5.1.1 Communication and Homeostasis

Question 1

Sensory Neurones

Answer 1

They have short dendrites and one long dendron to carry impulse from receptor cells to the cell body. Also has one short axon that carries impulse from the body to the CNS.

Question 2

Motor Neurones

Answer 2

Motor neurones have many short dendrites that carry nerve impulses from the CNS to the cell body. Then has one long axon that carries nerve impulses from the cell body to the effector cells.

Question 3

Relay Neurones

Answer 3

Relay neurones have many short dendrites that carry nerve impulses from the sensory neurone to the cell body. It also has one axon that carries the impulse from the cell body to the motor neurones.

Question 4

Neurone Cell Membrane

Answer 4

++++++++ outside cell

Resting potential =-70mV
maintained by Na+ and K+ pumps and K+ channels
Makes outside the cell positive

• • • • • • • inside cell

Question 5

Stimulus

Answer 5

excited neurone cell membrane causing Na+ channels to open. Membrane becomes more permeable to Na+ so they diffuse into the neurone. Makes the inside of the neurone less negative

Question 6

Depolarisation

Answer 6

If potential difference reaches the threshold (around -55mV), voltage gates Na+ channels open. More Na+ diffuses into the neurone. This is positive feedback.

Question 7

Repolarisation

Answer 7

At around 30mV the Na+ channels close and the voltage gates K+ channels open. Cell membrane is more permeable to K+ ions. So K+ diffuses out of the neurone. This is negative feedback.

Question 8

Hyperpolarisation

Answer 8

K+ channels are slow to close so often too many diffuse out. The potential gets more negative (below -70mV)

Question 9

Resting Potential

Answer 9

Ion channels reset and potential difference returns to -70mV.

Question 10

All or Nothing

Answer 10

Once the threshold is reached, an action potential will always fire with the same change in voltage no matter how big the stimulus is. If the threshold isn’t reached, an action potential won’t fire. A bigger stimulus won’t cause a bigger action potential but will cause them to fire more frequently.

Question 11

Myelinated Neurones

Answer 11

The Myelin Sheath is an electric insulator

In the PNS, the myelin sheath is made from Schwann cells

Between Schwann cells, there are areas of bare membrane called nodes of Ranvier. Na+ channels are concentrated here.

Depolarisation only happens are the nodes of ranvier where Na+ can get through the membrane.

Neurones cytoplasm conducts enough charge to depolarise the next node. This is called salatory conduction.

Question 12

Non-Myelinated Neurones

Answer 12

In non-myelinated neurones, impulse travels as a wave along the whole length of the axon membrane.

Question 13

Synapses part 1

Answer 13

An AP arrives at the pre-synaptic neurone and moves to the synaptic knob. The AP stimulates the voltage gates calcium ion channels in the presynaptic neurone to open. The calcium ions diffuse into the synaptic knob and they will be pumped back out when the AP has finished via active transport.

Question 14

Synapses part 2

Answer 14

The influx of calcium ions into the synaptic knob causes the synaptic knob causes the synoptic vesicles to fuse with the presynaptic membrane. Vesicles release acetylcholine by exocytosis

Question 15

Synapses part 3

Answer 15

ACh diffuses across the synaptic cleft and binds to cholinergic receptors. Causes Na+ channels in post neurone to open. Influx of Na+ causes depolarisation. AP generated if threshold is reached

Question 16

Synapses part 4

Answer 16

ACh is removed from the cleft so response doesn’t keep occurring. Broken down by the enzyme acetylcholinesterase. Once broken down, the products are reabsorbed by the presynaptic neurone