Chapter 10
Nervous System I: Basic Structure and Function
1. Distinguish between neurons and neuroglial cells.
Neurons are the structural and functional cells reacting to the physical and chemical changes in their environment. Neuroglia are the supporting cells necessary for nourishing and maintaining the neurons, among other functions.
2. Explain the relationship between the central nervous system and the peripheral nervous system.
The central nervous system (CNS) is composed of the brain and the spinal cord. The peripheral nervous system (PNS) is composed of all of the peripheral nerves that connect all of the parts of the body with the CNS.
3. List three general functions of the nervous system.
The nervous system functions in three ways:
Sensory—The sensory function is accomplished by means of sensory receptors that note changes in their environment.
Integrative—The CNS can take the impulses from all of the sensory receptors and combine them to make perceptions and sensations about the environment.
Motor—The CNS can send impulses along some peripheral nerves to effectors in the muscles and glands in response to changes in the internal and external environment.
4. Describe the generalized structure of a neuron.
All neurons have a cell body and nerve fibers that are responsible for nerve impulse conduction to and from the cell body. The cell body (soma or perikaryon) contains granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules. Neurofibrils extend in a network throughout the microtubules and support them. Also found in the cytoplasm are Nissl bodies, which are membranous sacks of rough endoplasmic reticulum. Cytoplasmic inclusions contain glycogen, lipids, or pigments. The nucleus is large and spherical with a conspicuous nucleolus. The two kinds of nerve fibers extending from the cell body are called dendrites and axons. The many dendrites are typically highly branched to provide receptive surfaces (dendritic spines) for other neurons to communicate. A neuron usually has only one axon arising from the axon hillock. It is a nearly smooth cylindrical process with uniform diameter. It is filled with cytoplasm containing many mitochondria, microtubules, and neurofibrils. It is specialized to conduct nerve impulses away from the cell body. The axon starts as a single fiber but may branch off into collaterals, which may end in presynaptic terminals.
5. Define myelin.
Surrounding larger axons and dendrites of peripheral nerves are sheaths of neuroglial cells called Schwann cells. These cells are wound tightly around the fibers and, as a result, the cell membranes are layered closely together with little or no cytoplasm between them. The layers are composed of a lipoprotein called myelin, which forms a myelin sheath on the outside of the fibers. The outermost Schwann cells contain most of the cytoplasm and their nuclei remain outside the myelin sheath. This layer is known as neurolemma or neurolemmal sheath.
6. Distinguish between myelinated and unmyelinated nerve fibers.
A myelinated nerve fiber is one, which is bound by Schwann cells longitudinally along its length. The Schwann cells wrap tightly around the nerve fiber and form a myelin sheath. Unmyelinated nerve fibers lack these sheaths. In this case, these Schwann cells are not wound around the axons but simply for a grove or valley in which the axon sits. Myelinated (medullated) nerve fibers appear white. Unmyelinated nerve fibers appear gray.
7. Explain how neurons are classified on the basis of their structure.
Nerve fibers can be classified into three main groups:
Bipolar—Bipolar neurons have only two nerve fibers, one is the axon and one is the dendrite. They are from opposite sides of the cell body.
Unipolar—Unipolar neurons have a single nerve fiber extending from the cell body. From there it branches in two directions; one branch extends into a peripheral body part and serves as a dendrite. The other extends into the CNS and acts like an axon.
Multipolar—Multipolar neurons have one axon and many other extensions from the cell body that serve as dendrites.
8. Explain how neurons are classified on the basis of their function.
Nerve fibers can be classified into three groups:
Sensory—Sensory (afferent) neurons sense changes inside or outside the body by means of receptors ends or nearby receptor cells. They send impulses to the CNS in response to these changes. Most of these neurons are unipolar, with some bipolar.
Interneurons—Interneurons (association or internuncial neurons) are multipolar neurons found in the CNS. They link with other neurons and send impulses from one part of the CNS to another.
