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.
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 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.
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.
Thread: Neglected Brother: Part I