Neglected Brother: Part I

**Jean Schernoff** · 05-28-2002, 04:33 PM

If you see most people who are part of us, bodybuilders/strength trainers, most of them neglect many important things that they must know in order to understand, and build their bodies the better way. Today's discussion is about human nervous system. The reason this article is called Neglected Brother, is because most athletes and bodybuilders neglect the nervous system. And today we are going to discuss the physiology, while the second part of the article that is going to come out soon is going to be about training-to-failure affecting it. This article is written mostly for advanced bodybuilders/athletes who know some things about physiology. But newbies are always welcome.

Basic Stuff You Must Know
Our nervous system is divided into two principal divisions called the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of brain and spinal cord and serves as a control center for the entire body. Sections of the PNS integrate incoming information and determine appropriate responses.
The PNS is composed of receptors in the sense organs and nerves that communicate between the CNS and the sense organs. The PNS contains 12 pairs of cranial nerves and 31 pairs of spinal nerves. It informs the CNS of changing conditions inside the body and at the body's surface and then transmits CNS responses to the appropriate muscles and glands that bring about adjustments.

The PNS may be further divided into somatic division and the autonomic division. The somatic division of the PNS is concerned with changes in the external environment, while the autonomic division is concerned with changes in the internal body environment.
Both the CNS and PNS are composed of two types of nerves known as sensory and motor nerves. Sensory, or afferent, nerves transmit messages from the body's receptors to the CNS. But when there is a response to stimuli, small guys called motor nerves transmit messages from the CNS to the muscles or other structures.
The autonomic division of the PNS has two different types of motor nerves. One type, called sympathetic nerves, carries impulses to the body organs and mobilizes the response to stress. The second type called parasympathetic nerves, return the organs to a quiet, calm state.

Glial Cells
There are two unique types of cells that are found in the nervous system. They are neurons, nerve cells that receive and transmit biochemical information; and glial cells, which are also referred to neuralgia, provide structural support for the neurons. There are about ten times as many glial cells as neuron cells in the nervous systems. Neuralgia are a type of connective tissue cells. There are several types of glial cells. One type is called oligodendrocites. These cells wrap their plasma membranes above neurons and form sheaths. The sheaths are composed of fatty material called myelin. Another type of glial cells is the astrocytes, which have cytoplasm that extends itself into numerous elongated processes and gives the cells a star shape. Astrocytes help form the blood-brain barrier, which prevents or slows the flow of unwanted substances into the brain tissue. Astrocytes also help seal off damaged nerve tissue.
There are also neuroglia cells called microglia, which are scattered throughout the tissue of the brain and spinal cord. The cells act during responses to inflammation or injury, when they become mobile and actively phagocytize invading organisms.
And finally, the last type of cells is called Schwann cells. They wrap themselves around neurons located outside the CNS. They are found in myelin sheaths covering the neurons in the PNS. Schwann cells form cellular sheaths around the myelin sheaths.

Neurons (Nerves)
Neurons are cells specialized to receive and transmit information in the nervous system. The neuron is the the structural and functional unit of the nervous system. It is distinguished by the cytoplasmic extenxions usually associated with the cell.
A neuron may be classified by structure or function. Structurally, neurons are described as mutlipolar neurons, bipolar neurons, and unipolar neurons. Multipolar neurons have many short extensions called dendrytes (sp?) and a single long extension called the axon. Most of the CNS neurons are of this type. Bipolar neurons have only one dendrite and one axon. They are located in the retina of the eye, the inner ear, and the olfactory nerves. Unipolar neurons have only a single extension that functions as both axon and dendrite. Most sensory neurons are unipolar. Neurons may be classified functionally as sensory neurons, motor neurons, and interneurons. Sensory neurons transmit information from receptors to the CNS, and motor neurons relay messages from the CNS to the muscles and glands. Interneurons link sensory and motor neurons to one another. Interneurons lie within the CNS, where they receive information from the sensory neurons and send out messages via the motor neurons.

