c. Neurilemma. The neurilemma is the nucleated cytoplasmic layer of the
Schwann cell. The neurilemma allows damaged nerves to regenerate. Nerves in the
brain and spinal cord DO NOT have a neurilemma and, therefore, DO NOT recover
TYPES OF NEURONS
Neurons in the body can be classified according to structure and function.
Classification by structure is based on the number of processes (projections or
protrusions) extending from the cell body. Neurons are classified by functions
according to the direction in which the neurons transmit impulses.
a. Classification of Neurons by Structure. Classifications are as follows:
multipolar neurons, bipolar neurons, and unipolar neurons. Multipolar neurons have
one axon and several dendrites. Brain and spinal cord neurons are generally multipolar
neurons. Bipolar neurons have one axon and one dendrite. Most of these neurons are
found in the retina of the eye, the inner ear, and the olfactory area. Neurons with only
one process extending from the cell body are termed unipolar neurons. The one
process divides with one part acting as an axon and the other part functioning as a
dendrite. Unipolar neurons are found in the posterior (sensory) root ganglia of the
b. Classification of Neurons by Function. According to function, there are
two types of neurons: sensory neurons (also called afferent neurons) and motor
neurons (also called efferent neurons). Sensory neurons conduct impulses from the
receptors in the skin, sense organs, and viscera (the large internal organs) to the brain
and the spinal cord. These neurons conduct impulses from receptors to the central
nervous system and are usually unipolar. Impulses transmitted from the brain and
spinal cord to either muscles or glands are carried by motor neurons. These neurons
conduct information away from the central nervous system to the skin, muscles, glands,
and organs of the body.
An impulse follows a conduction pathway from its origin in the dendrites or
neuron cell body in one part of the body to the impulse's end somewhere else in the
body. One pathway is called a reflex arc and is a functional unit of the nervous system.
The basic parts of a reflex arc are a receptor, a sensory neuron, a center, a motor
neuron, and an effector.
a. Function of Components of a Reflex Arc. Functions are:
(1) Receptor. The receptor activates a nerve impulse in a sensory neuron
in response to a change in the body's internal or external environment.