Anatomy and Physiology | Tutorial Notes
Nervous System I
After study of this chapter, the student should be able to:
1. Describe the general functions of the nervous system
2. Classify the general divisions of the nervous system
3. Identify the two types of cells that comprise the nervous system
4. Describe the parts of a neuron and indicate the function of each part.
5. Compare and contrast myelination in the PNS and the CNS
6. Distinguish between the sources of gray matter and white matter
7. Identify the 3 structural types of neurons
8. Identify the 3 functional types of neurons
9. Identify the 4 types of neuroglia in the CNS and indicate the function of each type.
10. Identify the 2 types of neuroglia in the PNS and indicate the function of each type.
11. Explain the major events that occur during synaptic transmission from a presynaptic neurons to a postsynaptic cell.
12. Define “membrane potential” and indicate what factors cause a membrane potential. Explain which cells have a membrane potential.
13. Define “resting membrane potential” and indicate which cells have a resting membrane potential.
14. Define “polarized”, “depolarized”, and “hyperpolarized”
15. Compare and contrast graded potentials with action potentials.
16. Describe the events leading to the generation of an action potential.
17. Explain the 3 phases of an action potential. Name the event that causes each phase.
18. Explain how an action potential is propagated along an axon.
19. Describe what is meant by “all-or-none” response
20. Explain how the strength of a stimulus affects the strength and frequency of action potentials.
21. Compare and contrast impulse conduction in myelinated and unmyelinated neurons.
22. Explain 2 ways excitatory postsynaptic potentials (EPSPs) exert their effect on a postsynaptic cell.
23. Explain 2 ways inhibitory postsynaptic potentials (IPSPs) exert their effect on a postsynaptic cell.
24. Describe how EPSPs and IPSPs summate and indicate where summation occurs on the postsynaptic neuron.
23. Explain 2 ways postsynaptic cells are prevented from being continuously stimulated.
24. Indicate the function of acetylcholinesterase and monoamine oxidase
25. Compare and contrast convergent pathways and divergent pathways with regards to neuronal pools.
I. Functions of the Nervous System
A. Maintains homeostasis
B. Receives sensory input
C. Initiates motor output
D. Integrates information into meaningful messages
E. Higher Cognitive Activity: critical thinking, judgment, memory, problem solving, etc.
II. Divisions of the Nervous System
A. Central Nervous System
2. Spinal Cord
B. Peripheral Nervous System
1. 12 pairs of cranial nerves
2. 31 pairs of spinal nerves
III. Neurons of the Peripheral Nervous System (PNS)
A. Sensory (Afferent) – transmits information from sensory receptors in the PNS towards the CNS.
B. Motor (Efferent) – transmits information from the CNS towards effectors (muscles & glands) in the PNS
IV. Divisions of the Peripheral Nervous System
A. Somatic Nervous System – under voluntary control.
1. Skeletal muscles
B. Autonomic Nervous System – under involuntary control
1. Smooth Muscles
2. Cardiac Muscles
V. Cells of the Nervous System
A. Neurons – transmit impulses
B. Neuroglia – provide functional and structural support
VI. Parts of a Neuron: 1. Dendrites 2. Cell Body 3. Axon
A. Dendrites – receive input from other cells or from environment.
1. Dendrites transmit input towards the cell body.
2. Dendritic Spines – tiny processes that serve as contact points.
Neurons can adjust their sensitivity by adding or removing dendritic spines.
3. Cells may have no dendrites or thousands of dendrites.
B. Cell Body (Soma or Perikaryon)
1. Contains organelles similar to most cells: cytoplasm, nucleus, mitochondria, lysosomes, Golgi Apparatus, etc.
2. Chromatophilic Substance (Nissl bodies): mostly rough Endoplasmic Reticulum.
Ribosomes on the Rough ER = site of protein synthesis.
3. Nucleus: contains…
Nucleolus – synthesizes ribosomes
Chromatin – DNA + packaging proteins. Encodes the genetic information for protein synthesis
4. Neurofibrils (bundles of neurofilaments)
Forms cytoskeleton of the neuron
Extends into and supports the axon.
Play a role in axonal transport of proteins from the cell body to the axon terminal.
1. Transmits impulses away from the cell body.
Neurons have at most one axon.
