Page 74 - 84 human physiolofy part-1

Chapter 4

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
autoimune 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. Because
different nerves are affected at different times, MS symptoms often worsen (exacerbate), improve, and
develop in different areas of the body. Early symptoms of the disorder may include vision changes
(blurred vision, blind spots) and muscle weakness. MS can progress steadily or cause acute attacks
(exacerbations) followed by partial or complete reduction in symptoms (remission). Most patients with
the disease have a normal lifespan.

There are different types of MS
Multiple sclerosis is classified according to frequency and severity of neurological symptoms, the
ability of the CNS to recover, and the accumulation of damage.

74 | Human Physiology