Motor—Motor (efferent) neurons are multipolar, and send impulses from the CNS to muscles or glands. There are two types of motor neurons that control smooth or cardiac muscle. Accelerator neurons increase muscle activity, while inhibitory neurons decrease muscle activity.
9. Discuss the functions of each type of neuroglial cell.
The PNS has only one type of neuroglial cell: the Schwann cell. The CNS has four different types of neuroglial cells. They are:
Astrocyte—Astrocytes are star-shaped cells located between neurons and blood vessels. They provide structural support and transport substances between the neurons and blood vessels. Astrocytes are joined together by gap junctions, providing a channel for circulating calcium ions. Other duties include metabolism of substances (such as glucose), keeping the synaptic clefts free of excess ions and neurotransmitters, and scar tissue formation after injury.
Oligodendrocytes—Oligodendrocytes function in myelin production. By extending numerous cellular processes, an oligodendroctye can provide myelin sheaths to several different axons. Because of this arrangement, no neurilemmal sheaths are formed.
Microglia—Microglia are small cells found throughout the CNS. They provide support and phagocytize bacteria and debris.
Ependyma—Ependyma are ciliated cuboidal or columnar cells found as the inner lining of the central canal and as a single-layered membrane covering the ventricles of the brain. They are joined together by gap and tight junctions, and provide a porous layer for substances to diffuse between the interstitial fluids of the brain and cerebrospinal fluid in the ventricles.
10. Describe how an injured nerve fiber may regenerate.
If the axon of a peripheral nerve is separated from the cell body, the distal portion deteriorates and the fragments are removed by macrophages. The proximal end then develops new sprouts, and nerve growth factors from surrounding neuroglia cause the sprouts to grow. At the same time, remaining Schwann cells proliferate and surround the new axon. If a sprout grows into the remaining basement membranes of the original tract, the new fiber may rejoin with its original connection. If the injured axon is from a neuron in the CNS, the lack of a myelin sheath prevents the new fiber from being guided to its original connection. Therefore, regeneration in the CNS is very unlikely. An injury to the cell body of a neuron usually causes death to the entire fiber, and no regeneration will occur.
11. Explain how a membrane may become polarized.
A cell, in its normal state, has a negatively charged interior with respect to the exterior. This is accomplished by transport mechanism channels that let potassium ions move easily in and out, and keep sodium and calcium ions under tight control.
12. Define resting potential.
Because the movement of sodium ions into the cell is slower than the movement of potassium out of the cell, there are more positive ions (cations) outside and more negative ions (anaions) inside. The difference in the charges between the two sides of the cell’s membrane is about –70 millivolts (in relation to the interior charge). This difference in charge is defined as the resting potential because it shows a potential to do the work (send a message).
13. Distinguish between depolarizing and hyperpolarizing.
If, in response to a stimulus, the resting potential becomes more negative, the cell is said to be hyperpolarizing. If the stimulus causes the resting potential to become less negative, the cell is said to be depolarizing.
14. List the changes that occur during an action potential.
When enough stimuli have accumulated to cause the threshold potential to be released, the area stimulated opens its sodium channels. As the sodium ions rush in, the inside of the cell becomes momentarily positive. At the same time, potassium channels open to allow the potassium ions out. This causes the inside of the cell to return to a negative charge (repolarization). The entire sequence takes less than 1/1,000th of a second.
15. Distinguish between action potentials and nerve impulses.
An action potential occurs at a specific site. When an action potential occurs at the trigger zone of a nerve cell, it sends an electrical impulse to the adjacent membrane. This causes an action potential at the next site. This occurs in a wavelike sequence, without losing amplitude, from the beginning of the fiber to the end, and is known as a nerve impulse.
16. Define refractory period.
After an action potential passes, the fiber needs time to return to its resting potential. This time is called the refractory period. The refractory period has two parts. The first is the absolute refractory period. This lasts about 1/2,500th of a second, and is the period when no new impulses can be sent. The second is called the relative refractory period. During this time, the fiber still has not returned to its resting potential, but enough sodium and potassium ions have moved to allow an action potential to occur to very strong stimuli.
