Neuronal Communication Flashcards
Sensory Receptors
Light sensitive receptors in retina
Temperature receptors in skin and hypothalamus
Pacinian corpuscles in the skin detect pressure
Vibration receptors in the cochlea detect sound
Chemical receptors on the tongue
Creating a nerve impulse
The cell is polarised when the inside is negative compared with the outside.
When sodium ion channels open sodium ions move down the concentration gradient into the cell.
the potential difference changes and the cell is now depolarised and a generator potential is produced.
Sufficient generator potentials will create an action potential.
Motor neurones
Carry action potentials from the CNS to an effector e.g a muscle
Sensory neurones
carry action potentials from sensory receptor to the CNS
Relay neurones
Connect sensory and motor neurones
Features of neurones
- Very long
- Plasma membrane has many gated ion channels controlling the entry and exit of ions
- Sodium/potassium pumps use ATP to actively transport ions
- Maintain potential difference across membrane
- Dendrites to connect to other neurones
- Axon carries impulse away from cell body
- Insulated by fatty myelin sheath
Advantages of myelination
- Transmit action potentials much faster
- More rapid response to stimulus
- Carry action potentials over long distances
- Non-myelinated neurones tend to be shorter, the action potential moves along the neurone as a wave
The resting potential
- Sodium/potassium pumps use ATP to pump 3 Na ions out for every 2 K ions in
- Some potassium channels are open so they can diffuse out of the cell
- Cell also contains anions e.g DNA, proteins
- Therefore, interior maintains a negative resting potential compared with the outside
Stage of an action potential
- Membrane is polarised at its resting potential - around -60mV, higher [Na] outside than inside and higher [K] inside than outside
- Sodium ion channels open and Na+ move into cell
- Membrane depolarises and reaches the threshold vale of around -50mV
- Positive feedback causes voltage gated sodium channels to open and sodium ions flood in making the inside more positive
- The potential difference reaches +40mV
- Sodium ion channels close, potassium ion channels open
- Potassium ions diffuse out of the cell, this is repolarisation.
- Potential difference overshoots and it becomes hyperpolarised
- Resting potential restored
The refractory period
After an action potential the ions are in the wrong places. Concentrations are restored by sodium/potassium ion pumps.
The refractory period is the time after an action potential when another can’t be generated and it ensures the action potential is transmitted in one direction.
Local Currents
- When an action potential occurs the sodium ion channels open at that point in the neurone
- Sodium ions diffuse down the concentration gradient into the cell
- Na ions diffuse sideways along the neurone, away from the region of increased concentration
- Movement of charged particles is a local current
- Local current cases slight depolarisation further along neurone which opens voltage gated sodium ion channels causing a full depolarisation
Saltatory Conduction
Ionic movements can only occur at the nodes of Ranvier. Local currents are elongated, sodium ions diffuse along neurone from one node to the next. Action potential appears to ‘jump’. This speeds up transmission.
Features of the pre-synaptic bulb
- Many mitochondria for ATP
- Lots of smooth endoplasmic reticulum to package neurotransmitter into vesicles
- lots of vesicles containing acetylcholine
- voltage gated calcium ion channels on plasma membrane
Transmission across synapse
- Action potential arrives at pre-synaptic bulb
- Voltage gated calcium ion channels open and calcium ions diffuse in
- Calcium ions case vesicles to move to and fuse with the pre-synaptic membrane
- Acetylcholine released by excocytosis and diffuses across cleft
- Acetylcholine binds to receptor sites on sodium ion channels in post-synaptic membrane
- sodium ion channels open and sodium ions diffuse into post-synaptic neurone
- Generator potential created
- If sufficient generator potentials, they combine to reach the threshold potential and create an action potential.
The role of acetylcholinesterase
It is an enzyme that hydrolyses acetylcholine into ethanoic acid and choline. This stops the transmission of signals after an action potential has been produced in the post synaptic neurone. The products are recycled back to the enzyme to be used again.
