In addition to conducting tissue, the nervous system contains cells that serve for support and protection. Collectively, these cells are called neuroglia or glial cells, from a Greek word meaning “glue.” There are different types of neuroglia, each with specialized functions, some of which are the following:
* Protect nervous tissue;
* Support nervous tissue and bind it to other structures;
* Aid in repair of cells;
* Act as phagocytes to remove pathogens and impurities;
* Regulate the composition of fluids around and between cells.
Neuroglia appear throughout the central and peripheral nervous systems. The Schwann cells that produce the myelin sheath in the peripheral nervous system are one type of neuroglia. Another example is shown in Figure 5-6. These cells are astrocytes, named for their starlike appearance. In the brain they attach to capillaries (small blood vessels) and help protect the brain from harmful substances. Unlike neurons, neuroglia continue to multiply throughout life. Because of their capacity to reproduce, most tumors of the nervous system are tumors of neuroglial tissue and not of nervous tissue itself.
Figure 5-6 Examples of neuroglia. (A) Astrocytes in the white matter of the brain. (B) Astrocytes attach to capillaries and help to protect the brain from harmful substances.
The Nervous System at Work
The nervous system works by means of electrical impulses sent along neuron fibers and transmitted from cell to cell at highly specialized junctions.
The Nerve Impulse
The mechanics of nerve impulse conduction are complex but can be compared with the spread of an electric current along a wire. What follows is a brief description of the electrical changes that occur as a resting neuron is stimulated and transmits a nerve impulse. The plasma membrane of an unstimulated (resting) neuron carries an electrical charge, or potential. This resting potential is maintained by ions (charged particles) concentrated on either side of the membrane.
At rest, the inside of the membrane is negative as compared with the outside. In this state, the membrane is said to be polarized. As in a battery, the separation of charges on either side of the membrane creates a possibility (potential) for generating energy. If there is a way for the charges to move toward each other, electricity will be generated. A nerve impulse starts with a local reversal in the membrane potential caused by changes in the ion concentrations on either side. This sudden electrical change at the membrane is called an action potential. A simple description of the events in an action potential is as follows (Fig. 5-7):
* The resting state. In addition to an electrical difference on the two sides of the plasma membrane at rest, there is also a slight difference in the concentration of ions on either side. At rest, sodium ions (Na+) are a little more concentrated at the outside of the membrane. At the same time, potassium ions (K+) are a little more concentrated at the inside of the membrane.
* Depolarization. A stimulus of adequate force, such as electrical, chemical, or mechanical energy, causes specific channels in the membrane to open and allow Na+ ions to flow into the cell. (Remember that substances flow by diffusion from an area where they are in higher concentration to an area where they are in lower concentration.) As these positive ions enter, they raise the charge on the inside of the membrane, a change known as depolarization
(see Fig. 5-7).
* Repolarization. In the next step of the action potential, K+ channels open to allow K+ to leave the cell. As the electrical charge returns to its resting value, the membrane is undergoing repolarization. At the same time that the membrane is repolarizing, the cell uses active transport to move Na+ and K+ back to their original concentrations on either side of the membrane so that the membrane can be stimulated again. This activity is described as the Na+/K+ pump. The action potential occurs rapidly-in less than 1/1000 of a second, and is followed by a rapid return to the resting state (Fig. 5-8). However, this local electrical change in the membrane stimulates an action potential at an adjacent point along the membrane. In scientific terms, the channels in the membrane are “voltage dependent,” that is, they respond to an electrical stimulus. And so, the action potential spreads along the membrane as a wave of electrical current. The spreading action potential is the nerve impulse, and in fact, the term action potential is used to mean the nerve impulse. A stimulus is any force that can start an action potential by opening membrane channels and allowing Na+ to enter the cell.
Figure 5-7 The action potential. In depolarization, Na+ membrane channels open and Na+ enters the cell. In repolarization, K+ membrane channels open and K+ leaves the cell. During and after repolarization, the Na+/K+ pump returns ion concentrations to their original concentrations so the membrane can be stimulated again.
