1. C 2. A 3. B 4. D 5. A 6. C 7. A 8. D 9. B 10. C 11. A 12. C
True or False Questions
1. T 2. T 3. F 4. F 5. F 6. T 7. T 8. F 9. T 10. T
Explanation of Terms
1. It is the basic structural and functional unit of the nervous system. The functions of neurons are to receive and integrate incoming information from sensory receptors or other neurons and to transmit information to other neurons or effector organs.
2. It is a process extending from the nerve cell body. Most neurons give rise to a single axon. It's the conducting (propagating or transmitting) part of the neuron, and has one or more collateral branches.
3. The processes extend from the nerve cell body. The receptive part of the neuron is the dendrite, or dendritic zone. Some dendrites give rise to dendritic spines, which are small mushroom-shaped projections that act as fine dendritic branches and receive synaptic inputs.
4. In the nervous system, axons with their surrounding myelin sheath and/or nerve membrane are usually termed nerve fibers.
5. Or glial cells, it outnumber neurons in the brain and spinal cord 10: 1. These cells appear to play a number of important roles, including myelin formation, guidance of developing neurons, maintenance of extracellular K+ levels, and reuptake of transmitters after synaptic activity. Classically, glial cells are grouped into two categories in the CNS, macroglia and microglia. The term macroglia refers to astrocytes and oligodendrocytes, in the broad sense, including ependymal cells. And in the PNS, the glial cells include Schwann cells, and the satellite cells of peripheral sensory ganglia.
6. In the CNS, some regions are relatively enriched in nerve cell bodies (e. g. , the central portion of the spinal cord, the surface of the cerebral hemisphere) and are called gray matter.
7. In the CNS, out of cortex, nerve cell bodies serving a common function, often with a common target, are frequently grouped together into nucleus.
8. In the PNS, nerve cell bodies with similar functions and connections that are grouped together outside the CNS are called ganglia.
9. Among the more conspicuous tracts and nuclei throughout the CNS are extensive fields of intermingled neurons and nerve fibers collectively termed the reticular formation.
10. Communication between neurons usually occurs from the axon terminal of the transmitting neuron (presynaptic side) to the receptive region of the receiving neuron (postsynaptic side). This specialized interneuronal complex is a synapse.
11. Current passes directly from cell to cell through specialized junctions called electrical synapses, or gap junctions. Electrical synapses are most common in invertebrate nervous systems, although they are found in a small number of sites in the mammalian CNS.
Answer the Following Questions
1. The human nervous system is a control system that regulates and coordinates all functions of the organism. Besides, it is also the seat of all mental activity, including consciousness, memory, and thinking. The human nervous system is the most complex and elegant computing device that exists. It receives and interprets an immense array of sensory information, controls a variety of simple and complex motor behaviors, and engages in deductive and inductive logic. The nervous system performs these functions as an information processing system.
2. The central portion of the nervous system consists of the brain and the elongated spinal cord. The brain can be subdivided into the cerebrum, the brain stem, and the cerebellum. The cerebrum (forebrain) consists of the telencephalon and the diencephalon; the telencephalon includes the cerebral cortex (gray matter), subcortical white matter, and the basal ganglia. The major subdivisions of the diencephalon are the thalamus and hypothalamus. The brain stem consists of the midbrain (mesencephalon), pons, and medulla oblongata. The cerebellum includes the vermis and two hemispheres. The brain, which is hollow, contains a system of spaces called ventricles; the spinal cord has a narrow central canal that is largely obliterated in adulthood. These spaces are filled with cerebrospinal fluid.
3. The peripheral nervous system (PNS) consists of spinal nerves, cranial nerves, and their associated ganglia. The nerves contain nerve fibers that conduct information to (afferent) or from (efferent) the CNS. In general, efferent fibers, or motor nerves, are involved in motor functions, such as the contraction of muscles or secretion of glands; afferent fibers, or sensory nerves, usually convey sensory stimuli from the skin, mucous membranes, and deeper structures. Visceral motor nerves are divided into sympathetic and parasympathetic nerves.
4. Communication between neurons usually occurs from the axon terminal of the transmitting neuron (presynaptic side) to the receptive region of the receiving neuron (postsynaptic side). This specialized interneuronal complex is a synapse, or synaptic junction. Synapses are composed of three basic elements: the presynaptic element, the synaptic cleft, and the postsynaptic element. Some synapses are located between an axon and a dendrite (axodendritic synapses, which tend to be excitatory), whereas others are located between an axon and a nerve cell body (axosomatic synapses, which tend to be inhibitory). Still other synapses are located between an axon terminal and another axon; these axoaxonic synapses modulate transmitter released by the postsynaptic axon. Synaptic transmission permits information from many presynaptic neurons to converge on a single postsynaptic neuron. Some large cell bodies receive several thousand synapses.
5. Anatomically, the human nervous system is a complex of two subdivisions.
(1) Central nervous system (CNS)-The CNS consists of the brain and spinal cord, both of them are encased in bone and protected by coverings (meninges) as well as cerebrospinal fluid (CSF). (2) Peripheral nervous system (PNS)--The PNS is formed by twelve pairs of cranial nerves and 31 pairs of spinal nerves.
Functionally, the nervous system can also be subdivided into two systems.