17. Define saltatory conduction.
The Schwann cells surrounding a myelinated nerve fiber serve as an insulator and prevent most ions from passing through. Between adjacent Schwann cells is a small gap called a node of Ranvier, where the nerve fiber is exposed. When a nerve impulse is conducted along a myelinated fiber, it “jumps” from node to node. This type of conduction is called saltatory conduction.
18. Define synapse.
The ends of axons and dendrite are not directly connected to other neurons or effects. Instead, they terminate very close to them, leaving a space. The terminal end of the axon is called a presynaptic terminal and it communicates with a postsynaptic neuron. This “connection” is called a synapse.
19. Explain how a nerve impulse is transmitted from one neuron to another.
When a nerve impulse reaches the end of the presynaptic neuron, it enters several synaptic knobs that contain synaptic vesicles filled with neurotransmitters. The neurotransmitters are released into the synaptic cleft and trigger an action potential in the postsynaptic neuron.
20. Explain the role of calcium in the release of neurotransmitters.
An action potential moving across a synaptic knob’s membrane causes calcium channels to open and increases the membrane’s permeability. As the calcium ions move inward, some of the synaptic vesicles fuse with the presynaptic membrane and release their neurotransmitters. The number of vesicles that fuse to the membrane is directly related to the amount of calcium ions that move inward.
21. Define neuropeptide.
Neuropeptides are chains of amino acids found in the CAN and/or PNS that act as neurotransmitters or neuromodulators (substances that block neurotransmitters or alter a neuron’s response). Among the neuropeptides are two found only in the CNS that are potent natural pain relievers (enkephalins and beta endorphin) and one found throughout both the CNS and PNS that transmits pain impulses (substance P).
22. Distinguish between excitatory and inhibitory postsynaptic potentials.
Different neurotransmitters cause distinctly different responses in the postsynaptic neuron. If a neurotransmitter binding to the postsynaptic neuron causes sodium ion channels to open, the ions move inward and depolarize the membrane, possibly causing an action potential. Because this reaction causes the membrane to be closer to the threshold potential, it is said to be an excitatory postsynaptic potential (EPSP). If the neurotransmitter causes the potassium receptors to open, the postsynaptic membrane becomes hyperpolarized in response to an influx of potassium ions. This reaction makes an action potential less likely and is called an inhibitory postsynaptic potential (IPSP).
23. Describe the “trigger zone” of a neuron.
A trigger zone is an area at the proximal end of an axon at the axonal hillock that starts a nerve impulse from the action potential.
24. Describe the relationship between an input nerve fiber and its neuronal pool.
A neuronal pool is a group of neurons in the CNS that has special characteristics. It responds to impulses from input (afferent) nerve fibers. These fibers branch many times upon entering the neuronal pool to form hundreds of synapses with dendrites and cell bodies in a certain region of the pool.
25. Define facilitation.
Because a neuron in a region of a neuronal pool may receive excitatory and inhibitory impulses at the same time, the net effect may be either excitatory or inhibitory. If the net effect is excitatory enough to pass the threshold potential, an action potential will occur. If the net effect is excitatory but subthreshold, the neuron becomes more excitable and easier to push over the threshold. This state is called facilitation.
26. Distinguish between convergence and divergence.
When input nerve fibers from different areas of the body send impulses to the same neuron, they are said to converge. When a neuron sends impulses out on an output (efferent) nerve fiber, the impulse may stimulate others. These, in turn, may stimulate still others, and so on. Divergence works with input nerve fibers as well. For instance, the impulse form a sensory receptor may diverge and reach several different areas of the CNS for processing.
27. Explain how nerve impulses are amplified.
When a nerve impulse is diverged, it is carried on several nerve fibers. This amplifies its effect and causes a more forceful response.
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