosum; different functions are associated with the two hemispheres.
The outer portion is a highly convoluted cerebral cortex consisting of gray matter containing cell bodies and short unmyelinated fibers.
The cerebral cortex in each hemisphere contains four surface lobes: the frontal, parietal, occipital, and temporal lobes.
Different functions are associated with each lobe.
The cerebral cortex contains motor, sensory, and association areas.
The human hand takes up a large proportion of the primary motor area.
The ventral to the primary motor area is a premotor area that organizes motor functions before the primary area sends signals to the cerebellum.
The left frontal lobe has Broca’s area for our ability to speak.
Sensory information from the skin and skeletal muscles arrives at a primary somatosensory area.
The primary visual area in the occipital lobe receives information from the eyes; a visual association area associates new visual information with old information.
The primary auditory area in the temporal lobe receives information from our ears.
The primary taste area is in the parietal lobe.
The somatosensory association area processes and analyzes sensory information from skin and muscles.
A general interpretation area receives information from all of the sensory association areas and allows us to quickly integrate signals and send them to the prefrontal area for immediate response.
The prefrontal area in the frontal lobe receives input from other association areas and reasons and plans.
1) White matter in the CNS consists of long myelinated axons organized into tracts.
2) Descending tracts from the primary motor area communicate with lower brain centers.
3) Ascending tracts from lower brain centers send sensory information up to the primary somatosensory area.
4) These tracts cross over near the brain; therefore the left side of the brain controls the right side of the body.
1) Aside from the tracts, there are masses of gray matter located deep within the white matter.
2) These basal nuclei integrate motor commands; malfunctions cause Huntingdon and Parkinson disease.
4. The Diencephalon
The hypothalamus and thalamus are in a portion of the brain known as the diencephalon, where the third ventricle is located.
The hypothalamus forms the floor of the third ventricle.
The hypothalamus maintains homeostasis.
It is an integrating center that regulates hunger, sleep, thirst, body temperature, water balance, and blood pressure.
It controls the pituitary gland and thereby serves as a link between the nervous and endocrine systems.
The thalamus consists of two masses of gray matter in the sides and roof of the third ventricle.
It is the last portion of the brain for sensory input before the cerebrum.
It is a central relay station for sensory impulses traveling up from the body or from the brain to the cerebrum.
Except for smell, it channels sensory impulses to specific regions of the cerebrum for interpretation.
The pineal gland, which secretes the melatonin hormone, is in the diencephalon.
5. The Cerebellum
The cerebellum is separated from the brain stem by the fourth ventricle.
The cerebellum is in two portions joined by a narrow median portion.
The cerebellum integrates impulses from higher centers to coordinate muscle actions, maintain equilibrium and muscle tone, and sustain normal posture.
It receives information from the eyes, inner ear, muscles, etc. indicating body position, integrates the information and sends impulses to muscles maintaining balance.
The cerebellum assists in the learning of new motor skills, as in sports or playing the piano; it may be important in judging the passage of time.
6. The Brain Stem
The brain stem contains the medulla oblongata, pons, and midbrain.
Besides acting as a relay station for tracts passing between the cerebrum and spinal cord or cerebellum, the midbrain has reflex centers for visual, auditory, and tactile responses.
The pons (“bridge”) contains bundles of axons traveling between the cerebellum and rest of the CNS.
The pons functions with the medulla to regulate the breathing rate.
It has reflex centers concerned with head movements in response to visual or auditory stimuli.
The medulla oblongata lies between the spinal cord and the pons, anterior to the cerebellum.
It contains vital centers for regulating heartbeat, breathing, and vasoconstriction.
It contains reflex centers for vomiting, coughing, sneezing, hiccuping, and swallowing.
It contains nerve tracts that ascend or descend between the spinal cord and the brain’s higher centers.
7. The Limbic System
The limbic system is a complex network of tracts and nuclei that incorporate medial portions of cerebral lobes, subcortical nuclei, and diencephalon.
It blends higher mental functions and primitive emotions.
Its two major structures are the hippocampus and amygdala, essential for learning and memory.
The hippocampus makes prefrontal area aware of past experiences stored in association areas.
The amygdala causes experiences to have emotional overtones.
Inclusion of the frontal lobe in the limbic system allows reasoning to keep us from acting out strong feelings.
Learning and Memory
Memory is the ability to hold thoughts in the mind and to recall past events.
Learning takes place when we retain and utilize past memories.
The prefrontal area in the frontal lobe is active in short-term memory (e.g., telephone numbers).
Long-term memory is a mix of semantic memory (numbers, words) and episodic memory (persons, events).
Skill memory is the ability to perform motor activities.
The hippocampus serves as a go-between to bring memories to mind.
The amygdala is responsible for fear conditioning and associates danger with sensory stimuli.
Long-term potentiation (LTP) is an enhanced response at synapses within the hippocampus.
LTP is essential to memory storage; excited postsynaptic cells may die due to a glutamate neurotransmitter.
