Vertebrates have four primary types of tissues namely epithelial, connective, muscle, and nerve tissues. The nerve tissues are made up of neurons and neuroglia (the supporting cells). The neurons, are specialized cells that produce and conduct electrochemical events, or “impulses”. Neuroglia provides support and insulation to the neurons and eliminate foreign materials in and around neurons and maintain overall homeostasis around the neurons.
- The Nervous System
The nervous tissues make up the nervous system, which is divided into the central nervous system (CNS, includes the brain and spinal cord) and the peripheral nervous system (PNS, includes nerves and ganglia (cluster of cell bodies). (see fig 1)
The peripheral nervous system (PNS) of vertebrates are further constituted by sensory and motor neurons.
Motor neurons are further of two types, Somatic and Autonomic motor neuron.
Somatic motor neurons stimulate skeletal muscles and hence control voluntary activities (moving hands, legs, etc).
Autonomic motor neurons regulate the activity of the smooth muscles, cardiac muscle, and glands, which perform involuntary functions (beating of heart, digestion, etc).
The autonomic motor neurons (i.e. involuntary) are further subdivided into the sympathetic (which prepares for action, utilizes energy) and parasympathetic (brings homeostasis after action, preserves energy) systems, which act to counterbalance each other.
Based on their function and location, neurons can be classified into three classes (fig 2):
- Sensory (or afferent) neurons
Sensory neurons carry impulses from sensory receptors to the central nervous system i.e. brain and spinal cord of vertebrates. Hence gives information about the environment to the body.
- Association neurons
These are found in the brain and spinal cord and often provide links between sensory and motor neurons and help provide more complex reflexes and higher associative functions, including learning and memory.
- Motor neurons
These neurons carry impulses from the CNS to the muscles and glands (effectors) and hence bring about the reaction to the environment.
Despite their varied appearances, most neurons have the same functional architecture. The overall structure of each neuron consists of three distinct parts (fig 3):
Dendrites
Most neurons have multiple dendrites, which extend outward from the cell body and are specialized to receive chemical signals (neurotransmitters) form the axon termini of other neurons or sensory organs. Dendrites convert these signals into small electric impulses and transmit them in the direction of the cell body. Then these electric disturbances g spread to the axon hillock and then axons.
Cell body:
Cell body is the enlarged region containing a nucleus, endoplasmic reticulum, ribosomes, Golgi apparatus, mitochondria, Lysosome and the other organelles that are essential for the functioning of all the cells.
All the neuronal and membrane proteins are synthesized here. These materials are assembled into membranous vesicles or multiprotein particles. These assembled structures are inserted into the plasma membrane or other organelles.
Axon:
Axons are unique extension of cytoplasm in neurons. They are designed specifically for the conduction of electric impulse called an action potential (explained in the next post), outward away from the cell body toward the axon terminus. In humans, axons may be more than meter long, yet it takes only a few milliseconds for an action potential to move along their length.
Many axons are encased by a myelin sheath, which provides insulation and facilitate a more rapid conduction of impulses. The sheath is interrupted at regular intervals by small gaps called nodes of Ranvier. In the peripheral nervous system, myelin sheaths are formed by neuroglia cells: Schwann cells (in PNS) or oligodendrocytes (in CNS).
The information from neuronal dendrites is integrated and “read out” at the origin of the axon called axon hillock and is then actively conducted down the axon finally into the axon terminals (diagram), the small branches of the axon which end into axon buttons.
Axon terminals form connections called synapses, with other cells. A single axon in the CNS can synapse with many neurons and induce responses in all of them simultaneously. The target cells or post synaptic cell of neurons include other nerve cells in the brain, spinal cord, and autonomic ganglia, and the cells of muscles and glands throughout the body.
Most abundant type of synapse in the nervous system are chemical synapses. The axon buttons contain secretory organelles called synaptic vesicles, which are filled with neurotransmitter molecules. The neurotransmitters bind to the neurotransmitter receptors on the target postsynaptic cells. Hence allows nerve cells to communicate the next cell, either a neuron, muscles or gland
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Ref
- Biology (6th ed) Raven PH, Johnson GB. McGraw-Hill, Boston 2002.
- Molecular cell biology (4th ed) Lodish H, Berk A, Zipursky New York 2000.
- Nerve Cells (2nd ed) Purves D, Augustine GJ, Fitzpatrick D, et al.Sunderland (MA): Sinauer Associates 2001.
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