(1) Somatic nervous system--This innervates the structures of the body wall (muscles, skin, and mucous membranes).
(2) Autonomic (visceral) nervous system (ANS)--The ANS contains portions of the central and peripheral systems. It controls the activities of the cardiac muscles, smooth muscles and glands of the internal organs (viscera) and the blood vessels and returns sensory information to the brain.
6. According to the numbers of processes, neurons can be divided into three types: pseudounipolar, bipolar, and multipolar neurons. Pseudounipolar neurons appear T- shaped, in which the initial portion of the two processes approximate and eventually fuse. Such sensory neurons convey nerve impulses from a variety of receptors. Bipolar neurons that have two processes, are sensory in function and transmit impulses generated by sensory receptor endings. Multipolar neurons transmit both sensory and motor nerve impulses, and are-characteristic of the brain, spinal cord, and peripheral autonomic nervous system.
According to the function and the conducting direction, neurons can be grouped into three categories: sensory, motor, and interneurons or association neurons. Afferent or sensory neurons convey information from the periphery to the CNS, whereas efferent or motor neurons carry impulses away from the CHIS. Interneurons, the remaining cells, are the most abundant signaling elements in the CNS and transmit impulses locally.
7. Impulse transmission at most synaptic sites involves, the release of a chemical transmitter substance; at other sites, current passes directly from cell to cell through specialized junctions called electrical synapses, or gap junctions. Electrical synapses are most common in invertebrate nervous systems, although they are found in a small number of sites in the mammalian CNS. Chemical synapses have several distinctive characteristics- synaptic vesicles in the presynaptic element, a synaptic cleft, and a postsynaptic element. The dense of the cell membrane is thick on both the presynaptic side and the postsynaptic side. Synaptic vesicles contain neurotransmitters, and each vesicle contains a small packet, or quanta, of transmitter. Synapses are very diverse in their shapes and other properties. Some are inhibitory and some excitatory; in some, the transmitter is acetylcholine; in others, it is a catecholamine, amino acid, or other substance. Some synaptic vesicles are large, some small; some have a dense core, whereas others do not. Flat synaptic vesicles appear to contain an inhibitory mediator; dense-core vesicles contain catecholamines. A large number of molecules act as neurotransmitters at chemical synapses. These neurotransmitters are present in the synaptic terminal (terminal bouton), a bulb at the end of an axon in which neurotransmitter molecules are stored and released. The action of the neurotransmitters may be blocked by pharmacologic agents. Some presynaptic neurons can release more than one transmitter; differences in the frequency of nerve stimulation probably control which transmitter is released.
8. Glial cells or neuroglia outnumber neurons in the brain and spinal cord 10:1. These ceils appear to play a number of important roles, including myelin formation, guidance of developing neurons, maintenance of extracellular K+ levels, and reuptake of transmitters after synaptic activity. Classically, glial cells are grouped into two categories in the CNS, macroglia and microglia. The term macroglia refers to astrocytes and oligodendrocytes.
There are two broad classes of astrocytes, protoplasmic and fibrous. Protoplasmic astrocytes are more delicate, confined to the grey matter and their many processes are branched. Fibrous astrocytes are more fibrous, situated chiefly in the white matter, and their processes (containing glial fibrils) are seldom branched. Astrocytic processes radiate in all directions from a small cell body. They provide structural support to nervous tissue and act during development as guidewires that direct neuronal migration. They also maintain appropriate concentrations of ions such as K+within the extracellular space of the brain and spinal cord. Astrocytes may also play a role in synaptic transmission.
Oligodendrocytes predominate in white matter; they form myelin in the CNS and may provide some nutritive support to the neurons they envelop. A single oligodendrocyte may wrap myelin sheaths around many axons. An oligodendrocyte may myelinate up to 40 to 50 axons. In peripheral nerves, by contrast, myelin is formed by Schwann cells. Each Schwann cell myelinates a single axon and remyelination can occur at a brisk pace after injury to the myelin in the peripheral nerves.
Microglial ceils (microglia) are the smallest of the glia cells, have an elongated nucleus; they are the macrophages, or scavengers, of the CNS. When an area of the brain or spinal cord is damaged or infected, microglias migrate to the site of injury to remove cellular debris. Some microglias are always present in the brain, but when injury or infection occurs, others enter the brain from blood vessels.
In the broad sense, the glial cells in the CNS should include ependymal cells, the cells lining the ventricles of the brain, which also belong to macroglia. And in the PNS, the glial cells include Schwann cells, and the satellite cells of peripheral sensory ganglia.
Multiple Choice Questions
1. The caudal extremity of the spinal cord in adults lies at the level of the inferior margin of thelumbar vertebra.
2. The caudal extremity of the spinal cord in a new-born infant lies at the level of the inferior margin of thelumbar vertebra.
3. Which of the following statements is correct?
A. The spinal cord occupies the upper one-thirds of the adult spinal canal within the vertebral column.
B. The spinal cord begins as a continuation of the medulla oblongata.
C. The length of the adult spinal cord ranges from 32 to 35 cm.
D. The diameter of the spinal cord is same at different levels.
E. The conus medullaris ends at the level of the intervertebral disc between the fourth and fifth lumbar vertebra.