Neuronal Communication

Question 1

Structure and function of sensory neurones

Answer 1

• Many dendrites, one long dendron and one short axon.
• Transmit action potentials from sensory receptors to the CNS.
• Myelinated
• Positioned just outside CNS in dorsal root ganglion.

Question 2

Structure and function of motor neurones

Answer 2

• Many dendrites, one long axon.
• No dendron
• Transmit action potentials from CNS to effectors e.g. muscles and glands.
• Positioned in CNS
• Myelinated

Question 3

Structure and function of relay neurones

Answer 3

• Transmit action potentials between sensory and motor neurones.
• No dendron
• Few to many dendrites and some long/short axons.
• Positioned in CNS
• Non-myelinated

Question 4

What is the resting potential of a neurone?

Answer 4

-60mV to -70mV

Question 5

How is the resting potential maintained?

Answer 5

• A sodium-potassium pump which uses ATP to pump out 3 sodium ions out for every two potassium ions pumped in.
• Voltage gated sodium ion channels closed, potassium ions channels open so they diffuse out,
  inside of cell more negative.

Question 6

What is an action potential?

Answer 6

A depolarisation of the membrane caused by an influx of sodium ions.

Question 7

How is an action potential generated?

Answer 7

• When neurone becomes stimulated, some voltage gated Na+ channels open, ions flow into cell.
• Inside of cell becomes less negative and potential difference rises (depolarisation).
• If enough flow in, potential reaches threshold potential is reached and more sodium channels are opened, causing an action potential.
• When membrane potential reaches +40mV, Na+ channels close, K+ channel opens, membrane more permeable to K+ ions.
• K+ ions diffuse out of axon and repolarisation happens.
• Lots of K+ ions diffuse out, inside axon more negative than normal (hyperpolarisation).
• Na+ channels close and sodium-potassium pump causes axon to return to resting state.

Question 8

What is a myelinated neurone?

Answer 8

Surrounded by myelin sheath created by Schwann cells.

Question 9

What are the gaps between Schwann cells called and how do these speed up transmission of nerve impulses?

Answer 9

Nodes of Ranvier, allow electrical impulses to jump along myelin sheath.

Question 10

What is a synapse?

Answer 10

A junction between a pre-synaptic neuron and a post-synaptic neuron.

Question 11

How are nerve impulses transmitted across a synapse?

Answer 11

• Action potential reaches end of presynaptic neuron and causes Ca2+ channels to open so calcium ions diffuse into the knob.
• Vesicles of ACh move towards membrane, fuse and exit by exocytosis.
• ACh molecules diffuse across cleft and bind to receptors on the postsynaptic neuron.
• Na+ channels in the postsynaptic membrane open and Na+ ions diffuse into post-synaptic neuron.
• Depolarisation of the membrane.
  Acetylcholinesterase breaks down ACh into choline and ethanoic acid.