SOMATIC NERVOUS SYSTEM | NEURON | TYPES OF NEURON


What is Somatic Nervous System? This system is under voluntary control. It controls the skeletal muscles of the body. We will discuss neuron, its structure and types.The peripheral nerves located throughout the body: Some carry information from each of the sensory receptors as in the eyes, ears, skin and so on to the brain. Some carry messages from the brain and spinal cord to the muscles and glands. Let’s discuss in detail Somatic Nervous System .

THE NEURON | SOMATIC NERVOUS SYSTEM

Neurons are the microscopically small cells that exist throughout our nervous system by the billion. They are tiny and complex. All behaviour begins with the action of neurons. A neuron is a cell specialized to receive process and transmit information to other cells within the body. Neurons forms the basic building blocks of the nervous system. They vary in shape, size, chemical composition and function. They have certain common characteristics also. A typical neuron gathers information at one end and transmits signals to the other.

STRUCTURE OF NEURON

A typical neuron has following parts.

Cell Body.All neurons are certain to have a cell body or soma. It contains the nucleus of the cell which contains the genetic information that keeps the cell functioning. With Nucleus, it contains the cytoplasm that sustains the cell’s life. The soma combines and averages all the information coming in from its dendrites and passes it to an extended fibre, the axon. Soma, often circular in shape, furnishes the cell with nutrients and manufactures chemicals used in transmitting information.
Dendrites It is derived from Greek word meaning tree. Dendrites are the branch-like extensions of a neuron’s cell body where most neural impulses are received. In a mature neuron, there may be thousands of thousands of dendrites, Typically dendrites reach out to receive messages, called neural impulses from other neurons.
Axon It is a long tail like extension of a neuron. It carries an impulse away from the cell body toward the synapse. Axons take the signal from the soma and conduct it along its length. This length varies; it can be more than three feet in the spinal cord and less than a millimetre in the brain. Axons end in swollen, bulb-like structures called terminal buttons. These provide the mechanism for getting the signals across from the axon of one neuron to the dendrites or soma of neighbouring neuron. Neurons transmit information in only one direction according to the law of forwarding conduction, from the axon of one neuron to the dendrites or cell bodies of the next.

Myelin Sheath. It is a white fatty covering found on some axons that serve to insulate and protect them while increasing the speed of impulse. Actually, neurons are covered with glial cells. They do not information as neurons, but they have three important functions conduct garbage removal, insulation and poison control. When neurons are damaged or dead glial cells in the area multiply and clean up the cellular junk left behind. The second function in the vertebrate brain is to form insulating glial cell covers called myelin sheath around the larger axons. Special of the embryo develops themselves around the axons as the nervous system.The fatty insulator greatly increases the speed of nerve signal conduction.  The third function of glial IS to prevent poisonous substance In the blood from reaching the delicate cells of the brain.

TYPES OF NEURONS


Three major types of neurons can be identified according to the types of cells to which they send, or, from which they receive information.

  1. Sensory neurons
  2. Motor neurons
  3. Interneuron’s

1.Sensory Neurons They are called afferent neurons as they carry messages in; towards the central nervous system from cells in the Periphery that are sensitive to light, sound, body position and the like.sensory neurons transmit impulses received by receptors to the central nervous system. The receptors are specialized cells in the sense organs, muscles, skin and joints that detect physical and chemical changes and translate these events into impulses that travel along the sensory neurons. The information travels from each sensory neuron’s dendrites to its soma, and then along its axon. After reaching the spinal cord, which itself contains neurons, the information is relayed up to a certain area of the brain.
2.Motor Neurons They are called efferent neurons. They carry messages out; away from the central nervous system to the muscles and glands.
3.Interneurons Sensory neurons rarely communicate directly with motor neurons. Between them IS, usually, the third category of neurons called interneurons. The interneurons are in billions, which make many contacts with each other before reaching a motor neuron. For every motor neuron in the body, there are as many as 5000 interneurons in the great intermediate net, which is the computational system of the brain. Interneurons are found only in brain and spinal cord.

SYNAPSE

There is a gap between the end of each neuron and the start of the next. This junction is known as synapse; it is the space where evolution has etched one of its most significant contributions to humanity. Much of the complexity of human consciousness and intelligence may be traced to the remarkable activity that takes place at the synapse. To bridge the gap and get the neural message across to the next neuron in line, electrical conduction in the axon is changed to chemical transmission. The signal is carried by chemical messengers from one side of the synapse, which is the presynaptic membrane of terminal button swellings filled with tiny packets or vesicles that store chemicals, across the gap to the postsynaptic membrane of the dendrites or some of the next neuron. This process of chemical transmission is carried by messengers called neurotransmitters. When a neural impulse travels down the axon and arrives at the axon terminals, it triggers the secretion of a chemical called a neurotransmitter. The neurotransmitter substance travels across the synaptic gap and stimulates the next neuron. Synaptic Transmission Synaptic junction is of tremendous importance because it is here that nerve cells transfer signals. A single neuron discharge, or “fires” when the stimulation reaches the threshold level. The neuron fires in a single, brief burst and are then inactive for a few thousands of a second. The size of neural impulse is constant and referred cannot be triggered by a stimulus unless it reached a threshold level. It is referred to as “all or by none”  principle of action.  The impulse’  once started travels down the axon to its many axon terminals.

REFLEX ARC

A nervous system not only transmits information but processes it also. Higher levels of complexity in information processing require circuits; a system of neurons functioning together to perform tasks that individual cells cannot carry out alone. The simplest circuits are reflex arcs, which may involve only a sensory neuron and motor neuron, or an interneuron between them in the spinal cord. Reflex arcs provide for automatic, rapid, simple responses. The body’s system of reflex arcs operates largely independent of brain involvement when the health and safety of the organism can benefit from a simple and swift response. A single reflex arc is a simple mechanism. A reflex action involves many reflex arcs and other interconnections and may be quite complex, Reflex action contains a number of neural circuits that perform certain responses quickly and automatically. These circuits contain chains of neurons, called reflex arcs, which are programmed to react automatically to certain stimuli, such as pain, noise or a tap on the knee. Reflex arcs involve sensory neurons, which travel to the spinal cord, and from there link up with motor neurons, which travel to the appropriate muscles or glands.