The Nervous System is a network of nerve cells (neruones) that transmit electrical signals between the Central Nervous System and the Peripheral Nervous System to coordinate actions like movement.
The Central Nervous System (CNS) consists of the brain and the spinal cord.
The Peripheral Nervous System (PNS) consists of the neurones in the rest of the body.
Neurones are specialised cells that transmit the electrical impulses around the body.
There are three types of neurone:
Neurones have a cell body, dendrites, a dendron, and an axon.
The cell body contains the organelle.
Dendrites are short, branched extensions of the cell body that receive impulses.
The dendron is a long extension of the cell body that receives impulses.
The axon is a long extension of the cell body that transmits impulses.
These can neurons work together to perform coordinated responses.
A reflex arc causes a rapid, involuntary response to a stimulus without conscious thought:
An impulse is initiated in a sensory neurone by an action potential.
An action potential is a change in the electrical potential across the membrane of a neurone.
When a neurone is not conducting an impulse, it is said to be at rest.
At rest, a neurone is maintaining a resting potential.
The resting potential is the electrical potential maintained across the membrane of a neurone whilst it is at rest.
The resting potential is -70mV.
Resting potential is maintained by:
All of the above contribute to causing the inside of a neurone to be more negative than the outside.
When a stimulus is detected, the membrane of the neurone becomes depolarised.
Depolarisation is a change in the electrical potential across the membrane of a neurone.
Depolarisation occurs when the membrane of the neurone becomes less negative.
Depolarisation is what generates the action potential - initiating an electrical impulse in the sensory neurone.
How depolarisation occurs:
Repolarisation is the process by which the membrane of a neurone returns to its resting potential.
Repolarisation occurs after depolarisation.
How repolarisation occurs:
Hyperpolarisation is the process by which the membrane of a neurone becomes more negative than the resting potential.
It's caused by the K+ channels being too slow to close, allowing more K+ out than neccessary.
During hyperpolarisation and repolarisation, the nerve cell cannor recieve another stimulus. This is known as the refractory period.
The cell re-establishes the resting potential and concentrtion gradients of K+ and Na+ using the sodium-potassium pump.
Receptors are specialised cells that detect stimuli.
Receptors act as transducers, meaning they convert one form of energy to another.
Receptors are most commonly found in sense organs: eyes, nose, mouth skin ears.
There are 4 types of receptor:
The pacinium corpuscle is an example a mechanoreceptor.
It responds to changes in mechanical pressure.
It is found in the skin.
Structurally, it is a capsule filled with connective tissues.
The pacianian corpuscle surrounds the endings of the sensory neurone.
When pressure is applied, the pacinian corpuscle changes shape, which stretches the membrane and the stretch-mediated Na+ channels.
As the stretch-mediated channels are opened, Na+ diffuses in, making the neurone less negative.
This can cause an action potential to be generated.
The All or Nothing principle states that all action potentials reach the same size and magnitude or else no size or magnitude at all.
The myelin sheath is a fatty layer that surrounds the axon of a neurone.
It is made up of Schwann cells.
The myelin sheath insulates the axon, preventing the electrical signal from leaking out.
This means that the myelin sheath allows impulses to travel faster.
The myelin sheath is not continuous. It is interrupted by gaps called nodes of Ranvier, where the axon is exposed. These nodes are where the electrical impulse is regenerated.
Impulses need to be able to be transmitted across different neurons.
This is done by the synapse.
The synapse is the junction between the axon of one neurone and the dendron of another neurone.
Here's how the synapse works:
Neurotransmitters are chemical messengers that transmit impulses across the synapse.
The pre-synaptic neurone is specialised for its role.
The pre-synaptic neurone has a large number of mitochondria to provide ATP for the sodium-potassium pump.
The pre-synaptic neurone contains SER and a golgi to synthesise the enzyme that breaks down neurotransmitters.
Summation is the process by which impulses are added together to produce a larger impulse.
There are two types of summation:
In temporal summation multiple impulses recieved in close succession add up to create a larger impulse.
In spatial summation multiple impulses recieved at the same time from multiple neurones add up to create a larger impulse in one neurone.