Figure 5-8 A nerve impulse. From a point of stimulation, a wave of depolarization followed by repolarization travels along the membrane of a neuron. This spreading action potential is a nerve impulse.
The Role of Myelin in Conduction
As previously noted, some axons are coated with the fatty material myelin. If a fiber is not myelinated, the action potential spreads continuously along the membrane of the cell (see Fig. 5-8). When myelin is present on an axon, however, it insulates the fiber against the spread of current. This would appear to slow or stop conduction along these fibers, but in fact, the myelin sheath speeds conduction. The reason is that the action potential must “jump” like a spark from node (space) to node along the sheath, and this type of conduction is actually faster than continuous conduction.
Neurons do not work alone; impulses must be transferred between neurons to convey information within the nervous system. The point of junction for transmitting the nerve impulse is the synapse, a term that comes from a Greek word meaning “to clasp” (Fig. 5-9).
Figure 5-9 A synapse. (A) The end-bulb of the presynaptic (transmitting) axon has vesicles containing neurotransmitter, which is released into the synaptic cleft to the membrane of the postsynaptic (receiving) cell. (B) Close-up of a synapse showing receptors for neurotransmitter in the postsynaptic cell membrane.
At a synapse, transmission of an impulse usually occurs from the axon of one cell, the presynaptic cell, to the dendrite of another cell, the postsynaptic cell. Information must be passed from one cell to another at the synapse across a tiny gap between the cells, the synaptic cleft. Information usually crosses this gap in the form of a chemical known as a neurotransmitter. While the cells at a synapse are at rest, the neurotransmitter is stored in many small vesicles (bubbles) within the enlarged endings of the axons, usually called end-bulbs or terminal knobs, but known by several other names as well. When a nerve impulse traveling along a neuron membrane reaches the end of the presynaptic axon, some of these vesicles fuse with the membrane and release their neurotransmitter into the synaptic cleft. The neurotransmitter then acts as a chemical signal to the postsynaptic cell. On the postsynaptic receiving membrane, usually that of a dendrite, but sometimes another part of the cell, there are special sites, or receptors, ready to pick up and respond to specific neurotransmitters. Receptors in the postsynaptic cell membrane influence how or if that cell will respond to a given neurotransmitter.
Neurotransmitters Although there are many known neurotransmitters, the main ones are epinephrine, also called adrenaline; a related compound, norepinephrine, or noradrenaline; and acetylcholine. Acetylcholine (ACh) is the neurotransmitter released at the neuromuscular junction, the synapse between a neuron and a muscle cell. All three of the above neurotransmitters function in the ANS. It is common to think of neurotransmitters as stimulating the cells they reach; in fact, they have been described as such in this discussion. Note, however, that some of these chemicals inhibit the postsynaptic cell and keep it from reacting, as will be demonstrated later in discussions of the autonomic nervous system. The connections between neurons can be quite complex. One cell can branch to stimulate many receiving cells, or a single cell may be stimulated by a number of different axons (Fig. 5-10). The cell’s response is based on the total effects of all the neurotransmitters it receives at any one time. After its release into the synaptic cleft, the neurotransmitter
may be removed by several methods:
* It may slowly diffuse away from the synapse.
* It may be destroyed rapidly by enzymes in the synaptic cleft.
* It may be taken back into the presynaptic cell to be used again, a process known as Reuptake. The method of removal helps determine how long a neurotransmitter will act.
Electrical Synapses Not all synapses are chemically controlled. In smooth muscle, cardiac muscle, and also in the CNS there is a type of synapse in which electrical energy travels directly from one cell to another. The membranes of the presynaptic and postsynaptic cells are close together and an electrical charge can spread directly between them. These electrical synapses allow more rapid and more coordinated communication. In the heart, for example, it is important that large groups of cells contract together for effective pumping action.
Figure 5-10 The effects of neurotransmitters on a neuron. A single neuron is stimulated by axons of many other neurons. The cell responds according to the total of all the excitatory and inhibitory neurotransmitters it receives.