Human Physiology/The Nervous System

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Nervous Tissue

The nervous system coordinates the activity of the muscles, monitors the organs, constructs and also stops input from

the senses, and initiates actions. Prominent participants in a nervous system include neurons and nerves, which play

roles in such coordination.Our nervous tissue only consists of two types of cells. These cells are neurons and

neuroglia cells. The neurons are responsible for transmitting nerve impulses. Neuroglia cells are responsible for

supporting and nourishing the neuron cells.

Types of Neurons

There are three types of neurons in the body. We have sensory

neurons, interneurons, and motor neurons. Neurons are a major class of

cells in the nervous system. Neurons are sometimes called nerve cells,

though this term is technically imprecise, as many neurons do not form

nerves. In vertebrates, neurons are found in the brain, the spinal cord

and in the nerves and ganglia of the peripheral nervous system. Their

main role is to process and transmit information. Neurons have

excitable membranes, which allow them to generate and propagate

electrical impulses. Sensory neuron takes nerve impulses or messages right from the sensory receptor and delivers it

to the central nervous system. A sensory receptor is a structure that can find any kind of change in it's surroundings

or environment.

Structure of a neuron

Neurons have three different parts to them. They all have an axon, a cell body and dendrites. The axon is the part of

the neuron that conducts nerve impulses. Axons can get to be quite long. When an axon is present in nerves, it is

called a nerve fiber. A cell body has a nucleous and it also has other organelles. The dendrites are the short pieces

that come off of the cell body that receive the signals from sensory receptors and other neurons.

Myelin Sheath

Schwann cells contain a lipid substance called myelin in their plasma membranes. When schwann cells wrap around

axons, a myelin sheath forms. There are gaps that have no myelin sheath around them; these gaps are called nodes of

Ranvier. Myelin sheathes make excellent insulators. Axons that are longer have a myelin sheath, while shorter axons

do not. The disease multiple sclerosis is an autoimmune disease where the body attacks the myelin sheath of the

central nervous system.

Case Study

A 35-year-old male in 1986 had been admitted to a hospital in Florida three weeks previous to being diagnosed, with

complaints of weakness and spasticity in the right leg, difficulties with balance, and fatigue and malaise. Tests

performed at the Florida hospital had revealed abnormalities in spinal fluid and MRI brain scan. The patient

complained of being severely depressed and anxious. He had anger at his circumstances and frequent crying spells.

One month previously he had noticed aching and loss of vision in the left eye that had since improved.

This man was diagnosed with Multiple Sclerosis. MS is a chronic, degenerative, and progressive disorder that affects

the nerve fibers in the brain and spinal cord. Myelin is a fatty substance that surrounds and insulates the nerve fibers

and facilitates the conduction of the nerve impulse transmissions. MS is characterized by intermittent damage to

myelin (called demyelination) caused by the destruction of specialized cells (oligodendrocytes) that form the

substance. Demyelination causes scarring and hardening (sclerosis) of nerve fibers usually in the spinal cord, brain

stem, and optic nerves, which slows nerve impulses and results in weakness, numbness, pain, and vision loss.