Extinction of too many cells in the hippocampus is the underlying cause of Alzheimer disease.
39.4 Peripheral Nervous System
1. The peripheral nervous system lies outside the CNS.
a. Cranial nerves connect to the brain.
b. Spinal nerves lie on either side of the spinal cord.
2. Axons in nerves are called nerve fibers.
3. The cell bodies of neurons are found in the CNS or in ganglia.
4. Ganglia are collections of cell bodies in the PNS.
5. Humans have 12 pairs of cranial nerves attached to the brain.
a. Sensory nerves only contain sensory nerve fibers.
b. Motor nerves only contain motor nerve fibers.
c. Mixed nerves contain both sensory and motor nerve fibers.
d. Cranial nerves mostly connect to the head, neck, and facial regions.
e. The vagus nerve also branches to the pharynx, larynx, and some internal organs.
6. Humans have 31 pairs of spinal nerves emerging from the spinal cord.
a. The paired spinal nerves leave the spinal cord by two short branches, or roots.
b. The dorsal root contains fibers of sensory neurons conducting nerve impulses to the spinal cord; the cell body of a sensory neuron is in the dorsal root ganglion.
c. The ventral root contains the axons of motor neurons that conduct nerve impulses away from the spinal cord.
d. All spinal nerves are mixed nerves that conduct impulses to and from the spinal cord.
e. Spinal nerves are mixed nerves with sensory and motor fibers; each serves its own region.
A. Somatic System
1. The somatic system includes the nerves that carry sensory information to the CNS and motor commands away from the CNS to skeletal muscles.
2. Any voluntary control of muscles involves the brain; reflexes, involuntary responses to stimuli, involve the brain or just the spinal cord.
3. Outside stimuli can initiate reflex actions, some of which involve the brain.
B. The Reflex Arc
1. Reflexes are automatic, involuntary responses.
2. A reflex arc involves the following pathway:
a. Sensory receptors generate an impulse in a sensory neuron that moves along sensory axons toward the spinal cord.
b. Sensory neurons enter the cord dorsally and pass signals to interneurons.
c. Impulses travel along motor axons to an effector, which brings about a response to the stimulus.
d. The immediate response is that muscles contract to withdraw from source of pain.
3. Reflex response occurs because the sensory neuron stimulates several interneurons.
4. Some impulses extend to the cerebrum, which makes a person conscious of the stimulus and the reaction.
C. Autonomic System
1. The autonomic system is a part of the PNS and regulates cardiac and smooth muscle and glands.
2. There are two divisions: the sympathetic and parasympathetic systems.
a. Both function automatically and usually in an involuntary manner.
b. Both innervate all internal organs.
c. Both utilize two neurons and one ganglion for each impulse.
1) The first neuron has a cell body within the CNS and a preganglionic fiber.
2) The second neuron has a cell body within the ganglion and a postganglionic fiber.
d. Breathing rate and blood pressure are regulated by reflex actions to maintain homeostasis.
3. Sympathetic Division
Most preganglionic fibers of the sympathetic system arise from the middle (thoracic‑lumbar) portion of the spinal cord and almost immediately terminate in ganglia that lie near the cord (thoracic-lumbar portion).
Therefore the preganglionic fiber is short, but the postganglionic fiber that contacts an organ is long.
The sympathetic system is especially important during emergency situations (the “fight or flight” response).
To defend or flee, muscles need a supply of glucose and oxygen; the sympathetic system accelerates heartbeat, and dilates bronchi.
To divert energy from less necessary digestive functions, the sympathetic system inhibits digestion.
The neurotransmitter released by the postganglionic axon is mainly norepinephrine, similar to epinephrine (adrenaline) used as a heart stimulant.
4. Parasympathetic Division
The parasympathetic system consists of a few cranial nerves, including the vagus nerve, and fibers that arise from the bottom craniosacral portion of the spinal cord.
In this case, the preganglionic fibers are long and the postganglionic fibers are short.
This system is a “housekeeper system”; it promotes internal responses resulting in a relaxed state.
The parasympathetic system causes the eye pupil to constrict, promotes digestion, and retards heartbeat.
The neurotransmitter released is acetylcholine.
Chapter 40 Sense Organs
This chapter describes the structures involved with the senses of taste, smell, vision, and hearing. The biochemistry involved with the sensory perceptions is described. A Health Focus box looks at “Protecting Vision and Hearing.”