2. Axon Hillock (trigger zone)
cone-shaped thickening where the axon meets the cell body
Action potential (electrical impulse) is initiated at the axon hillock.
3. Collaterals – branches of the axon
4. Axon Terminal – distal end of the axon.
5. Synaptic Knob – enlarged end of the axon terminal.
Contains secretory vesicles with neurotransmitters.
6. Synaptic Cleft – small space between synaptic knob and postsynaptic cell
7. Axoplasm – cytoplasm within the axon of a neuron
Axoplasmic transport – transports proteins, organelles, and other substances from the cell body to the axon.
VII. Myelination of axons
A. Myelin sheath
1. The myelin sheath
Myelin is a lipid rich coating covering the axon of many neurons.
The myelin sheath increases the speed of a nerve impulse.
Nodes of Ranvier – narrow gaps in the myelin sheath.
B. Myelination in the Peripheral Nervous System (PNS)
1. Schwann Cells form the myelin sheath in the PNS
Schwann cell membranes wind and wrap around axons forming a thick layer of insulation. = myelin.
Neurilemma – cytoplasm and nucleus of Schwann cells is pushed outward forming an outer layer, called the neurilemmal.
Several Schwann Cells myelinate each axon.
Schwann cells are separated by a small gap, called the node of Ranvier
Schwann Cells also enclose, but do not wrap around unmyelinated axons.
Remak Bundle – bundle of unmyelinated axons, bundled by Schwann Cells.
C. Myelination in the Central Nervous System (CNS)
1. Oligodendrocytes form the myelin sheath in the CNS.
The Cell Body sits in between axons, giving off cell processes that myelinate multiple axons.
White Matter – groups of myelinated axons within the CNS. The Myelin sheath imparts a whitish appearance.
Gray Matter – groups of unmyelinated axons and cell bodies within the CNS. Unmyelinated tissue imparts a grayish appearance.
VIII. Structural Classification of Neurons
A. Multipolar Neurons
a. Contain many dendrites but only one axon
b. Most neurons of the brain and spinal cord (CNS) are multipolar
c. Motor neurons are multipolar
B. Bipolar Neurons
a. Contains two processes: one dendrite and one axon.
b. Found in some special senses: eyes, nose, ears
C. Pseudounipolar (Unipolar) Neurons
a. One process attached to the cell body divides into two branches that act as a single axon.
b. Peripheral process – contains dendrites near the peripheral end
c. Central process – enters the brain or the spinal cord.
d. Most unipolar neurons are found in ganglia (eg. Dorsal root ganglia).
IX. Functional Classification of Neurons
A. Sensory (afferent) neurons
a. conduct impulses from the periphery towards the brain or spinal cord.
b. sensory neurons detect changes in the outside world or in the internal environment.
c. most sensory neurons are unipolar
B. Motor (efferent) neurons
a. conduct impulses from the CNS towards effectors (muscles or glands) in the PNS
b. Somatic motor – skeletal muscles, under voluntary control
c. Autonomic motor – smooth muscle, cardiac muscle, and glands – under involuntary control.
d. motor neurons are multipolar
C. Interneurons (association fibers)
a. lie completely within the brain or spinal cord
b. form links with other neurons
c. relay information from one part of the CNS to another part of the CNS
d. may relay incoming sensory information to the appropriate region for processing
A. Neuroglia within the CNS
1. Astrocytes “star cell”
a. Found between blood vessels and neurons
b. Astrocytes participate in the Blood-Brain-Barrier. They do not form the BBB, but transfer nutrients from the bloodstream to the neuron.
c. Astrocytes regulate ion concentrations, strengthen synapses, and prevent the spread of infection by depositing connective tissue (glial scars)
2. Ependymal Cells
a. Cuboid (or columnar) epithelium often ciliated.
b. Line ventricles of the brain and central canal of the spinal cord.
c. Regulate the composition of cerebrospinal fluid (CSF)
a. Small cells that are normally inactive.
b. When activated by infection or inflammation, microglia enlarge and become macrophages.
c. Microglia phagocytize bacterial cells and cell debris.
a. Myelinate axons in the CNS
b. Oligodendrocytes form rows in between neurons and form processes that myelinate multiple axons.
B. Neuroglia in the PNS
1. Satellite Cells
a. Surround bodies within ganglia
b. Provide metabolic and structural support.
