What are neurons?
Nerve cells - present in the nervous system
What are neurons specialised for?
communication
What do neurons connect with?
- Each other and with other ‘excitable cells’ e.g. muscles, glands
Name 6 different types of neuron?
- Purkinje cell
- Spinal interneuron
- Sensory neuron
- Pyramidal cell
- Motor neuron
- Bipolar cell
What is the resting membrane potential?
A potential difference existing across the membrane of all cells
What is the range of values for a resting membrane potential?
- 20-90mV
How are charged distributed across the membrane?
- The inside is negative with respect to the outside
- There are approximately equal numbers of positive and negative charges on each side of the membrane, but they are not evenly distributed
What is resting membrane potential determines by?
- Diffusion of K+ from cell interior through K+ channels
- The sodium potassium pump also continuously moving unequal amounts of Na & K (3 Na out to 2 K in)
What can the membrane potential be altered by?
- By applying an electric current (stimulus) to the cell
Explain the process of an action potential?
- If the MP is raised to around -55mV, a large depolarisation occurs
- The MP ‘overshoots’ zero, so that the polarisation is reversed
- This large change in MP is called the ‘action potential’
- The action potential is an ‘all or none’ event
- The AP amplitude is independent of the stimulus intensity
What happens ‘at threshold’ of an action potential?
- Voltage-gated Na+ channels open
- Na+ diffuse in –> depolarisation
What happens at the ‘peak’ of an action potential?
- Na+ channels close
- Voltage-gated K+ channels open
- K+ diffuse out –> repolarisation
What does local anaesthetic do?
Stops nerve conduction by blocking the Na+ channels (so will not allow the activity of pain)
What is the refractory period?
- After an AP is initiated, the neuron cannot regenerate another AP until the first one has ended
- This period of inexitability is called the refractory period
What causes the refractory period?
- It is due to the inactivation of voltage-gated sodium channels
What is action potential propagation (a wave of depolarisation)?
- An AP in one section of an axon depolarises adjacent ‘resting’ parts of the axon
- The AP is regenerated further along the axon
- Action potentials travel along the axon as waves of depolarisation
What increases the speed of AP propagation?
- Increasing the axon diameter
- Large axons conduct impulses more rapidly than small ones
- But rapid conduction is achieved only with very large axons
What can greatly increase the AP conducting speed of an axon?
- Wrapping a layer of myelin around the axon (myelin sheath)
What is myelin and how is it formed?
- Myelin is a fatty layer
- Formed by wrapping the membranes of ‘glial’ cells around the axon
- This in effect ‘insulates’ the axon, and improves conduction
What is a ‘node of Ranvier’?
- Intervals that interrupt the myelin sheath (areas with no insulation - exposure of membrane to ECF)
- Here, the axon membrane is exposed to the ECF, and ion flow can occur
What is Saltatory conduction?
- The rapid passage of an electric potential between the nodes of Ranvier in myelinated nerve fibres, rather than along the full length of the membrane