40.1 Chemical Senses
A. Chemoreceptors are responsible for taste and smell by being sensitive to chemicals in food, liquids, and air.
1. Chemoreception is found universally in animals; it is thought to be the most primitive sense.
2. Chemoreceptors are present all over a planarian but concentrated in the auricles at the side of the head.
3. Insects, such as houseflies, taste with their feet.
4. Crustacea have chemoreceptors on their antennae and appendages.
5. In amphibians, chemoreceptors are located in the nose, mouth, and all over the skin.
6. In mammals, receptors for taste are in the mouth, and receptors for smell are in the nose.
B. Sense of Taste
1. Human taste buds are located primarily on the tongue.
2. Many lie along the walls of papillae, the small elevations on the surface of the tongue.
3. Isolated ones are present on the surfaces of the hard palate, pharynx, and epiglottis.
4. Taste buds are embedded in tongue epithelium and open at a taste pore.
5. Taste buds have supporting cells and elongated taste cells that end in microvilli.
6. Microvilli bear receptor proteins for certain chemicals.
a. Molecules bind to receptor proteins and impulses are generated in associated sensory nerves
b. Nerve impulses go to the brain cortical areas which interpret them as tastes.
7. Humans have four primary types of taste.
a. Taste buds for each are concentrated in particular regions.
1) Sweet receptors are most plentiful near the tip of the tongue.
2) Sour receptors occur primarily along the margins of the tongue.
3) Salty receptors are most common on the tip and upper front portion.
Bitter receptors are located near the back of the tongue.
A fifth type, called umami, may exist for certain flavors (cheese, beef broth, seafood).
b. The brain appears to take an overall “weighted average” of taste messages as the perceived taste.
C. Sense of Smell
1. The sense of smell depends on olfactory cells located in the olfactory epithelium high in the roof of the nasal cavity.
2. Olfactory cells are modified neurons.
3. Each cell has a tuft of five olfactory cilia that bear receptor proteins for an odor molecule.
a. There are around 1,000 different types of odor receptors; many olfactory cells carry the same type.
b. Nerve fibers from like olfactory cells lead to the same neuron in the olfactory bulb.
c. An odor activates a characteristic combination of cells; this information is pooled in the olfactory bulb.
d. Interneurons communicate this information via the olfactory tract to areas of the cerebral cortex.
4. Olfactory bulbs are directly connected with the limbic system; smells associate with emotions and memory.
5. Taste and smell supplement each other.
a. “Smelling” food also involves the taste receptors.
b. Losing taste when you have a cold is usually due to a loss of smell.
40.2 Sense of Vision
A. Animals lacking photoreceptors, sensory receptors sensitive to light, depend on their senses of hearing and smell rather than sight.
B. Photoreceptors vary in complexity.
1. In its simplest form, a photoreceptor indicates only the presence of light and its intensity.
2. “Eyespots” of planaria allow flatworms to determine direction of light.
3. Image‑forming eyes occur in four invertebrate groups: cnidaria, annelids, molluscs, and arthropods.
4. Arthropods have compound eyes composed of many independent visual units (ommatidia), each possessing all of the elements needed for light reception.
a. The cornea and crystalline cone of each visual unit focus rays toward the photoreceptors.
b. Photoreceptors generate nerve impulses, which pass to the brain by way of optic nerve fibers.
c. The image resulting from all stimulated visual units is crude; the small size of compound eyes limits the number of visual units.
d. Insects have color vision but utilize a narrower range of the electromagnetic spectrum and can see some ultraviolet.
5. Some fishes, reptiles, and most birds are believed to have color vision, but among mammals, only humans and other primates have color vision; this is adaptive for day activity.
6. Vertebrates and certain molluscs (e.g., the squid and the octopus) have a camera-type eye.
a. Molluscs and vertebrates are not closely related; therefore this is convergent evolution.
b. A single lens focuses an image of the visual field on closely packed photoreceptors.
c. In vertebrates the lens changes shape to aid in focusing; in molluscs the lens move back and forth.
The human eye is considerably more complex than a camera.
Animals with two eyes facing forward have three-dimensional, or stereoscopic, vision.
Animals with eyes facing sideways (e.g., rabbits) have panoramic vision—the visual field is wide.
C. The Human Eye
The human eye is an elongated sphere 2.5 cm in diameter with three layers.
The sclera is the outer, white fibrous layer that covers most of the eye; it protects and supports the eyeball.
The cornea is a transparent part of the sclera at the front of the eye; it is the window of the eye.
The conjunctiva is a thin layer of epithelial cells that covers the sclera and keeps the eyes moist.
The middle, thin, dark‑brown layer is the choroid containing many blood vessels and pigments absorbing stray light rays.
To the front of the eye, the choroid thickens and forms a ring‑shaped ciliary body and finally becomes the iris that regulates the size of an opening called the pupil.
The lens divides the cavity into two portions: aqueous humor fills the anterior cavity and vitreous humor fills the posterior.
a. The inner layer is the retina that contains photoreceptors called rod cells and cone cells.
b. The fovea centralis is a small area of retina that contain only cones; this area produces acute color vision in daylight.
c. Cone cells are not very sensitive in low intensity light; at night, the rods are still active.
Focusing of the Eye
Light rays enter the eye through the pupil and are focused on the retina.
Focusing involves light passing through the cornea, the lens, and the humors.
Because of refraction, the image on the retina is inverted 180º from actual but is perceived righted in the brain.
