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The Nervous System. Introduction. During the first two weeks of life, cell division changes the fertilized egg or zygote into a group of cells known as blastula , which then becomes a two-layered hollow cup called gastrula .
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Introduction • During the first two weeks of life, cell division changes the 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.
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.