The cell body of the neuron contains a small percentage of the cell's total volume. It houses the nucleus and many other cellular organelles including the mitochondria, Golgi bodies, and lysosomes. A characteristic of the cell body is the Nissl body, an accumulation of an organelle known as the rough endoplasmic reticulum. Proteins are synthesized at this cellular location. The highly branched extensions of the cell body, the dendrites, are specialized to recieve nerve impulses and conduct them toward the cell body. Dendritic surfaces are dotted with thousands of spines where the dendrites form juctions with other neurons.

Impulses are transmitted away from the cell body by the axon. The axon arises from a thickened part of the cell body called the axon hillock. The cytoplasm within the axon is called axoplasm, and the membrane is the axolemma. The axon is microscopic in diameter, but it may extend several feet in length. For example, axons extending from the lower portion of the spinal cord down to the foot may be up to three feet long. Bundles of axons often travel together as a nerve fiber, commonly reffered to as nerve.
At the distal end of the axon are found thousands of microscopic branches called axon terminals. These branches are strudded with enlargements reffered to as synaptic knowbs aka terminal buttons. At the synaptic knobs, nerve cells release chemical substances called neurotransmitters. Neurotransmitters transmit nerve impulses from a neuron to a muscle or gland or to another neuron.

The Myelin Sheath and Neurilemma
The axons of many neurons of the PNS are covered by two sheaths: the myelin sheath and the neurilemma. The myelin sheath provides insulation to the axon. It is composed primarily of myelin, a white lipid-rich substance that is the principal component of the plasma membrane of the Schwann cell. Myelin insulates the electricity that speeds nerve impulses down the axon. Myelinated fibers conduct nerve impulses rapidly, while unmyelinated fibers conduct impulses slowly. Both myelinated and unmyelinated axons are found in the central nervous system.
The Schwann cell wraps it's plasma membrane around the axon to produce the myelin sheath. Between successive Schwann cells there are gaps called the nodes of Ranvier. At the nodes of Ranvier, the axon is not insulated within myelin. Myelin is responsible for the white color of the white matter in the brain and spinal cord. It also forms the white substance of myelinated peripheral nerves. Deterioration of patches of myelin can result in a condition called multiple sclerosis. The outer sheath surroundding th axon of PNS cells is called the neurilemma. The neurilemma functions in the regeneration of injured neurons. It is formed from the bulk of the Schwann cells remaining alongside the axon outside the myelin sheaths.

Nerves and Ganglia
A nerve consists of several bundles of axons, and each bundle is known as a fascice. Each fascicle in a nerve is surrounded by a sheath called a perineurium. Fibirous connective tissue called epineurium surrounds the nerve and binds the fascicles to one another. The cell bodies of neurons are generally grouped together in a mass called a ganglion (plural ganglia). Many glanglia exist outside the spinal cord. The axons of the cell bodies extend from these ganglia to other parts of the body.

Nerve Characteristics
The nervous system coordinates several activities that bring about a response to a stimulus. The first activity is reception, a process in which information is gathered from the external environment. The next activity is transmission, in which information is delivered by sensory neurons to the central nervous system. Then comes another activity called intergration, in which an appropriate response is determined. The final activity is response. In response, a nerve impulse is dispatched via motor neruons to skeletal muscle or glands that will regenerate a response to the stimulus. Muscles and glands are the body's primary effectors.