The shape of the lens is controlled by the ciliary muscle.
The lens is flatter when the ciliary muscle is relaxed when we are viewing distant objects.
The lens is naturally elastic and becomes rounder for viewing near objects where light rays must bend to a greater degree.
This change is called visual accommodation.
An aging lens loses its ability to accommodate for near objects and we may need reading glasses by middle age.
The lens is also subject to cataracts, or becoming opaque; surgery, using a cryoprobe, is the only current treatment.
Persons who can see well close up but not far away are nearsighted (myopia).
They often have an elongated eyeball that focuses a distant image in front of the retina.
They can wear corrective concave lenses to refocus the image on the retina.
Radial keratotomy is a new treatment that surgically cuts and flattens the cornea.
Persons who can see far away but not up close are farsighted (hyperopia)
They often have a shortened eyeball that focuses near images behind retina.
They can wear corrective convex lenses to refocus the image on retina.
When the cornea or lens is uneven, the image is fuzzy; this is astigmatism corrected by an unevenly ground lens to compensate for unevenness.
D. Photoreceptors of the Eye
1. Vision begins when light has been focused on photoreceptors in the retina.
2. Rod cells and cone cells have an outer segment joined to an inner segment by a stalk.
3. The outer segment contains stacks of membranous disks (lamellae) with many molecules of rhodopsin.
4. The rhodopsin molecules contain a protein opsin and the pigment molecule retinal derived from Vitamin A.
5. When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to a cascade of reactions and the closure of ion channels in the rod cell plasma membrane.
6. This stops the release of inhibitory molecules from the rod’s synaptic vesicles and starts signals that result in impulses to brain.
7. The rods are stimulated by low light and provide night vision.
8. Because rods are distributed throughout the retina, rods detect our peripheral vision and motion but not color or detail.
9. Cones located primarily in the fovea centralis are activated by bright light and detect detail and color.
10. The three kinds of cones contain either blue, green, or red pigment.
11. Each pigment is composed of retinal and opsin, but the structure of opsin varies among the three.
12. Combinations of cones are stimulated by intermediate colors; the combined nerve impulses are interpreted in the brain.
E. Integration of Visual Signals in the Retina
1. The retina has three layers of neurons.
a. The rods and cones are nearest the choroid.
b. Bipolar cells form the middle layer.
c. Ganglion cells, whose fibers become the optic nerve, form the innermost layer.
2. Since only rod and cone cells are sensitive to light, light must penetrate through the ganglion cells.
3. Rods and cones synapse with bipolar cells which pass the impulse to ganglion cells.
4. There are more rods and cones than nerve fibers leaving ganglionic cells.
5. Up to 150 rods may synapse with a ganglion cell; this results in indistinct vision.
6. Each cone synapses with one ganglionic cell; this accounts for the detailed images of cones, mostly found in the fovea.
7. Integration occurs as signals pass from the bipolar to the ganglion cells.
a. If all rod cells in a receptive field are stimulated, the ganglion cell is weakly stimulated or neutral.
b. If only the center is lit, it is stimulated; if only the edge is lit, it is inhibited.
c. Therefore considerable processing occurs in the retina before an impulse is sent to the brain.
8. The blind spot is an area where the optic nerve passes through the retina; it lacks rods and cones.
Senses of Hearing and Balance
The ear has two sensory functions: hearing and balance (equilibrium).
The sensory receptors for both are in the inner ear, and each consists of hair cells with stereocilia that are sensitive to mechanical stimulation; they are mechanoreceptors.
A. Anatomy of the Ear
1. The human ear has three divisions: an outer, middle, and inner ear.
2. The outer ear consists of the pinna (external flap) and the auditory canal.
a. The auditory canal opening is lined by fine hairs that filter air.
b. Modified sweat glands in the auditory canal secrete earwax to guard against foreign matter.
3. The middle ear begins at the tympanic membrane and ends at a bony wall with membrane‑covered openings (the oval window and the round window).
a. It contains small bones called ossicles: malleus (hammer), incus (anvil), and stapes (stirrup).
b. The malleus adheres to the tympanum; the stapes touches the oval window.
c. The auditory (eustachian) tube extends from the middle ear to the pharynx to equalize the inside and outside air.
4. The inner ear has three regions: the semicircular canals, vestibule, and cochlea.
5. The cochlea resembles a snail shell because it spirals.
B. Process of Hearing
1. The process of hearing begins when sound waves enter the auditory canal, causing the ossicles to vibrate.
2. Sound is amplified about 20 times by the size difference between the tympanic membrane and the oval window.
3. The stapes strikes the membrane of the oval window, passing pressure waves to the fluid in the cochlea.
4. Three canals are located within the cochlea: vestibular canal, cochlear canal, and tympanic canal.
5. The vestibular canal connects with the tympanic canal, which leads to the oval window membrane.
6. Along the basilar membrane are hair cells whose stereocilia are embedded in a tectorial membrane.
7. The hair cells of the spiral organ (organ of Corti) synapse with nerve fibers of the cochlear (auditory) nerve.
8. When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal and across the basilar membrane, and the round window bulges.
