Neurones and synapses

Three main types of neurone:

• Sensory neurone - carry impulses from receptors to the CNS
• Motor neurone - carry impulses from CNS to effectors
• Connector neurone - connect neurones within the CNS

All neurones have three main parts:

• a centron (cell body) - contains the nucleus
• dendrons/dendrites - cytoplasmic connections which transmit impulses to the cell body
• Axons - extensions that carry impulses Away from cell body

• All cells have a potential difference across their cell surface membranes, due to uneven distribution of ions.
• Neurones have a large excess of charged sodium ions (Na+) on the outside
• The membrane is said to be polarised
• The potential difference across the polarised membrane is called the resting potential, (-70mV)

Neurones are excitable - the potential difference can be reversed. When a neurone is stimulated:

• The membrane becomes permeable to sodium ions
• Sodium ions diffuse into the axon and the potential difference is reversed - inside becomes positively charged
• If a threshold level is reached, the ions will surge in and the membrane rapidly becomes depolarised
• If threshold is reached, the action potential will always be of equal magnitude and duration

Depolarisation = reversal of potential difference = action potential = (+40mV)

• The action potential is followed by a period when the membrane repolarises and recovers its resting potential - known as the refractory period
• During the refractory period, the membrane is unexcitable and cannot be restimulated

The depolarisation of one part of the membrane sets up local circuits with the areas either side of it.

Local circuits occur as positive ions are attracted by neighbouring negative regions and flow in both directions. One one side - membrane is in refractory period. On other excitable side - local circuit triggers depolarisation and formation of an action potential.

Factors affecting conduction of impulse

• Myelin sheath - depolatisation can only occur at nodes of Ranvier, impulse jumps from node to node = saltatory conduction. Faster transmission
• Diameter of axon - wide diameter offers less resistance to the movement of ions and less leakage of ions, so transmit impulses faster
• Temperature - increase in temp, increase in transmission

Transmission across a synapse:

• Synaptic knob contains vesicles
• When impulse arrives at synaptic bulb, membrane becomes permeable to calcium ions
• Ca ions stimulate movement of vesicles towards pre-synaptic membrane
• Vesicles fuse with membrane and release transmitter substance - acetylcholine by exocytosis into the synaptic cleft
• ACh diffuses across synaptic cleft (20nm wide)
• ACh attaches to receptors on post-synatptic membrane, causing depolarisation
• An EPSP is generated - if it reaches a threshold, an action potential is evoked
• Continued stimulation of the post-synaptic membrane is prevented by the enzyme cholinesterase which breaks down the ACh and release products into cleft
• Products - choline and ethanoic acid - diffuse across cleft and are reabsorbed into synaptic bulb where they are re-synthesised into ACh using energy from ATP