Human Nerve/Muscle Connection Model

Nerve Fibers

Two kinds of nerve fibers, called "dendrites" and "axons," extend from the cell bodies of most neurons. Although a neuron usually has many dendrites, it has a single axon. In most neurons, the dendrites are relatively short and highly branched. These processes, together with the membrane of the cell body, provide the main receptive surfaces of the neuron to which processes from other neurons communicate. Often the dendrites have tiny, thornlike spines on their surfaces, which serve as contact points for parts of other neurons. The axon, which usually arises from a slight elevation of the cell body (axon hillock), is a slender, cyclindrical process with a nearly smooth surface and uniform diameter. This cable is a one-way pipe from one nerve cell to the axon terminal. Each nerve cell has a single axon, but the axon may have several branches (collaterals). The axon terminal is a point where the electrical charge sent from one nerve cell to another is changed into a chemical signal to be sent away from the region of the cell body.

Brain

The brain is a jelly-like substance, which in adults weighs about three pounds. It is divided into three parts: the brain stem, which is an extension of the spinal cord, the forebrain (which consists mainly of the cerebruim) and the cerebellum. The forebrain and cerebellum are divided into two hemispheres which are linked by a thick band of nerve fibers and these hemispheres have areas, called "lobes," which perform specific functions. The brain's surface lies in rather ugly, wrinkled folds. Traditionally referred to as one's "gray matter," it does, indeed, contain gray nerve cell bodies which surround a smaller mass of white nerve fibers. The brain, like the heart, is protected by a buffer zone. This, in the form of fluid, may be the source of "water on the brain," but it is very necessary to our survival. Only these pools of fluid and the skull protect the brain from the bumps and grinds of daily living which would damage this fragile organ. With them, we are able to think, reason, love, forgive, create and remember, as well as to survive through automatic processes such as breathing and digesting, and we have reflexes which signal in case of "fight or flight" emergencies. Just think of it!

Push/Pull Muscles

The body is made up of a set of levers, whose movements copy the geometry of classical mechanics. These levers are powered by muscles, the efficient body components whose day-to-day operation we all take for granted. Each of more than 600 muscles is served by nerves. Linking muscles to the brain and spinal cord, a network of nerve circuits carries signals that direct the ebb and flow of muscular energy. Many muscles must work together to perform even the simplest jobs. Much of the muscular activity occurs outside the region of the conscious mind, as the body, through the neuromuscular network, manages its own motion. Within each motor unit, muscle fibers obey the "all or none" principle, meaning that all contract or none contracts. If the muscle fibers of a motor unit are stimulated enough by nerve impulses to contract at all, they contract to the maximum. Athletes display some of the wonderful shows of force that the human body is capable of performing. Yet such force is only possible through the arrangement of the muscles, bones and joints that make up the body's lever systems. Bones act as the levers, while joints perform as living fulcrums. Muscle, attached to bones by tendons and other connective tissue, exerts force by converting chemical energy into tension and contraction. When a muscle contracts, it shortens, in many cases pulling bone like a lever across its hinge. Muscles move and by their motions we move. We are capable of performing a wide variety of actions, but despite this, muscle itself moves only by becoming shorter. They shorten and then they rest - in other words, a muscle can pull but it cannot push. We can see whole muscles contracting in this way but, in reality, they consist of millions of tiny, finely tuned protein filaments working in total concert. Muscles produce large amounts of heat. Involuntary contractions of muscle releases chemical energy, which produces heat to warm the body - we call these contractions "shivering."

Muscle Cells

A muscle is composed of bundles of specialized cells capable of contraction and relaxation to create movement. There are three types of muscle in the body: skeletal, smooth, and cardiac. There are only three basic types of muscle: the striped, or striated, skeletal muscles that move the bones; the smooth, involuntary muscles that line the blood vessels, stomach, digestive tract, and other internal organs; and the cardiac muscles, which are a cross between the smooth and the striped muscles. If one were to slice through a muscle diagonally, he would find that it resembles a telephone cable. Inside is a bundle of smaller cables, and each bundle surrounds still smaller ones. The first and largest bundle is made up of muscle fibers in which there are nerves, blood vessels, and connective tissue. Each fiber is built up from smaller strands called myofibrils, and each myofibril contains interlaced filaments of muscle proteins. Visceral tissue is tissue associated with the internal organs of the body, especially those in the abdominal cavity.

Neural Vesicle

A neural vesicle is sac-like structure that contains fluid for chemical transportation of impulses.

Acetylchonline

Acetylcholine is a chemical neuron which transfers information from one nerve cell to another.

Schwann Cells

Larger axons passing through peripheral nerves commonly are enclosed in sheaths called "Schwann cells." These are tightly wound around the axons, somewhat like insulation on a wire. The smallest axons also are enclosed in Schwann cells, but they are not wound around the axons. These membranes are composed largely of lipid-protein that has a higher proportion of lipid (fat) than other cell surface membranes. This lipid-protein is called myelin, and it forms a "myelin sheath" on the outside of an axon. Axons that have myelin sheaths are called "myelinated" (or medullated) nerve fibers, while those that lack these sheaths are "unmyelinated" nerve fibers. Myelin serves as an insulator by preventing almost all flow of ions through the membrane. Considering this, it might seem that the myelin sheath would prevent the conduction of a nerve impulse altogether, and this would be true if the sheath were continuous. It is, however, interrupted by some constrictions called "nodes of Ranier," which occur between adjoining Schwann cells. At these nodes, the fiber membrane is especially permeable to sodium and potassium ions.

Synapse

Within the nervous system, nerve impulses travel from neuron to neuron along complex nerve pathways. The junction between the parts of two such neurons is called a "synapse." Actually, these cells, called "presynaptic" and "postsynaptic neurons," are not in direct contact at the synapse. There is a gap called a "synaptic cleft" between them, and for an impulse to continue along a nerve pathway it must cross this space. The typical one-way transmission from axon to cell body is due to the fact that axons usually have rounded "synaptic knobs" at their presynaptic terminals, which the cell bodies lack. These knobs contain numerous membranous sacs, called "synaptic vesicles," and when a nerve impulse reaches a knob, some of the vesicles respond by releasing a substance which diffuses across the synaptic cleft. If a sufficient amount of the substance (called "neurotransmitter") is released, the membrane is stimulated, and a nerve impulse is triggered.