9. The basilar membrane vibrates up and down bending the stereocilia of hair cells embedded in the tectorial membrane.
10. This generates nerve impulses in the cochlear nerve that travel to the brain stem.
11. When they reach the auditory areas of the cerebral cortex, this is interpreted as sound.
12. The spiral organ is narrow at its base and widens at its tip; each part is sensitive to different pitches.
13. Nerve fibers from each region (high pitch base or low pitch tip) lead to slightly different regions of the brain, producing the sensation of pitch.
14. Sound volume is caused by more vibration; the increased stimulation is interpreted as louder sound intensity.
15. Tone is an interpretation by the brain based on the distribution of the hair cells stimulated.
C. Sense of Balance
1. The sense of balance is divided into:
a. rotational equilibrium (angular or rotational movement of the head), and
b. gravitational equilibrium (vertical or horizontal movement).
2. Rotational equilibrium utilizes the semicircular canals.
a. The semicircular canals are oriented at right angles to one another in three different planes.
b. The enlarged base of each semicircular canal is called an ampulla.
c. Fluid flowing over and displacing a cupula causes the stereocilia of the hair cells to bend; the pattern of impulses carried by the vestibular nerve to the brain changes.
d. Continuous movement of the fluid in the semicircular canals causes vertigo motion sickness.
e. By spinning and stopping, we see a room still spin; this indicates that vision is also involved in balance.
3. Gravitational equilibrium utilizes the utricle and saccule.
a. A vestibule or space between the semicircular canals and the cochlea contains the utricle and the saccule.
b. The utricle and saccule are small membranous sacs, each of which contains hair cells.
c. Hair cell stereocilia are embedded within a gelatinous material called the otolithic membrane.
d. Calcium carbonate granules (otoliths) rest on this membrane.
e. The utricle is sensitive to horizontal movements; the saccule responds best to up‑down movements.
f. When the head is still, otoliths in the utricle and saccule rest on the otolithic membrane above the hair cells.
g. As the head bends or the body moves, otoliths are displaced and the otolithic membrane sags, bending the larger stereocilia (kinocillium) of hair cells beneath; this tells the brain the direction of movement.
D. Sensory Receptors in Other Animals
1. The lateral line system of fish and amphibians detects water currents and pressure waves.
2. Primitive fishes have the system on the surface; advanced fishes enclose it in a canal on the side.
3. The lateral line receptor is a collection of hair cells with cilia embedded in a mass of gelatinous material (cupula).
4. Static equilibrium organs called statocysts are found in cnidaria, molluscs, and crustacea.
5. A small particle called a statolith stimulates cilia that generate impulses, indicating the position of the head.
Chapter 41 Locomotion and Support Systems
A comparison of the diversity of animal skeletal systems precedes an examination of the human musculoskeletal system. The structure and function of whole muscles and muscle fibers are also detailed, as is the biochemistry of muscle contraction.
41.1 Diversity of Skeletons
1. Three types of skeletons occur in the animal kingdom.
2. A hydrostatic skeleton occurs in cnidarians, flatworms, roundworms and annelids.
3. An exoskeleton is found in molluscs and arthropods, respectively.
4. An endoskeleton is found in sponges, echinoderms, and vertebrates.
A. Hydrostatic Skeleton
1. A fluid‑filled gastrovascular cavity or coelom can act as a hydrostatic skeleton.
2. It offers support and resistance to the contraction of muscles for motility.
3. Many animals have hydroskeletons.
a. Hydras use a fluid‑filled gastrovascular cavity to support tentacles that rapidly contract.
b. Planaria easily glide over substrate with muscular contractions of body walls and cilia.
c. Roundworms have a fluid‑filled pseudocoelom and move when their longitudinal muscles contract against it.
Earthworms are segmented with septa dividing the coelom into compartments; circular and longitudinal muscles contract in each segment to coordinate elongation and contraction.
Animals with exoskeletons or endoskeletons move selected body parts by means of muscular hydrostats, i.e., fluid contained within certain muscle fibers assists movement of that part.
B. Exoskeletons and Endoskeletons
1. An exoskeleton is an external skeleton.
a. Molluscs have exoskeletons that are predominantly calcium carbonate (CaCO3).
b. Insects and crustacea have jointed exoskeletons composed of chitin, a strong, flexible, nitrogenous polysaccharide.
c. The exoskeleton provides protection against damage frm enemies and also keeps tissues from drying out.
d. Although stiffness provides support for muscles, the exoskeleton is not as strong as an endoskeleton.
e. The clam and snail exoskeletons grow with the animals; their thick nonmobile CaCO3 shell is for protection.
f. The chitinous exoskeleton of arthropods is jointed and moveable.
g. Arthropods must molt when their exoskeleton becomes too small; a molting animal is vulnerable to predators.
