ORGANIZATION
The nervous system is one of the major control systems of the body and plays a key role in maintaining homeostasis. Recall that in homeostasis a receptor responds to change in the homeostatic condition and transmits this information to a control center. The control center integrates the information and may either store the information ( memory) or generate a response through effector organs (muscle and glands).
A conceptual model of how the nervous system is organized would be as follows:
Stimuli can be either external (light, sound, heat, tactile, etc.) or internal (pressure, H+, O2, etc.). The stimuli acts on receptors which transduce the stimuli into a nerve impulse. The nerve impulse is conducted by the nerves to the Central nervous System (CNS - brain and spinal cord). The CNS receives the information conducted by the nerves and integrates the information. The CNS then sends nerve impulses via the efferent or motor peripheral nervous system to effector organs (muscle and glands). Skeletal muscle is controlled by the somatic nervous system while the visceral organs - smooth muscle, cardiac muscle and glands - are controlled by the autonomic (visceral) nervous system. The autonomic nervous system has two subdivisions: the parasympathetic and the sympathetic. The components of the afferent and efferent nervous systems are outside the CNS and are referred to as the peripheral nervous system. The receptors and the effectors are connected to the central nervous system by nerves.
The CNS, especially the brain, is the major integrating component of the nervous system. It is here that perception and thoughts are generated and memory formed. Integration can then lead to action by activating the efferent motor system. Although the many varied functions of the brain are awesome, it is still composed of cells like any other organ. There are two distinct types of nerve cells: neuroglia and neurons. Although the function of neuroglial cells is still undergoing intensive study, it is generally assumed that their primary function is one of support and nurture. It is the neurons which display the physiological functions of irritability and the ability to conduct impulses.
The neuron is comprised of a nerve cell body (NCB) and cell processes. The nerve cell body (soma, perikaryon) is the region of the neuron surrounding the nucleus. There may be from one to many dendrites on each NCB. Dendrites are generally short and may branch numerous times. Dendrites are the major sites for connections (synapses) with other neuron processes. In most neurons dendrites conduct nerve impulses toward the NCB. In contrast to the dendrites, only one axon process is associated with the NCB. Where the axon leaves the NCB there is usually an extended area which is termed the hillock. Although there is only one axon per NCB, the axon may give off branches (collaterals) at right angles to the original axon. The axon at its distal end may divide into several terminal (presynaptic) branches. Axons carry nerve impulses away from the NCB. Axons vary in length from very short to over two feet in length.
There are several organelles associated with the neuron. Chromatophilic NISSL bodies are located in the dendrites and the NCB. The Nissl bodies are aggregates of ribosomes and constitute the production sites for proteins. Notice that the axon does not possess Nissl bodies and is therefore dependent on the dendrites and NCB for proteins. There are two primary cytoskeletal structures in neurons: neurofibrils and neurotubules. Neurofibrils are solid microfibers and are thought to serve for support giving strength to the thin axons. Neurotubules are hollow microtubules and are thought to be associated with the transport of materials within the axon. As mentioned previously the axon cannot produce proteins and must receive proteins from the dendrites and NCB. The transport of these proteins is correlated with the presence of neurotubules. The more neurotubules present the faster the transport of proteins in the axon (axonal transport). Neurotubules may also transport materials from the axon to the NCB. Many neurons may possess pigment vacuoles in the NCB. There is debate about the function of these pigments but they may constitute metabolic byproducts which are trapped within the cell.
As mentioned above the axon can reach long lengths and is very thin. This structure necessitates forms of support to prevent breakage. The neurofibrils provide internal support within the neuron and there are also support cells associated with the axon. Inside the CNS a form of neuroglia cell, the oligodendroglia, surrounds the axon and provides support. In the peripheral nervous system, support is provided by the Schwann cells. Both the oligodendroglia and the Schwann cell can produce myelin, a lipoprotein, which is laid down between the support membranes and the axon. In the peripheral axons there are gaps between the adjacent Schwann cells which are termed the Nodes of Ranvier. The point where the proximal edge of the first Schwann cell originates on the axon hillock is referred to as the first segment. The first segment is important physiologically as this is the point of origin of the nerve impulse which travels down the axon.
On the basis of structure neurons can be classified into three types: 1) Unipolar; 2) Bipolar and 3) Multipolar. Unipolar neurons make up the afferent (sensory) component of them peripheral nervous system. One branch runs from the periphery of the body to the NCB and is termed the peripheral fiber. The second fiber runs from the NCB to the CNS and is termed the central fiber. The NCB of the unipolar neuron is located outside the CNS in the posterior root of the spinal nerves. NCBs outside the CNS are termed ganglia and in this case represent the posterior root ganglia
Unipolar neuron
Bipolar neurons are characterized by having one dendrite and one axon.
Bipolar neurons are restricted in distribution and generally associated with the special sense of the head Olfactory epithelium, hair cells of the organ of Corti, and bipolar neurons of the retina.
