The Nervous System

The basic structural unit of the nervous system is a nerve cell, or neuron. It consists of the following parts:

1. The cell body contains the nucleus and other cellular organelles.

2. The dendrite is typically a short, abundantly branched, slender extension of the cell body that receives stimuli.

3. The axon is typically a long, slender extension of the cell body that sends nerve impulses. A nerve impulse begins at the tips of the dendrite branches, passes through the dendrites to the cell body, then through the axon, and finally terminates at branches of the axon. Neurons can be classified into three general groups by their functions:

1. Sensory neurons (or afferent neurons) receive the initial stimulus. For example, sensory neurons embedded in the retina of the eye are stimulated by light, while certain sensory neurons in the hand are stimulated by touch.

2. Motor neurons (or efferent neurons) stimulate effectors, target cells that produce some kind of response. For example, efferent neurons may stimulate muscles (creating a movement to maintain balance or to avoid pain, for example), sweat glands (to cool the body), or cells in the stomach (to secrete gastrin in response to the smell of food, perhaps).

3. Association neurons (or interneurons neurons) are located in the spinal cord or brain and receive impulses from sensory neurons or send impulses to motor neurons. Interneurons are integrators, evaluating impulses for appropriate responses. The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of chemical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized, that is, there is a difference in electrical charge between the outside and inside of the membrane. In particular, the inside is negative with respect to the outside. Polarization is established by maintaining an excess of sodium ions (Na+) on the outside and an excess of potassium ions (K+) on the inside. A certain amount of Na+ and K+ is always leaking across the membrane, but Na+/K+ pumps in the membrane actively restore the ions to the appropriate side. Other ions, such as large, negatively charged proteins and nucleic acids, reside inside the cell. It is these large, negatively charged ions that contribute to the overall negative charge on the inside of the cell membrane compared to the outside. The following events characterize the transmission of a nerve impulse

1. Resting potential. The resting potential describes the unstimulated, polarized state of a neuron (at about –70 millivolts).

2. Action potential. In response to a stimulus, gated ion channels in the membrane suddenly open and permit the Na+ on the outside to rush into the cell. As the positively charged Na+ rush in, the charge on the cell membrane becomes depolarized, or more positive on the inside (from –70 toward 0 millivolts). If the stimulus is strong enough—that is, if it is above a certain threshold level—more Na+ gates open, increasing the inflow of Na+ even more, causing an action potential, or complete depolarization (about +30 millivolts). This, in turn, stimulates neighboring Na+ gates, further down the neuron, to open. In this manner, the action potential travels down the length of the neuron as opened Na+ gates stimulate neighboring Na+ gates to open. The action potential is an all-or-nothing event: when the stimulus fails to produce a depolarization that exceeds the threshold value, no action potential results, but when threshold potential is exceeded, complete depolarization occurs.

3. Repolarization. In response to the inflow of Na+, another kind of gated channel opens, this time allowing the K+ on the inside to rush out of the cell. The movement of K+ out of the cell causes repolarization by restoring the original membrane polarization. Unlike the resting potential, however, the K+ are on the outside and the Na+ are on the inside. Soon after the K+ gates open, the Na+ gates close.

4. Hyperpolarization. By the time the K+ gated channels close, more K+ have moved out of the cell than is actually necessary to establish the original polarized potential. Thus, the membrane becomes hyperpolarized (about –80 millivolts).

5. Refractory period.With the passage of the action potential, the cell membrane is in an unusual state of affairs. The membrane is polarized, but the Na+ and K+ are on the wrong sides of the membrane. During this refractory period, the neuron will not respond to a new stimulus. To reestablish the original distribution of these ions, the Na+ and K+ are returned to their resting potential location by Na+/K+ pumps in the cell membrane. Once these ions are completely returned to their resting potential location, the neuron is ready for another stimulus.

Some neurons possess a myelin sheath, which consists of a series of Schwann cells that encircle the axon. The Schwann cells act as insulators and are separated by gaps of unsheathed axon called nodes of Ranvier. Instead of traveling continuously down the axon, the action potential jumps from node to node (saltatory conduction), thereby speeding the propagation of the impulse. A synapse, or synaptic cleft, is the gap that separates adjacent neurons. Transmission of an impulse across a synapse, from presynaptic cell to postsynaptic cell, may be electrical or chemical. In electrical synapses, the action potential travels along the membranes of gap junctions, small tubes of cytoplasm that connect adjacent cells. In most animals, however, most synaptic clefts are traversed by chemicals, as follows:

1. Calcium (Ca2+) gates open. When an action potential reaches the end of an axon, the depolarization of the membrane causes gated channels to open and allow Ca2+ to enter the cell.

2. Synaptic vesicles release neurotransmitter. The influx of Ca2+ into the terminal end of the axon causes synaptic vesicles to merge with the presynaptic membrane, releasing molecules of a chemical called a neurotransmitter into the synaptic cleft.

3. Neurotransmitter binds with postsynaptic receptors. The neurotransmitter diffuses across the synaptic cleft and binds with proteins on the postsynaptic membrane. Different proteins are receptors for different neurotransmitters.

4. The postsynaptic membrane is excited or inhibited. Depending upon the kind of neurotransmitter and the kind of membrane receptors, there are two possible outcomes for the postsynaptic membrane.

• If Na+ gates open, the membrane becomes depolarized and results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated.

• If K+ gates open, the membrane becomes more polarized (hyperpolarized) and results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.

5. The neurotransmitter is degraded and recycled. After the neurotransmitter binds to the postsynaptic membrane receptors, it is broken down by enzymes in the synaptic cleft. For example, a common neurotransmitter, acetylcholine, is broken down by cholinesterase. Degraded neurotransmitters are recycled by the presynaptic cell. Some of the common neurotransmitters and the kind of activity they generate are summarized below:

1. Acetylcholine is commonly secreted at neuromuscular junctions, the gaps between motor neurons and muscle cells, where it stimulates muscles to contract. At other kinds of junctions, it typically produces an inhibitory postsynaptic potential.

2. Epinephrine, norepinephrine, dopamine, and serotonin are derived from amino acids and are mostly secreted between neurons of the central nervous system.

3. Gamma aminobutyric acid (GABA) is usually an inhibitory neurotransmitter among neurons in the brain. The nervous systems of humans and other vertebrates consist of two parts, as follows:

1. The central nervous system (CNS) consists of the brain and spinal cord.

2. The peripheral nervous system consists of sensory neurons that transmit impulses to the CNS and motor neurons that transmit impulses from the CNS to effectors. The motor neuron system can be divided into two groups, as follows: • The somatic nervous system directs the contraction of skeletal muscles. • The autonomic nervous system controls the activities of organs and various involuntary muscles, such as cardiac and smooth muscles.

There are two divisions of the autonomic nervous system:

1. The sympathetic nervous system is involved in the stimulation of activities that prepare the body for action, such as increasing the heart rate, increasing the release of sugar from the liver into the blood, and other activities generally considered as fight-or-flight responses (responses that serve to fight off or retreat from danger).

2. The parasympathetic nervous system activates tranquil functions, such as stimulating the secretion of saliva or digestive enzymes into the stomach. Generally, both sympathetic and parasympathetic systems target the same organs but often work antagonistically. For example, the sympathetic system accelerates the cardiac cycle, while the parasympathetic slows it down. Each system is stimulated as is appropriate to maintain homeostasis. A reflex arc is a rapid, involuntary response to a stimulus. It consists of two or three neurons— a sensory and motor neuron and, in some reflex arcs, an interneuron. Although neurons may transmit information about the reflex response to the brain, the brain does not actually integrate the sensory and motor activities.