It forms the organs of the nervous system, influencing the behavior that is constituted by the bodies of the nerve cells housed in it.
The nervous system consists of organs composed primarily of nervous tissue supported and protected by connective tissues.
Composition and function
Two types of cells make up nervous tissue: neurons and neuroglia.
Neurons, or nerve cells, are the structural and functional units of the nervous system. They are delicate cells that are specialized to generate and transmit nerve impulses. Neurons can vary in size and shape, but they have many characteristics in common.
The cell body is the portion of a neuron that contains the sizeable spherical nucleus. The cell body also has the usual cytoplasmic organelles. Two neuronal processes extend from the cell body: dendrites and axons.
A neuron can have many dendrites, but it only has one axon.
Dendrites are generally short, highly branched, and tapering processes that receive impulses ( electrochemical signals ) from other neurons and sensory receptors. The dendrites carry impulses to the cell body or the axon.
An axon, or nerve fiber, is a long, thin process in a neuron. It may have one or more lateral branches, called axon collaterals. It also forms a series of short and delicate terminal arborization components at its distal tip.
The slightly increased tips of the terminal arborization are the terminal buttons, which form junctions (synapses) with other neurons, muscles, adipose tissue, or glands. An axon carries nerve impulses away from the cell body or dendrites.
Some axons are encased in an insulating myelin sheath of special neuroglia. These axons are known as myelinated axons. The myelin sheath increases the speed of nerve impulse transmission.
The small spaces between adjacent myelin-forming cells, where the axon is exposed, are known as myelin sheath spaces (or nodules of Ranvier).
Axons that lack a myelin sheath are called unmyelinated axons and have a much slower rate of nerve impulse transmission.
Types of neurons
Neurons can be classified according to their anatomy or their function. There are three basic types of neurons structurally: multipolar, bipolar, and unipolar neurons.
- Multipolar neurons have several dendrites and a single axon that extends from the cell body. Most neurons whose cell bodies are found in the brain and spinal cord are multipolar neurons.
- Bipolar neurons only have two processes: a dendrite and an axon that extend from opposite ends of the cell body. Bipolar neurons occur in the sensory portions of the eyes, ears, and nose.
- Unipolar neurons have a single process extending from the cell body. This process quickly divides into two branches extending opposite directions, with both departments functioning as a single axon.
One end of the axon ends in a terminal arborization, while the other ends in dendrites. Unipolar neurons carry nerve impulses from sensory receptors to the CNS.
Clusters of cell bodies of unipolar neurons often form ganglia (singular, ganglion) located in the PNS.
Functionally, there are three basic types of neurons: sensory neurons, interneurons, and motor neurons.
Sensory neurons carry nerve impulses from peripheral parts of the body to the CNS. Its dendrites are associated with sensory receptors or are specialized to detect changes directly.
Nerve impulses are carried through an axon within the cranial or spinal nerves to the CNS.
The cell bodies of sensory neurons are located outside the CNS in the ganglia. Structurally, most sensory neurons are unipolar, although bipolar neurons are found in unique sense organs.
Interneurons are located entirely within the CNS and form synapses with other neurons. They are responsible for the processing and interpretation of nerve impulses by the CNS.
Interneurons receive nerve impulses from sensory neurons and transmit them from one place to another within the CNS. They also activate motor neurons, resulting in the stimulation of effectors. Interneurons are multipolar neurons.
Motor neurons carry nerve impulses from the CNS to effectors to produce an action. Their cell bodies and dendrites are found within the CNS, while their axons are located in the cranial and spinal nerves. Motor neurons are multipolar neurons.
The neuroglia provides support and protection for neurons. One type of neuroglia – Schwann cells – occurs in the PNS. There are four types of glia in the CNS, where they are even more numerous than neurons.
Schwann cells form the myelin sheath around the myelinated axons of PNS. They are wrapped tightly around an axon many times so that the nucleus and most of the cytoplasm are compressed into the superficial layer.
The deep layers of plasma membrane layers constitute the myelin sheath. The most superficial layer forms the neurilemma, essential for axon regeneration after injury.
Oligodendrocytes form the myelin sheath of myelinated axons within the CNS, but they do not create a neurilemma. The lack of a neurilemma contributes to the inability of axons in the brain and spinal cord to regenerate after injury.
Astrocytes are the primary supporting cells for neurons in the CNS. They stimulate the growth of neurons and influence synaptic transmission.
Astrocytes also bind with the epithelium of blood vessels to form the blood-brain barrier, which protects neurons by tightly regulating the exchange of materials between blood and neurons.
Microglial cells are found throughout the CNS, where they keep tissues clean by engulfing and digesting cellular debris and pathogens.