The Nervous System

Question 1

What is the CNS

Answer 1

Brain and spinal cord

Question 2

How is a reflex response activated

Answer 2

Receptor cells detect the stimulus and the nerve impulse is carried along the sensory neurone and enters the spinal cord via the dorsal root. This then passes along the relay neurone and out of the spinal cord through the ventral root on the motor neurone which causes the effect in the effector.

Question 3

What is the sensory neurone

Answer 3

Carries the impulse from the sensory organ to the CNS

Question 4

What is the relay neurone

Answer 4

Found in the CNS and connects the sensory and motor neurones

Question 5

What is the motor neurone

Answer 5

Carries the response impulse from the CNS to the effector

Question 6

What is the grey matter of the spinal cord

Answer 6

It contains cell bodies and synapses.

Question 7

What is the white matter of the spinal cord

Answer 7

It contains axons coated in fatty myelin

Question 8

List three characteristics of a reflex

Answer 8

Automatic
Protective
Fast

Question 9

Describe the nervous system of hydra (nerve net)

Answer 9

no recognisable CNS and fewer types of receptor cells and therefore responds to a limited number of stimuli. Hydra cannot detect the direction of a stimulus. The greater the intensity of the stimulus the more nerve cells are triggered initiating a greater response. Impulses travel more slowly and in all directions

Question 10

Describe resting potential

Answer 10

No impulse is being transported and the resting potential difference is -70mv. This is achieved by sodium-potassium pumps pumping 3Na+ out and only 2K+ in making the outside of the cell membrane is more positive than inside, making the membrane polarised.

Question 11

Describe action potential

Answer 11

The presence of a stimulus opens Na+ channels allowing them to flood the axon, depolarising the membrane and creating a potential difference of +40mv. The Na+ channels then shut

Question 12

Describe hyper-repolarisation

Answer 12

When the Na+ channels close, the K+ channels open. K+ exit the axon. A potential difference of -80mv is achieved because the membrane is very permeable to K+ and too many leave. The sodium-potassium pump restores the resting potential and the membrane
becomes polarised again.

Question 13

Describe the refractory period

Answer 13

No further action potentials can be propagated along the axon until polarisation has been restored. No action potential can occur during depolarisation and repolarisation as they cause an absolute refractory period, but during hyper-repolarisation an action potential can be generated if the stimulus is strong enough

Question 14

Describe the all or nothing law

Answer 14

If the intensity of a stimulus does not reach a certain threshold, no action potential will be generated. This prevents the brain from being overloaded by small unnecessary stimuli

Question 15

What factors affect the speed of nerve transmission

Answer 15

Temperature – Increases in temperature increase kinetic energy and therefore speeds up the transport of ions; this speeds up nerve transmission.
Diameter of the axon – The greater the diameter of the axon the lower the resistance to the movement of ions. Giant squids have axons up to 1mm in diameter allowing them to react quickly in low temperatures.
Myelination - Schwann cells wrap around the axon and secrete myelin which is an electrical insulator. Therefore the impulse jumps between nodes of Ranvier (gaps in the myelin) which speeds up transmission

Question 16

Describe synaptic transmission

Answer 16

When an action potential reaches the presynaptic membrane Ca2+ channel open allowing Ca2+ to enter the synaptic knob. Synaptic vessels, containing neurotransmitter, migrate towards the presynaptic membrane and fuse with it, then releasing the neurotransmitter by exocytosis.The neurotransmitter diffuses across the synaptic cleft and binds to receptor
proteins on the post synaptic membrane. This opens Na+ channel allowing Na+ to flood into the post synaptic neurone. If sufficient Na+ enters the post synaptic neuron an action potential will be generated in that nerve cell.

Question 17

Why does the synaptic knob have many mitochondria

Answer 17

If the neurotransmitter stays in the synaptic cleft it would keep initiating impulses. They are hydrolysed down to their component parts and diffuse back to the synaptic knob where they are repackaged into synaptic vessels. ATP is required which is why there are lots of mitochondria.

Question 18

List the functions of a synapse

Answer 18

Transmit information form neurone to neurone
Pass impulses in one direction
Act as junctions
Prevents overstimulation
Filter out low level stimuli

Question 19

Layers of the meninges from top down

Answer 19

Dura mater
Arachnoid Mater
Pia Mater