2. Vertebrates have an endoskeleton composed of bone and cartilage that grows with the animal.
a. The endoskeleton does not limit the space available for internal organs and it can support greater weight.
b. Soft tissues surround the endoskeleton to protect it; injuries to soft tissue are easier to repair.
c. Usually an endoskeleton has elements that protect vital internal organs.
d. The jointed exoskeleton of arthropods and endoskeletons of vertebrates allow flexibility and helped arthropods and vertebrates colonize land.
41.2 The Human Skeletal System
1. Skeletons protect organs: skull (brain), vertebral column (spinal cord), and rib cage (heart and lungs).
2. The large, heavy leg bones support the body against the pull of gravity.
3. Leg and arm bones permit flexible body movement.
4. The flat bones of the skull, ribs, and breastbone contain red bone marrow that manufactures blood cells.
5. All bones store inorganic calcium and phosphorous salts.
A. Bone Growth and Renewal
1. The prenatal human skeleton is cartilaginous; cartilage structures serve as “models” for bone construction.
a. The cartilaginous models are converted to bones when calcium salts are deposited in the matrix, first by cartilaginous cells and later by bone‑forming cells called osteoblasts.
b. Conversion of cartilaginous models to bones is called endochondral ossification.
c. Some bones (e.g., facial bones) are formed without a cartilaginous model.
2. During endochondral ossification, there is a primary ossification center at the middle of a long bone; latter secondary centers form at the ends.
3. A cartilaginous growth plate occurs between primary and secondary ossification centers.
4. As long as the growth plate remains between the two centers, bone growth occurs.
5. The rate of growth is controlled by hormones, including growth hormone (GH) and sex hormones.
6. Eventually plates become ossified and bone stops growing; this determines adult height.
7. In adults, bone is continually being broken down and built up again.
a. Bone‑absorbing cells (osteoclasts) break down bone, remove worn cells, and deposit calcium in the blood.
b. Osteoblasts form new bone, taking calcium from the blood.
c. Osteoblasts become entrapped in the bone matrix and become osteocytes in the lacunae of osteons.
d. This continual remodeling allows bones to gradually change in thickness.
e. Osteoclasts also determine the calcium level in the blood; calcium level is important for muscle contraction and nerve conduction and levels are controlled by the hormones PTH and calcitonin.
8. Adults need more calcium in the diet than do children to promote the work of osteoblasts.
B. Anatomy of a Long Bone
1. A long bone illustrates the principles of bone anatomy.
a. A long bone consists of a central medullary cavity surrounded by compact bone.
b. Ends are composed of spongy bone surrounded by a thin layer of compact bone and covered with hyaline cartilage.
c. Compact bone contains many osteons (Haversian systems); bone cells in tiny chambers (lacunae) are arranged in concentric circles around central canals.
d. Central canals contain blood vessels and nerves.
e. The lacunae are separated by a matrix that contains protein fibers of collagen and mineral deposits.
2. Spongy bone has numerous plates and bars separated by irregular spaces.
a. Spongy bone is lighter but designed for strength; solid portions of bone follow the lines of stress.
b. Bone spaces are often filled with red bone marrow, a specialized tissue that produces blood cells.
C. The axial skeleton lies at the midline of the body and consists of the skull, vertebral column, sternum and ribs.
The skull is formed by the cranium and the facial bones.
Newborns have membranous junctions called fontanels that usually close by the age of two.
The bones of the cranium contain sinuses, air spaces lined with mucous membrane that reduce the weight of skull and give a resonant sound to the voice.
Two mastoid sinuses drain into the middle ear; mastoiditis is an inflammation that can lead to deafness.
The cranium is composed of eight bones: a frontal, two parietal, an occipital, two temporal, a sphenoid, and an ethmoid.
The spinal cord passes through the foramen magnum, an opening at the base of the skull in the occipital bone.
Each temporal bone has an opening that leads to the middle ear.
The sphenoid bone completes the sides of the skull and forms the floors and walls of the eye sockets.
The ethmoid bone is in front of the sphenoid, part of the orbital wall, and a component of the nasal septum.
Fourteen facial bones include: mandible, two maxillae, two palatine, two zygomatic, two lacrimal, two nasal, and vomer.
The mandible or lower jaw is the only movable portion of the skull; it contains tooth sockets.
The maxilla forms the upper jaw and the anterior of the hard palate; it also contains tooth sockets.
The palatine bones make up the posterior portion of the hard palate and the floor of the nasal cavity.
The zygomatic gives us our cheekbone prominences.
Nasal bones form the bridge of the nose.
Other bones make up the nasal septum which divides the nose cavity into two regions.
The ears are elastic cartilage and lack bone; the nose is a mixture of bone, cartilage, and fibrous connective tissue.
The Vertebral Column and Rib Cage
The vertebral column supports the head and trunk and protects the spinal cord and the roots of the spinal nerves.