2. Schwann Cells
a. Forms myelin sheath in the PNS
b. Several Schwann Cells myelinate one axon
C. Properties of Neuroglia
1. Neuroglia outnumber neurons 10 to 1.
2. Many diseases of the nervous system originate from neuroglia, not neurons.
Autoimmune disorder – immune system attacks myelin sheath in the CNS.
During repair, neuroglia deposit Connective Tissue in place of myelin forming several Scars (Scleroses)
Muscles innervated by scarred motor neurons stop contracting and eventually atrophy.
Most Primary Brain Tumors are due to neuroglia that divide too often.
Microglia release a toxin that damages neurons.
Causes uncontrollable movements and cognitive impairment.
D. Neuroglia and Axonal Regeneration
1. Damage to neuron’s cell body usually kills the neuron, because mature neurons do not divide. But a damaged peripheral axon may regenerate.
2. After damage distal portion of axon and its myelin sheath degenerate.
3. Macrophages remove the fragments of myelin and other debris.
4. Nearby neuroglia secrete growth factors that guide developing sprouts from the cell body into a tube formed by the remaining Schwann Cells.
5. Schwann cells along the regenerating axon form new myelin.
6. The developing axon often ends up at the wrong place, so full function often does not return.
XI. The Synapse
A. Presynaptic Neuron – sends the signal
1. Synaptic Knob – contains secretory vesicles with neurotransmitters
2. Synaptic Cleft – gap between neurons
3. Neurotransmitters diffuse across the synaptic cleft and bind to receptors on postsynaptic cell
B. Postsynaptic Neuron – receives the signal
C. Synaptic Transmission
1. Action potential (electrical signal) travels along presynaptic neuron to axon terminal.
2. Calcium channels on synaptic knob open, Calcium diffuses into synaptic knob.
3. The Presynaptic Neuron releases neurotransmitters into the synaptic cleft via exocytosis.
4. Neurotransmitters diffuse across synaptic cleft and bind to receptors on postsynaptic cell.
5. Depending on the receptor and the neurotransmitter, the neurotransmitter may either excite the postsynaptic cell or inhibit the postsynaptic cell.
By Nrets [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
XII. Membrane Potential
A. Cell membranes are polarized (electrically charged).
B. Inside of cell is negatively charged compared to the outside of the cell.
C. Membrane Potential – refers to the electrical charge across the cell membrane.
D. All Cells have a membrane potential.
E. Membrane potential is maintained by three factors…
1. Na+/K+ ATPase (pumps)
Transports 3 Na+ out of the Cell, but only 2 K+ into the cell.
As positive charges leave the cell, inside of cell becomes negatively charged.
2. Potassium Leak (non-gated) channels
Makes cells very leaky to potassium, but not to sodium.
Even as Na+/K+ ATPases pump potassium into the cell, intracellular potassium continues to leak back out of the cell making the inside even more negative.
3. Intracellular Proteins and DNA
Negatively Charged proteins and DNA contribute to the membrane potential making the inside of a neuron more negative.
XIII. Resting Membrane Potential (RMP)
A. RMP = membrane potential of an excitable cell at rest.
B. Neuron cells and muscle cells are excitable.
C. RMP of a neuron = -70mV (inside the cell)
D. Changes in the Membrane Potential
1. Depolarization – inside of the cell becomes less negative than RMP.
2. Hyperpolarization – inside of the cell becomes more negative than RMP.
XIV. Non-Gated and Gated Channels
A. Non-gated “Leak” channels.
1. Non-gated channels are always open, allowing specific ions to freely move through the cell membrane.
2. Cells have many potassium leak channels, making them very leaky to potassium.
B. Gated Channels – open/close in response to a stimulus.
1. Mechanically gated channel
a. Opens in response to mechanical deformation of the cell membrane.
b. Examples include: touch, pressure, vibrations, hearing, etc.
2. Ligand (chemical) gated channel.
a. Opens when a chemical (ligand) binds to a receptor.
b. Ligands may be hormones, neurotransmitters, drugs, toxins, etc.
3. Voltage-gated channels
a. Open/Close in response to changes in the membrane potential.
b. Voltage-Gated Sodium Channels
open when the membrane potential = -55mV