A human body is composed of trillions of cells. But here I will consider the cells of the nervous system, which are called neurons. They are messengers and carry message through an electrochemical process. The human brain has approximately 100 billion neurons and they carry message by action potential (which would be discussed briefly).
A typical neuron consists of 3 major parts i.e. cell body or soma, dendrites and axons. The cell body contains the nucleus, which stores the cell’s genes; the axon is a long slender cable that carries electrical signals known as action potentials away from the cell body toward other neurons; and the dendrites are shorter branching fibers that receive signals from other neurons.
Every neuron has its own shape, position & connections in the nervous system. They vary in size and shape and the smallest neuron can be 4 micron wide whereas the largest neuron can be of 100 micron wide.
Basic structure of a neuron
The basic characteristics of neurons are similar to other cells in the body because
- Neurons are surrounded by cell membrane.
- Neurons have a nucleus that contains genes.
- Neurons contain cytoplasm, mitochondria and other organelles.
- Neurons carry out basic cellular processes like protein synthesis and energy production.
But the neurons are different from other cell bodies because
- Neurons have specialized cell parts called dendrites and axons.
- Neurons communicate with each other by an electrochemical process.
- Neurons contain some specialized structures and chemicals.
The important components of a neuron along with other features are briefly discussed below:
Soma: The soma also known as the cell body of a neuron contains the nucleus and other structures common to living cells. The structures within the soma support the chemical processing of the neuron and production of neurotransmitters.
Axon: They are long slender projections of nerve cell or neuron, which conducts electrical impulses away from neuron’s cell body or soma. They are different from dendrites in several features like shape, length and function. Axons make contact with other cells at junctions called synapses. An axon’s branching tips communicate with the dendrites, axons and cell bodies of other neurons across tiny gaps called synapses.
Dendrites: Dendrites are extensions of the cell body of the neuron specialized for receiving and processing the vast majority of excitatory synaptic inputs. Synapses occur directly on the shaft of some dendrites, but other dendrites have specialized enlargements and protrusions to receive synaptic input. Dendrites make relatively local connections as compared with the axon. The axon, emerging either from the soma or a dendrite, may extend to distant targets, up to a meter or more away from the cell body in some cases. Dendrites are rarely longer than 1-2 mm, even in the largest neurons, and are often much smaller.
Difference between axons and dendrites are summarized below
Synapse: The junction between the axon terminals and the receiving cell of a neuron is called a synapse. There are two types of synapse i.e. chemical and electrical synapse. Synapses play a major role in memory formation.
Axon hillock: It is located at the end of the soma and is involved in firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire an action potential down the axon.
Myelin sheath: The myelin sheath is a greatly extended and modified plasma membrane wrapped around the nerve axon in a spiral fashion. The periodic interruptions where short portions of the axon are left uncovered by myelin are the nodes of Ranvier, and they are critical to the functioning of myelin.
Nodes or Ranvier: Two adjacent segments of myelin on one axon are separated by a node of Ranvier. In this region, the axon is not covered by myelin. Fat serves as a good insulator, the myelin sheaths speed the rate of transmission of an electrical impulse along the axon. The electrical impulse jumps from one node to the next at a rate as fast as 120 meters/second. This rapid rate of conduction is called saltatory conduction.
Terminal buttons: The Terminal Buttons of a neuron are the small knobs at the end of an axon that release chemicals called neurotransmitters. The terminal buttons form the presynaptic neuron of the synapse.
Classification of Neurons
Based on structure:
- Unipolar neurons: Have a short extension and divides into a axon and a dendrite.
- Bipolar neurons: Have two main extensions of similar length.
- Multipolar neurons: Have short dendrites originating from cell body and one along axon.
Based on function:
- Sensory neurons: Receive sensory signals from sensory organs and send them using axons to CNS.
- Motor neurons: Carries motor commands from cortex to spinal cord or from spinal cord to muscles.
- Interneurons: Sends information between sensory neurons and motor neurons.
In future posts, I would take the topics of action potential, neurotransmitters and how neurons communicate with each other.
- Dendrite Structure: John C. Fiala, Kristen M. Harris
- Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors.