M
ost of us can probably remember being told, when we were children, not to touch the stove or some other source of potential harm. Because children are curious, such warnings often go unheeded. The result? Touching a hot stove brings about an immedi- ate response of pulling away and a vivid memory of painful fingers. This simple and familiar experience illustrates the functions of the nervous system:1. To detect changes and feel sensations 2. To initiate appropriate responses to changes 3. To organize information for immediate use and
store it for future use
The nervous system is one of the regulating sys- tems (the endocrine system is the other and is dis- cussed in Chapter 10). Electrochemical impulses of the nervous system make it possible to obtain infor- mation about the external or internal environment and do whatever is necessary to maintain homeostasis.
Some of this activity is conscious, but much of it hap- pens without our awareness.
Yet another type of glial cell is the astrocyte(liter- ally, “star cell”). In the embryo, these cells provide a framework for the migrating neurons that will form the brain. Thereafter, the extensions of astrocytes are wrapped around brain capillaries and contribute to the blood–brain barrier, which prevents potentially harmful waste products in the blood from diffusing out into brain tissue. These waste products are normal in the blood and tissue fluid, but brain tissue is much
more sensitive to even low levels of them than are other tissues such as muscle tissue or connective tis- sue. The capillaries of the brain also contribute to this barrier, because they are less permeable than are other capillaries. A disadvantage of the blood–brain barrier is that some useful medications cannot cross it, and the antibodies produced by lymphocytes cross only with difficulty. This becomes an important considera- tion when treating brain infections or other diseases
Afferent (sensory) neuron Axon terminal
Axon
Nucleus
Cell body
Functional dendrite
Myelin sheath
Receptors
Dendrites
Nucleus
Axon terminal
Efferent (motor) neuron
Cell body
Axon
Schwann cell nucleus Myelin sheath
Node of Ranvier
A B
Schwann cell Axon
Neurolemma Layers of myelin sheath
C Figure 8–1. Neuron struc-
ture. (A) A typical sensory neu- ron. (B) A typical motor neuron.
The arrows indicate the direc- tion of impulse transmission.
(C) Details of the myelin sheath and neurolemma formed by Schwann cells.
QUESTION: The axon terminal of the motor neuron would be found at what kinds of effec- tors?
or disorders (Table 8–1 summarizes the functions of the neuroglia).
SYNAPSES
Neurons that transmit impulses to other neurons do not actually touch one another. The small gap or space between the axon of one neuron and the dendrites or cell body of the next neuron is called the synapse.
Within the synaptic knob (terminal end) of the presy-
naptic axon is a chemical neurotransmitter that is released into the synapse by the arrival of an electrical nerve impulse (Fig. 8–2). The neurotransmitter dif- fuses across the synapse, combines with specific recep- tor sites on the cell membrane of the postsynaptic neuron, and there generates an electrical impulse that is, in turn, carried by this neuron’s axon to the next synapse, and so forth. A chemical inactivatorat the cell body or dendrite of the postsynaptic neuron quickly inactivates the neurotransmitter. This pre- vents unwanted, continuous impulses, unless a new impulse from the first neuron releases more neuro- transmitter.
Many synapses are termed excitatory, because the neurotransmitter causes the postsynaptic neuron to depolarize (become more negative outside as Na⫹ions enter the cell) and transmit an electrical impulse to another neuron, muscle cell, or gland. Some synapses, however, are inhibitory, meaning that the neurotrans- mitter causes the postsynaptic neuron to hyperpolar- ize (become even more positive outside as K⫹ ions leave the cell or Cl⫺ions enter the cell) and therefore not transmit an electrical impulse. Such inhibitory synapses are important, for example, for slowing the heart rate, and for balancing the excitatory impulses transmitted to skeletal muscles. With respect to the skeletal muscles, this inhibition prevents excessive contraction and is important for coordination.
168 The Nervous System
BOX 8–1 MULTIPLE SCLEROSIS
protect the axon. Because loss of myelin may occur in many parts of the central nervous system, the symptoms vary, but they usually include muscle weakness or paralysis, numbness or partial loss of sensation, double vision, and loss of spinal cord reflexes, including those for urination and defecation.
The first symptoms usually appear between the ages of 20 and 40 years, and the disease may progress either slowly or rapidly. Some MS patients haveremissions, periods of time when their symp- toms diminish, but remissions and progression of the disease are not predictable. There is still no cure for MS, but therapies include suppression of the immune response, and interferon, which seems to prolong remissions in some patients. The possibility of stimulating remyelination of neurons is also being investigated.
Multiple sclerosis (MS) is a demyelinating dis- ease; that is, it involves deterioration of the myelin sheath of neurons in the central nervous system.
Without the myelin sheath, the impulses of these neurons are short-circuited and do not reach their proper destinations, and the neuron axons are damaged and gradually die.
Multiple sclerosis is an autoimmune disorder that may be triggered by a virus or bacterial infec- tion. Research has also uncovered a genetic com- ponent to some clusters of MS cases in families.
Exactly how such genes would increase a person’s susceptibility to an autoimmune disease is not yet known. In MS, the autoantibodies destroy the oligodendrocytes, the myelin-producing neu- roglia of the central nervous system, which results in the formation of scleroses, or plaques of scar tissue, that do not provide electrical insulation or
Table 8–1 NEUROGLIA
Name Function
Oligodendrocytes
Microglia
Astrocytes
Ependyma
• Produce the myelin sheath to electrically insulate neurons of the CNS.
• Capable of movement and phagocytosis of pathogens and damaged tissue.
• Support neurons, help main- tain K⫹level, contribute to the blood–brain barrier.
• Line the ventricles of the brain; many of the cells have cilia; involved in circulation of cerebrospinal fluid.
One important consequence of the presence of synapses is that they ensure one-way transmission of impulses in a living person. A nerve impulse cannot go backward across a synapse because there is no neuro- transmitter released by the dendrites or cell body.
Neurotransmitters can be released only by a neuron’s axon, which does not have receptor sites for it, as does the postsynaptic membrane. Keep this in mind when we discuss the types of neurons later in the chapter.
An example of a neurotransmitter is acetylcholine, which is found at neuromuscular junctions, in the CNS, and in much of the peripheral nervous system.
Acetylcholine usually makes a postsynaptic membrane more permeable to Na⫹ ions, which brings about depolarization of the postsynaptic neuron. Cholin-
esteraseis the inactivator of acetylcholine. There are many other neurotransmitters, especially in the cen- tral nervous system. These include dopamine, GABA, norepinephrine, glutamate, and serotonin. Each of these neurotransmitters has its own chemical inactiva- tor. Some neurotransmitters are reabsorbed into the neurons that secreted them; this process is called reuptakeand also terminates the effect of the trans- mitter.
The complexity and variety of synapses make them frequent targets of medications. For example, drugs that alter mood or behavior often act on specific neu- rotransmitters in the brain, and antihypertensive drugs affect synapse transmission at the smooth mus- cle of blood vessels.
Na+
Na+
Na+
Axon of presynaptic neuron
Vesicles of neurotransmitter Receptor site Inactivator (cholinesterase)
Dendrite of postsynaptic
neuron
Inactivated neurotransmitter Neurotransmitter
(acetylcholine) Mitochondrion
Figure 8–2. Impulse transmission at a synapse. The arrow indicates the direction of the electrical impulse.
QUESTION:Is this an excitatory synapse or an inhibitory synapse? Explain your answer.