**Jean Schernoff** · 05-30-2002, 10:02 PM

Nerve Activity
During nerve activity, nerve impulses travel over a sequence of neurons. The sensory neurons, interneurons, and motor neurons are generall involved. These nerurons are organized into circuits called neural cicuits. In a neural circuit, meurons are arranged so that the axon of one neuron comes close to but does not join directly with the dendrite of the next neuron in the circuit. The junction between two close neurons is called synapse.
The reflex arc is the simplest unit of nerve activity. It is typified by the knee-jerk reflex, in which the patellar ligament is stuck and the lower leg raises up; and by the withdrawal refles, in which the finger is touched to something painful and immediately withdrawn. A reflex arc begins when a stimulation is detected in the receptor portion at the end of a sensory neuron. A nerve impulse is generated, and the impulse travels over the sensory neuron to interneurons in the central nervous system serving as a processing center. The interneurons communicate with motor neurons, and an impulse is generated for transmission to an effector muscle or gland that will make an appropriate response. In the wuthdrawal reflex, for example, the finger is pulled away from the pain as the muscles contract. The reflex arc is automatic and unconscious; it does not involve the brain or any mental activity. It helps maintain homeostasis in the body during such activities as sneezing, coughing, and swallowing, and it represents the most simple act that the nervous system can perform.

The Nerve Impulse
In many cases, a unit of nerve activity is initiated by stimulating a receptor at the body's surface. Among the familiar receptors are the sense organs such as eyes. ears. nose, and taste buds. Other receptor in the skin respond to pressure, light, touch, warmth, and cold. Once the receptor has been stimulated, the neural message is transmitted to the CNS over a sensory neuron. The nature of the nerve impulse is an electrochemical event arising from changes in ion distribution in the nerve cell.

As the name implies, a resting neuron is not transmitting an impulse. In a resting neuron, the outer surface of the plasma membrane carries a positive charge as compared to the cytoplasm inside the membrane of the nerve cell. The resting neuron is said to be polarized, that is, the inside and outside regions of the membrane have different electrical charges. Electrical charge separated in this way have the potential to do work should the difference in charges disappear. The difference in potential is called resting potential.

The resting potential is an imbalance in the electrical charges existing on either side of the plasma membrane. In a resting nerve cell, the resting potential is about 70 millivolts (mV). By convention this resting potential is expressed as -70mV ("negative"), because the inner surface of the plasma membrane is negatively charged relative to the positively charged region outside the membrane. The resting potential results from an excess of positively charged ions outside the plasma membrane as compared to inside the plasma membrane. There is also a slight excess of negative ions inside the cytoplasm. Outside the plasma membrane the concentration of sodium ions is over 10 times greater than inside plasma membrane. This gives the outside of the cell an overall positive charge as compared to the inside.

The ionic imbalance in a nerve cell is brought about by several factors. For example, the plasma membrane has a very efficient sodium-potassium pump. This pump actively transports sodium ions out of the cell and brings a small amount of potassium ions into the cell. The pump workds against the concentration gradient and, therefore, much ATP is required to maintain the pump. For every three sodium ions pumped out of the cell, two potassium ions are brought into the cell. Because more positive ions are pumped out than are brought in, a positive charge develops outside the cell.

The ion imbalance is also encouraged by ions dffusing from areas of high to areas of low concentration thorugh channels in the cell membrane. Some of these channels consist of charged protein molecules that undergo a structural chage when the membrane is stimulated. Channels such as these are called ion gates because they act as active gates and are specific to different types of ions. Sodium gates and potassium gates are examples of ion gates.
Contributing to the ionic imbalane are large numbers of negatively charged proteins within the cytoplasm of the neuron. Organic phosphate ions are also negatively charged, and thei accumulation within the cytoplasm adds further to the negative charge. Large negatively charged ions also exist within the cell and add to the ionic imbalance.

A nerve impulse is called an action potantial. When an action potential is generated, an electrical, chemical, or mechanical stimulus alters the resting potential by increasing the permeability of the plasma membrane to sodium. The sodium gates open, and the membrane of the neuron depolarizes. During depolarization, the resting potential drops to about -50mV, the threshold level of a nerve impulse. At that point the sodium ions channels open, and sodium ions flow rapidly into the cell. The flow continues for a thousandth of a second, then the channels close. The resting potential continues to drop to zero at this point and overshoots to about +35mV ("positive"), so that a momentary reversal in polarity takes place. The action potential is sufficiently strong to depolarize the adjacent area of the membrane, and that area depolarizes the next area, and so on. A wave of depolarization thus spreads like a chain reaction from area to adjacent area. The wave "travels" down the neuron at a constant velocity; the wave is the action potential. It is the nerve impulse.