Multipolar neurons are the most common form of neuron. The neurons comprising the efferent component of the peripheral nervous system, many of the interconnecting neurons within the CNS, and the majority of the neurons in the brain are multipolar neurons.
Neurons may also be classified by the function. The simplest functional classification would be:
1) Sensory (afferent) neurons - carry information from the body to the CNS - somatic are all unipolar.
2) Interneurons - within the CNS and interconnect neurons - multipolar
3) Efferent neurons - carry information from the CNS to the effectors - multipolar
NEURON SYNAPSES
Neurons carry out their functions by interacting with other cell: nerve cells; muscle cells; and gland cells. The interface between the neuron and the cell it interacts with is termed the synapse. On the basis of the cell type the neuron interacts with, the synapse may be classified as follows:
Neuron gland -------------------------- Neuroglandular
Neuron - muscle ------------------------- Neuromuscular (myoneural)
Neuron - neuron ------------------------- Neurosynapses
All synapses have the same basic components. There is a presynaptic component which generally conducts the nerve impulse toward the synapse. A postsynaptic component is activated by the presynaptic component and may conduct the nerve impulse away from the synapse. The presynaptic component is usually an axon and, in the nervous system, the postsynaptic component may be either a dendrite or the NCB. Outside the nervous system the presynaptic component could also terminate with either a muscle cell or a gland cell. In this case, the postsynaptic component would be the membrane of the muscle or gland cell. The pre- and postsynaptic membranes do not join but are separated by gap referred to as the synaptic cleft.
There are two types of physiological synapses:
1) Electrical 2) Chemical
Electric synapses are also called symmetrical synapses. They are symmetrical because the pre-and postsynaptic components have similar structure. There are no chemical transmitters present rather the pre-and postsynaptic membrane are joined together by gap junctions. When a nerve impulse reaches the end of the presynaptic axon it is directly transmitted to the postsynaptic membrane through the gap junctions. Because these synapses do not involve chemical transmitters, the impulse can also pass through the gap junctions from the postsynaptic component to the presynaptic component. Electric synapses have very restricted distribution in the human body. In mammals electrical synapses have been found in : 1) the lateral vestibular nucleus; 2) the mesencephalic portion of the V cranial nerve; 3) the retina of Primates; and 4) between neuroglia cells.
Chemical synapses are the dominant type of synapses in the human body. They are termed asymmetrical synapses because the structure and function of the presynaptic component is different from the postsysnaptic component. Synaptic vesicles which contain chemical transmitters are located within the presynaptic component. Most axons contain one chemical transmitter but some may contain more than one chemical transmitter. When a nerve impulse reaches the end of the presynaptic component (axon) the impulse initiates the release of the chemical transmitter into the synaptic cleft. The chemical transmitter diffuses across the synaptic cleft and interacts with a reeptor on the postsynaptic membrane. Interaction of the chemical transmitter with the postsynaptic membrane receptor initiates a response in the postsynaptic component. The postsynaptic membrane usually contains an enzyme which degrades the chemical transmitter allowing the postsynaptic component to return to the resting condition. In some synapses the presynaptic component may also contain degrading enzymes. Some synaptic membranes may also contain ÒpumpsÓ to remove the chemical transmitter, or its breakdown products, from the synaptic cleft. In some types of synapses, removal of the chemical transmitter by ÒpumpsÓ in the presynaptic membrane (Re-uptake) is the primary form of chemical transmitter removal from the synaptic cleft. Due to the chemical transmitter only being located in the presynaptic component, the nerve impulse can only pass from the presynaptic component to the postsynaptic component. Thus, chemical synapses are unidirectional. In addition, the time needed for the impulse to release the chemical transmitter, the time necessary for the chemical transmitter to diffuse across the synaptic cleft and interact with the postsynaptic receptor imposes a time delay between the impulse reaching the end of the presynaptic component and the generation of a response by the postsynaptic component. This synaptic delay averages approximately 0.5 mS.
The neurons comprising the CNS can be interconnected in many different ways and these patterns of neuron interaction may determine their function. The ways that the neurons are interconnected is termed their circuitry. Convergence is a form of circuitry where several presynaptic neurons terminate on one postsynaptic component.
Convergence circuits are important in the integration of information within the CNS. The output neuron receives input from several different sources ( i.e. several bits of information) and whether there is an output depends on the interaction of the various inputs on the output neuron. Another function of convergence circuits is the ability to receive weak stimuli. A single input may be too weak to generate an output by itself but when combined with other weak inputs, the combined input may be sufficient to fire the output neuron. An example of convergence occurs in the retina of the eye where several rods converge on one bipolar neuron. A single rod responding to weak light may not be sufficient to fire the bipolar neuron, but several weak impulses from several rods converging on the bipolar neuron can generate an response. This convergence allows the reception of very weak light by the retina.
The opposite phenomenon to convergence is divergence. In divergence, one input generates multiple outputs.
