Introduction
 During the first two weeks of life, cell division changes the
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fertilized egg or zygote into a group of cells known as
blastula, which then becomes a two-layered hollow cup
called gastrula.
From about the third to the ninth week, the cell colony is
called an embryo.
Ectoderm – (outer layer) will form the sense organs, skin
and nervous system.
Mesoderm – (middle layer) will form the blood, bone and
muscles.
Endoderm – (inner layer) will form the digestive system.
Nervous System
Central Nervous
System
Brain
Thalamus
Spinal Chord
Peripheral
Nervous System
Somatic Nervous
System
Autonomic
Nervous System
Cerebral Cortex
Sympathetic
Nervous System
Cerebellum
Parasympathetic
Nervous System
Hypothalamus
Brain Stem
Neurons (Nerve Cells)
 The neuron, or nerve cell, is the basic building block of
the nervous system.
 A neuron is highly specialized and amitotic.
 Structure of a Typical Neuron
 Cell Body – contains the nucleus of the cell, and two
types of fibers that branch off the cell body. It has a
nucleus with at least one nucleolus and contains many
of the typical cytoplasmic organelles. It lacks
centrioles, however. Because centrioles function in cell
division, the fact that neurons lack these organelles is
consistent with the amitotic nature of the cell.
 Dendrites - Dendrites and axons are cytoplasmic
extensions, or processes, which project from the cell
body. They are sometimes referred to as fibers.
Dendrites are usually, but not always, short and
branching, which increases their surface area to receive
signals from other neurons. They are called afferent
processes because they transmit impulses to the
neuron cell body.
 Axon - There is only one axon that projects from each
cell body. It is usually elongated and because it carries
impulses away from the cell body, it is called an
efferent process. An axon may have infrequent
branches called axon collaterals. Axons and axon
collaterals terminate in many short branches or
telodendria. The distal ends of the telodendria are
slightly enlarged to form synaptic bulbs. Many axons
are surrounded by a segmented, white, fatty substance
called myelin or the myelin sheath.
Types of Neuron
 Sensory or afferent (incoming) neurons – receive the
stimuli and carry them to the brain (sensation) for
interpretation (perception). The cell body of the
sensory neuron is located in the nerve root, which lies
outside the spinal cord. It receives external stimuli
through its dendrite fibers and relays the impulses
through the cell body into the spinal cord. Once inside
the spinal cord, impulses either travel to the brain or
pass directly to the motor neurons, which transmit the
impulses to the muscles and glands (effectors).
 Motor Neurons – relay the messages from the brain to
the muscles or glands. In these neurons, the cell body
is located in the spinal cord and the axon is long
enough to reach the neighboring neuron or even as far
as the muscle or gland to which it sends impulse.
 Interneuron or association neurons - located in the
brain and spinal cord. They connect the impulse from
the axon fibers of the sensory neuron to the dendrite
fibers of the motor neuron. They also provide nerve
impulses with alternate circuits nor pathways.
 The interneuron serve only as conductors. They do not
accept sensory stimuli as the sensory neurons do, nor do
they stimulate the glands or muscle cells as the motor
neurons do.