Nervous Transmission

The nervous system is comprised of the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord, while the PNS includes all other nerves in the body.
The nervous system works by transmitting signals, known as nerve impulses, through a network of special cells called neurons.
Neurons are composed of a cell body, an axon, and several dendrites. The cell body contains the cell’s nucleus, the axon conducts nerve impulses, and the dendrites receive incoming nerve impulses.
There are three types of neurons: sensory neurons, motor neurons, and interneurons. Sensory neurons transmit impulses from sensory organs to the CNS, motor neurons transmit impulses from the CNS to muscles or glands, and interneurons connect sensory and motor neurons.

The Synapse

The point at which a nerve impulse is transmitted from one neuron to another is called a synapse.
A synapse is composed of a pre-synaptic neuron, a synaptic cleft, and a post-synaptic neuron. The pre-synaptic neuron generates the nerve impulse, while the post-synaptic neuron receives this impulse.
A nerve impulse is transferred across a synapse via neurotransmitters, chemical substances released by the pre-synaptic neuron. These neurotransmitters diffuse across the synaptic cleft and bind to receptors on the post-synaptic neuron, triggering a new impulse.

A nerve impulse is initiated by a change in potential difference across a neuron’s membrane. This is triggered by a stimulus, such as a change in temperature, pressure, or light.
At rest, a neuron’s membrane is polarised, meaning that the inside of the cell is negative relative to the outside. This is maintained by the sodium-potassium pump, which pumps sodium ions out of the cell and potassium ions into the cell against their concentration gradients.
When a stimulus is detected, sodium channels in the neuron’s membrane open, allowing sodium ions to diffuse into the cell. This influx of positive ions causes the neuron to depolarise, triggering a nerve impulse.
After an impulse has passed, potassium channels in the neuron’s membrane open, allowing potassium ions to diffuse out of the cell. This restores the resting potential, readying the neuron for another impulse. This is known as repolarisation.

Some neurons are encased in a myelin sheath, a protective layer that speeds up the transmission of nerve impulses. This sheath is produced by specialised cells, known as Schwann cells.
The myelin sheath insulates the neuron and prevents nerve impulses from spreading sideways. Instead, impulses jump from gap to gap in the sheath, a process known as saltatory conduction, which greatly enhances the speed of transmission.
Multiple Sclerosis (MS) is a condition related to the myelin sheath, where the immune system mistakenly attacks and damages it, slowing down or blocking nerve transmission.