Once the action potential has shot down the axon, the membrane begins to repolarize. The sodium gates close and potassium gates open, causing potassium to move out of the membrane. The leakage of potassium returns the exterior region to a positive state and repolarizes the membrane. The entire depolarization and repolarization can occur in less than one millisecond.

The return to the normal resting condition requires that sodium be pumped out of the neuron, and this pumping occurs over the next few seconds. In its depolarized state, the neuron is refractory; that is, it cannot transmit another action potential unless the stimulation is intense. However, once sodium ions have been pumped out and the ionic imbalance is once again established, the nerve cell is free to undergoa new action potential. Thus, the nerve cell conforms to all-or-none law: a stimulus strong enough to depolarize the neuron to its critical threshold results in a nerve impulse; a stronger stimulus results in the same impulse; the neuron either propagates the nerve impulse or it does not; there is no variation in strength of an impulse.

**Jean Schernoff** · 05-30-2002, 10:04 PM

The Synapse
What is a synapse you're asking? A synapse is a junction between two neurons or between a neuron and its effector muscle or gland. When the synapse exists between a neuron and a muscle cell, it is called neuromuscular junction, or you can also call it a motor end plate. The space itself is called the synaptic cleft. When an impulse reaches the end of an axon it is unable to jump the synaptic cleft. A chemical mechanism is needed to bridge the gap and conduct the message across to the next neuron or muscle/gland.

On arriving at the synaptic knobs at the axon's end, an impulse stimulates a release of chemical substances called neurotransmitters. Neurotransmitters swiftly diffuse across the synaptic cleft and change the permeability of the next neuron. If sufficient neurotransmitters are available, the dendrite of the next neuron is depolarized, and a nerve impulse is propagated. Neurotransmitters are continually synthesized in the synaptic knobs of axons. They are stored in membrane-bound sacs called synaptic vesicles within the synaptic knobs. When a nerve impulse reaches the synaptic knob, calcium ions from the surrounding tissue pass into the axon terminal and encourage the synaptic vesicles to release their contents into the synaptic cleft. This process occurs by exocytosis from the presynaptic membrane.
After diffusing across the synaptic cleft, neurotransmitters combine with receptor sites on membranes of the dendrites of the next neuron. These recpetors are protein molecules that form ion channels. The channels then open and permit ions to pass through the membrane into the dendrite. If ion passage is sufficiently intense, depolarization takes place, and a new nerve impulse is generated.

More than 50 different kinds of neurotransmitters are known to exist. One neurotransmitter extensively studies is acetylcholine. Acetylcholine is released from the neurons that innervate skeletal muscles, and it triggers muscle contraction. Acetylcholine is released by certain neurons in the autonomic division in the PNS and by certain neurons in the brain. After acetylcholine has combined with its receptors, the excess is removed by an enzyme called cholinesterase. Another well known neurotransmitter is norepinephrine. Norepinephrine is released by sympathetic neurons and many neurons in the brain and spinal cord. Other neurotransmitters are epinephrine and dopamine. The three compounds belong to a class of organic substances callec catecholamines.

To conclude this article, there are other neurotransmitters including serotonin, glutamate, glycine, and endorphins. Different neurotransmitters play different roles in our body. Different neurotransmitters are produced in different tissues of the body; for instance, glutamate is produced in the celebral cortex, while glycine is released in the spinal cord. Certain neurotransmitters excite the second neuron and lead to depolarization and nerve impulses called excitatory postsynaptic potentials (EPSPs). Other neurotransmitters inhibit the development of nerve impulses in the second neuron by decreasing the membrane permeability to sodium ions, which leads to inhibitory postsynaptic potentials (IPSPs).

Have fun, and remember to read the second part of this article which is also coming soon.

Jean,