Neuron Structure & Function

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Neurons

Neurons are the main building blocks of the nervous system and they have a structure that is specialised to their function.

Neurons are the central building blocks of the nervous system. There are 100 billion present at birth.
Like all cells, neurons are made up of several different parts. Each part has a specialised function.

A neuron’s outer surface is made up of a semipermeable membrane.
This membrane allows smaller molecules and molecules without an electrical charge to pass through it. It also stops larger or highly charged molecules.

The nucleus of the neuron is located in the soma, or cell body.
The soma has branching extensions known as dendrites.
The neuron is a small information processor. Dendrites act as input sites where signals are received from other neurons.

Electrical signals received from other neurons are transmitted electrically across the soma and down a major extension from the soma known as the axon. The axon ends at multiple terminal buttons.
The terminal buttons contain synaptic vesicles that house neurotransmitters, the chemical messengers of the nervous system.

The action potential from an electrical impulse moves rapidly down the axon to the terminal buttons.
At the terminal button, synaptic vesicles release neurotransmitters into the synapse.

Synapses and neurotransmitters are essential components of communication between neurons.

A synapse is a very small space between two neurons. It is an important site where communication between neurons happens.
The synaptic cleft or gap is the space between two neurons.
The synaptic knob is the bulb shaped end of a neuron where it meets another neuron.
The synaptic vesicles are in the synaptic knob and are where neurotransmitters are stored before release into the synaptic cleft.
A neurotransmitter is a chemical messenger that carries information from one neuron to another.

Once neurotransmitters are released into the synapse, they travel across the small space and bind with corresponding receptors on the dendrite of an adjacent neuron.

Receptors (proteins on the cell surface where neurotransmitters attach) vary in shape. Different shapes “match” different neurotransmitters.
The neurotransmitter and the receptor have what is known as a lock-and-key relationship - specific neurotransmitters fit specific receptors similar to how a key fits a lock.

The neurotransmitter binds to any receptor that it fits.
This binding triggers an action potential in the adjacent neuron, and the electrical impulse (i.e. the information) is transferred.

You need to understand the role of sensory, relay and motor neurons.

They carry nerve impulses (information) from sensory receptors (e.g. vision, taste, touch) to the spinal cord and towards the brain. Sensory receptors are found in various locations in the body, for example in the eyes, tongue and skin.
Sensory neurons convert information from these sensory receptors into neural impulses. When these impulses reach the brain, they are translated into sensations of, for example, visual input, heat, pain etc., so that the organism can respond appropriately.

Not all sensory information travels as far as the brain, with some neurons terminating in the spinal cord.
This allows reflex actions to occur quickly without the delay of sending impulses to the brain.

Most neurons are neither sensory nor motor, but lie somewhere between the sensory input and the motor output. Relay neurons allow sensory and motor neurons to communicate with each other. These relay neurons (or interneurons) lie wholly within the brain and spinal cord.

The term motor neuron refers to neurons located in the CNS that project their axons outside the CNS and directly or indirectly control muscles. Motor neurons form synapses with muscles and control their contractions.
When stimulated, the motor neuron releases neurotransmitters that bind to receptors on the muscle and trigger a response which leads to muscle movement. When the axon of a motor neuron fires, the muscle it has formed synapses with contracts. Muscle relaxation is caused by inhibition of the motor neuron.