How is a nerve impulse created?
The permeability of the nerve cell membrane to sodium ions is altered.
As the sodium ion channels open, the membrane permeability is increased and sodium ions can move across teh membrane down their concentration gradient into the cell.
The movement of ions across the membrane creates a change in potential difference across the membrane. The inside of the cell becomes less negative than usual compared to the outside.
This is called depolarisation.
What?Its a small depolarisation caused by sodium ions entering the cell.
How?Receptor cells respond to the changes in the environment
The gated sodium ion channels open and allow sodium ions to diffuse across the membrane into the cell.
The larger the stimulus, the more gated channels will open.
If enough sodium ions enter the cell the potential difference changes significantly and will initiate an impulse or action potential.
what? Its achieved when the membrane is depolarised to a value of about +40mV.Its an all or nothing response.
How? In the events leading up to an action potential, the membrane depolarises and reaches a threshold level, then lots of sodium ions enter the axon and an action potential is reached.
Sensory and motor neurones
- sensory neurones - from sensory receptor to CNS
- motor neurones - from CNS to an effector
- Relay neurones - connect sensory and motor neurones
Neurones are specialised cells with the following features:
- very long so they can transmit the action potential over a long distance
- cell surface plasma membrane has many gated ion channels that control the entry/exit of sodium/potassium/calcium ions.
- sodium/potassium ion pumps that use ATP to activly transport sodium ions out of the cell and potassium ions into the cell
- maintain a potential difference across the cell surface membrane
- surrounded by a fatty sheath called the myelin sheath(consists of a series of schwann cells) that insulates the neurone from the electrical activity in nearby cells.There are gaps where the schwann cells meet called nodes of Ranvier.
- cell body contains a nucleus, many mitochondria and ribosomes.
- numerous dendrites connected to other neurones.
Motor neurones VS sensory neurones
- cell body is in the CNS
- long axon that carries the action potential to the effector
- cell body is just outside the CNS
- long dendron that carries the action potential from a sensory receptor to the cell body
- short axon carries the action potential into the CNS