Electrochemical signals are sent, received, and transmitted throughout the body via neurones, the brain’s information processing units.
In essence, neurons—also referred to as nerve cells—are the cells that comprise the nervous system and the brain. Although neurones do not come into contact with one another, a synapse is created when two neurones approach one another.
There are approximately 86 billion neurones in the human brain, according to recent studies (Herculano-Houzel, 2009). Unlike other cells, these cells cannot procreate or regenerate when they die, although they do mature fully around the time of birth.
How Are Neurones Operational?
Although they are not linked, neurones are located next to one another. A synapse is a very small space between neurones. Neurones are responsible for sending nerve impulses from one neurone to the next along their length and across the synapse. Action potentials are the electrical impulses that neurones send out. To go to or from the central nervous system, the electrical signal must pass across the synaptic gap. Chemicals that diffuse over the space between the two neurones are used to accomplish this. We refer to these substances as neurotransmitters. The action potential, which is an electrical impulse, causes the pre-synaptic neuron’s synaptic vesicles to release neurotransmitters, which are chemical messages, during synaptic transmission.
These neurotransmitters attach to specific receptor sites on the post-synaptic neurone after diffusing over the synaptic gap, which is the space between the pre- and post-synaptic neurones. The neighbouring cell will then experience an electrical impulse as a result.
These information-processing neurones are found in both the peripheral nervous system, which is made up of sensory and motor nerve cells, and the central nervous system, which includes the brain and spinal cord.
What Makes up a Neurone?
The neurone is made up of dendrites, which are tree-like structures that receive signals from other neurones, and the soma, or cell body, from which the axon, a nerve fibre that conducts electrical impulses, extends. The axon is surrounded by an insulating layer called the myelin sheath, which speeds up nerve impulse transmission. The axon of one neurone and the dendrite of the next are separated by a space known as the synapse; neurones do not come into contact with one another.
Dendrites
Typically, shorter and more numerous than axons, dendrites are the tree-root-shaped portion of the neurone. They provide electrical impulses to the cell body and receive information from other neurones.
Because dendrites are coated in synapses, other neurones can send messages to them. The dendrites of certain neurones are shorter than those of others.
Neurones in the central nervous system can receive messages from several other neurones because of their vast length and intricate branching.
For example, the cerebellum has Purkinje cells, which have highly developed dendrites that allow them to receive messages from thousands of other cells.
Soma (Cell Body)
In essence, the soma, or cell body, is the centre of the neurone. The soma’s job is to keep the cell and the neurone in good working order (Luengo-Sanchez et al., 2015).
A membrane enclosing the soma provides protection and permits interaction with its immediate environment.
The cell nucleus in the soma generates genetic material and controls protein synthesis. The function of other neuronal components depends on these proteins.
Axon
The neuron’s axon, also known as a nerve fibre, is a structure that resembles a tail and connects to the cell body at a point known as the axon hillock.
In order to send electrical impulses to other neurones, the axon must transfer messages from the cell body to the terminal buttons. The axon serves as a conduit, sending these messages to muscles, glands, or other neurones. An axon is a neuron’s long projection that sends electrical impulses to target cells or other neurones from the neuron’s cell body.
One axon, which can be anywhere from 0.1 millimetres to more than three feet in length, is present in the majority of neurones (Miller & Zachary, 2017). A fatty material known as myelin coats certain axons, insulating them and facilitating faster signal transmission. The cytoplasm of an axon is called axoplasm. It is in charge of moving proteins, organelles, and other biological elements from the cell body of the neurone to the synaptic terminals and back again. The axon’s upkeep and functionality depend on this transport.
At specialised junctions known as synapses, neurotransmitters are released by synaptic terminals at the end of the axon to facilitate contact with target cells.
Myelin Sheath
Neurones’ axons are covered in a coating of fatty substance called the myelin sheath. Its function is to insulate nerve cells from one another and to stop one neuron’s impulse from interfering with another’s.
Accelerating the conduction of nerve impulses along the axon is the myelin sheath’s second purpose.
The glial cells (also called Schwann cells and oligodendrocytes) envelop the axons to produce the myelin sheath.
The function of the myelin sheath that envelops these neurones is to shield and insulate the axon. Transmission to other neurones occurs far more quickly as a result of this protection than it does for unmyelinated neurones.
The nodes of Ranvier are fragmented spaces that make up the myelin sheath. The ability of electrical impulses to hop between Ranvier nodes aids in accelerating signal transmission.
Axon Terminals
The axon terminals, also known as terminal buttons, are found at the end of the neurone and are in charge of sending messages to other neurones.
There is a synapse, or gap, at the end of the terminal button. Neurotransmitter-containing vessels are located in terminal buttons. The terminal buttons release neurotransmitters into the synapse, which then convey messages to neighbouring neurones. During this process, the electrical signals are transformed into chemical signals.
The terminal buttons are then in charge of reabsorbing any extra neurotransmitters that were not transferred to the subsequent neurone.
Neurone Types
Neurones may be divided into three fundamental classes based on their function, despite the fact that there are billions of neurones and significant variation: sensory neurones (long dendrites and short axons), motor neurones (short dendrites and long axons), and relay neurones (short dendrites and either short or long axons).
Sensory Neurons
Nerve cells called sensory neurons—also known as afferent neurons—transmit nerve impulses from sense receptors to the brain and central nervous system. These nerve impulses are converted into “sensations” like vision, hearing, taste, and touch once they arrive at the brain.
This sensory data can be chemical, such as taste or smell, or physical, such as sound, heat, touch, and light. This can happen, for instance, when you contact a really hot surface. Following this, the sensory neurones will communicate the information they have received to the central nervous system.
The majority of sensory neurones are classified as pseudo unipolar. This indicates that they have two branches from a single axon.
Motor Neurons
The nerve cells that transmit signals from the central nervous system to muscles in order to produce movement are known as motor neurones, or efferent neurones. To set off reactions that result in muscular contraction, they produce neurotransmitters.
Motor neurones connect to muscles, glands, and organs all throughout the body and are found in the brainstem or spinal cord, which are components of the central nervous system.
These neurones either directly or indirectly regulate muscle movements by sending signals to the skeletal and smooth muscles from the brainstem and spinal cord.
For example, your palm contacting a heated surface sends a signal to the sensory neurones. The hand then moves away from the heated area as a result of the motor neurones.
Motor neurones come in two varieties:
- Neurones that go from the spinal cord to the body’s muscles are known as lower motor neurones.
- Neurones that go from the brain to the spinal cord are known as upper motor neurones.
- Multipolarity is a characteristic of motor neurones. This indicates that many dendrites and one axon protrude from the cell body.
Relay Neurons
A relay neurone, sometimes referred to as an interneuron, facilitates communication between motor and sensory neurones. Because of their small axons, relay neurones are easily identifiable and connect different neurones in the brain and spinal cord.
Multipolar interneurons are similar to motor neurones. This indicates that they have several dendrites and one axon.
In addition to serving as a link between neurones, interneurons may also build circuits of varying complexity that allow them to interact with one another.
The communication between interneurons assists the brain in completing complex functions such as learning and decision-making, as well as playing a vital role in reflexes and neurogenesis – which means the regeneration of new neurons.