The vertebral column serves as an anchor for all of the other bones of the skeleton.
Seven cervical vertebrae are located in the neck.
Twelve thoracic vertebrae are in the thorax or chest.
The lumbar vertebrae are in the small of the back.
One sacrum is formed from five fused sacral vertebrae.
One coccyx is formed from four fused coccygeal vertebrae.
Normally, the spinal column has four normal curvatures that provide strength and resiliency in posture.
Scoliosis is an abnormal sideways curvature; hunchback and swayback are also abnormal.
Intervertebral disks between the vertebrae act as a padding to prevent the vertebrae from grinding against each other, and to absorb shock during running, etc.; they weaken with age.
Vertebral disks allow motion between vertebrae for bending forward, etc.
The rib cage: all twelve pairs of ribs connect directly to the thoracic vertebrae in back; seven attach directly to the sternum.
Three pairs connect via cartilage to the sternum at front.
The two ribs totally unattached to the sternum are called “floating ribs.”
The rib cage protects the heart and lungs, yet is flexible enough to allow breathing.
3. The Appendicular Skeleton
a. The appendicular skeleton consists of the bones within the pectoral girdle and upper limbs and the pelvic girdle and lower limbs.
b. The pectoral girdle and arms are specialized for flexibility; the pelvic girdle and legs is built for strength.
c. The components of the pectoral girdle are only loosely linked by ligaments.
1) The clavicle (“collarbone”) connects with the sternum in front and the scapula (“shoulderblade”) in back.
2) The scapula connects with the clavicle; it is freely movable and held in place only by muscles.
d. The humerus is the long bone of the upper arm; its smoothly rounded head fits into a socket of the scapula.
e. The radius is the more lateral of the bones of the lower arm; it articulates with the humerus at the elbow joint, a hinge joint, and the radius crosses in front of the ulna for easy twisting.
f. The ulna is the more medial of the two bones of the lower arm; its end is the prominence in your elbow.
g. The many hand bones increase its flexibility.
1) The wrist has eight carpal bones which look like small pebbles.
2) Five metacarpal bones fan out to form the framework of the palm.
3) The phalanges are the bones of fingers and thumb.
h. The pelvic girdle consists of two heavy, large coxal (hip) bones.
1) The coxal bones are anchored to the sacrum; together with the sacrum they form a hollow cavity that is wider in females than in males; it transmits weight from the vertebral column via the sacrum to the legs.
2) The femur is the largest bone of the body; it is limited in the amount of weight that it can support.
3) The tibia has a ridge called the “shin”; its end forms the inside of the ankle.
4) The fibula is the smaller of the two bones; its end forms the outside of the ankle.
5) Seven tarsal bones are in each ankle; one receives the weight and passes it to the heel and ball of foot.
6) The metatarsal bones form the arch of the foot and provide a springy base.
7) The phalanges are the bones of the toes, which are stouter than the fingers.
D. Classification of Joints
1. Bones are joined at joints that are classified as fibrous, cartilaginous, or synovial.
2. Fibrous joints, such as those between the cranial bones, are immovable.
3. Cartilaginous joints, such as those between the vertebrae, are slightly moveable; the two hipbones are slightly movable because they are ventrally joined by cartilage and respond to pregnancy hormones.
4. Synovial joints are freely movable.
a. Most joints are synovial joints, with the two bones separated by a cavity.
b. Ligaments are fibrous connective tissue that bind bone to bone, forming a joint capsule.
c. In a “double‑jointed” individual, the ligaments are unusually loose.
d. The joint capsule is lined with a synovial membrane that produces a lubricating synovial fluid.
e. The knee represents a synovial joint.
1) Knee bones are capped by cartilage; a crescent‑shaped piece of cartilage, the meniscus, is between the bones.
2) Athletes who injure the meniscus have torn this cartilage.
3) The knee joint also contains 13 fluid‑filled sacs called bursae to ease friction between the tendons and ligaments and tendons and bones.
4) Inflammation of the bursae is bursitis; “tennis elbow” is a form of bursitis.
5) The knee and elbow are hinge joints; the shoulder and hip are ball-and-socket joints.
f. Synovial Joints
1) Synovial joints are subject to arthritis.
2) In rheumatoid arthritis, the synovial membrane becomes inflamed and thickened.
3) The joint degenerates and becomes immovable and painful.
4) This is likely caused by an autoimmune reaction.
5) In osteoarthritis from old age, the cartilage at the ends of bones disintegrates; the bones then become rough and irregular.
41.3 The Human Muscular System
1. Skeletal muscle contraction assists homeostasis by helping maintain constant body temperature.
2. Skeletal muscle contraction also causes ATP breakdown, releasing heat that is distributed about the body.
A. Macroscopic Anatomy and Physiology
1. Skeletal muscles are attached to the skeleton by tendons made of fibrous connective tissue.
2. When muscles contract, they only shorten or pull; therefore, skeletal muscles must work in antagonistic pairs.
a. One muscle of an antagonistic pair bends the joint and brings a limb toward the body.
b. The other one straightens the joint and extends the limb.
3. If a muscle is given a rapid series of stimuli, it responds to the next stimulus before completely relaxing.
4. Muscle contraction summates until it reaches a maximal sustained contraction, called tetanus.
5. Even at rest, muscles maintain tone by some fibers contracting; this is essential to maintaining posture.
B. Microscopic Anatomy and Physiology
1. A whole skeletal muscle consists of a number of muscle fibers in bundles.
2. Each muscle fiber is a cell with some special features.
a. A plasma membrane called the sarcolemma forms a T (transverse) system.
1) Transverse (T) tubules penetrate down into the cell and contact with, but do not fuse with, the modified endoplasmic reticulum (the sarcoplasmic reticulum).
2) Expanded portions or sacs of the sarcoplasmic reticulum are modified for Ca2+ ion storage; this encases hundreds and sometimes thousands of myofibrils.
b. The myofibrils are contractile portions of fibers that lie parallel and run the length of the fiber.
c. A light microscope shows light and dark bands called striations.
d. An electron microscope shows that these striations of myofibrils are formed by placement of protein filaments within sarcomeres.
e. The two protein filaments are either thick (made of myosin) or thin (made of actin).
f. A sarcomere has repeating bands of actin and myosin that occur between two Z lines in a myofibril.
1) The I band contains only actin filaments.
2) The H zone contains only myosin filaments.
3. Sliding Filament Model
a. As a muscle fiber contracts, sarcomeres within the myofibrils shorten.
b. As a sarcomere shortens, actin filaments slide past the myosin; the I band shortens and the H zone disappears.
c. Sliding filament model: actin filaments slide past myosin filaments because myosin filaments have cross‑bridges that pull actin filaments inward, toward their Z line.
d. The contraction process involves the sarcomere shortening although the filaments themselves remain the same length.
e. ATP supplies the energy for muscle contraction.
f. Myosin filaments break down ATP to form cross‑bridges that attach to and pull the actin filaments toward the center of the sarcomere.
a. Muscle cells contain myoglobin that stores oxygen; cellular respiration does not immediately supply all of the ATP needed.
b. Muscle fibers rely on a supply of stored creatine phosphate (phosphocreatine), a storage form of high-energy phosphate.
c. Creatine phosphate does not directly participate in muscle contraction but regenerates ATP rapidly: creatine — P + ADP → ATP + creatine
d. This reaction occurs in the midst of sliding filaments and is speedy.
e. When all creatine phosphate is depleted, and if O2 is in limited supply, fermentation produces a small amount of ATP, but this results in a buildup of lactate.
f. The buildup of lactate partially accounts for muscle fatigue and represents oxygen debt.
g. Lactate is transported to the liver; 20% is completely broken down to CO2 and H2O in aerobic respiration.
h. The ATP gained from this respiration is then used to reconvert 80% of the lactate to glucose.
i. In persons who train, the number of mitochondria increases, reducing the need for fermentation.
C. Muscle Innervation
Muscles are stimulated to contract by motor nerve fibers.
The neuromuscular junction is a region where an axon bulb is in close association with the sarcolemma of a muscle fiber.
An axon bulb contains synaptic vesicles filled with the neurotransmitter acetylcholine.
When nerve impulses travel down a motor neuron to the axon bulb, vesicles merge with the presynaptic membrane and acetylcholine molecules are released into the synaptic cleft.
Acetylcholine rapidly diffuses to and binds with receptors on the sarcolemma.
The sarcolemma generates impulse spreading down the T tubule system to the sarcoplasmic reticulum where it triggers the release of Ca2+ ions out amongst the myofilaments.
The Ca2+ ions then initiate muscle contraction.
Ca2+ ions bind to troponin, which causes tropomyosin threads to shift position.
The change in the structure of tropomyosin exposes the myosin heads with ATP binding sites.
The myosin heads function as ATPase enzymes, splitting ATP into ADP and Pi .
After attaching to actin filaments, the myosin cross‑bridges bend forward and the actin filament is pulled along.
While ATP and Ca2+ ions are available, cross‑bridges attach; as ADP and P are released, the cross-bridges change their positions and cause a power stroke as filaments pull together.
When another ATP molecule binds to the myosin head, the cross-bridge detaches and the cycle begins again.
When a nerve impulse ceases, active transport proteins in the sarcoplasmic reticulum pump calcium ions back into calcium storage sites and muscle relaxation occurs.
Chapter 42 Hormones and Endocrine Systems
This chapter has been entirely rearranged from the previous edition. The categories of hormones are described as are the biochemical activities elicited by the individual hormones. Each glands’ hormones are described individually. Many endocrine-related disorders are described, including a detailed discussion of diabetes mellitus. A Science Focus box discusses “Isolation of Insulin.”