Neurons are the basic unit of the nervous system and nervous tissue. All cells of the nervous system are comprised of neurons. The nervous system helps us to sense and respond to our environment and can be divided into two parts: the central nervous system and the peripheral nervous system.
The central nervous system consists of the brain and spinal cord, while the peripheral nervous system consists of sensory and motor nerve cells that run throughout the rest of the body. Neurons are responsible for sending, receiving, and interpreting information from all parts of the body.
Parts of a Neuron
A neuron consists of two major parts: a cell body and nerve processes.
Neurons contain the same cellular components as other body cells. The central cell body is the process part of a neuron and contains the neuron’s nucleus, associated cytoplasm, organelles, and other cell structures. The cell body produces proteins needed for the construction of other parts of the neuron.
Nerve processes are “finger-like” projections from the cell body that are able to conduct and transmit signals. There are two types:
- Axons typically carry signals away from the cell body. They are long nerve processes that may branch out to convey signals to various areas. Some axons are wrapped in an insulating coat of glial cells called oligodendrocytes and Schwann cells. These cells form the myelin sheath which indirectly assists in the conduction of impulses as myelinated nerves can conduct impulses quicker than unmyelinated ones. Gaps between the myelin sheath are called Nodes of Ranvier. Axons end at junctions known as synapses.
- Dendrites typically carry signals toward the cell body. Dendrites are usually more numerous, shorter, and more branched than axons. They have many synapses in order to receive signal messages from nearby neurons.
Information is communicated among nervous system structures through nerve signals. Axons and dendrites are bundled together into what are called nerves. These nerves send signals between the brain, spinal cord, and other body organs via nerve impulses. Nerve impulses, or action potentials, are electrochemical impulses that cause neurons to release electrical or chemical signals that initiate an action potential in another neuron. Nerve impulses are received at neuronal dendrites, passed through the cell body, and are carried along the axon to the terminal branches. Since axons can have numerous branches, nerve impulses can be transmitted to numerous cells. These branches end at junctions called synapses.
It is at the synapse where chemical or electrical impulses must cross a gap and be carried to the dendrites of adjacent cells. At electrical synapses, ions and other molecules pass through gap junctions allowing for the passive transmission of electrical signals from one cell to the other. At chemical synapses, chemical signals called neurotransmitters are released which cross the gap junction to stimulate the next neuron. This process is accomplished by exocytosis of the neurotransmitters. After crossing the gap, neurotransmitters bind to receptor sites on the receiving neuron and stimulate an action potential in the neuron.
Nervous system chemical and electrical signaling allow for quick responses to internal and external changes. In contrast, the endocrine system, which uses hormones as its chemical messengers, is typically slow-acting with effects that are long-lasting. Both of these systems work together to maintain homeostasis.
There are three main categories of neurons. They are multipolar, unipolar, and bipolar neurons.
- Multipolar neurons are found in the central nervous system and are the most common of the neuron types. These neurons have a single axon and many dendrites extending from the cell body.
- Unipolar neurons have one very short process that extends from a single cell body and branches into two processes. Unipolar neurons are found in spinal nerve cell bodies and cranial nerves.
- Bipolar neurons are sensory neurons consisting of one axon and one dendrite that extend from the cell body. They are found in retinal cells and olfactory epithelium.
Neurons are classified as either motor, sensory, or interneurons. Motor neurons carry information from the central nervous system to organs, glands, and muscles. Sensory neurons send information to the central nervous system from internal organs or from external stimuli. Interneurons relay signals between motor and sensory neurons.