Neurones, reflexes and the nervous system: exam questions

Answer: B (1) Cell bodies of relay and motor neurones are in the grey matter.

• Schwann cells wrap around the axon (1)
• many times / many layers of (lipid-rich) plasma membrane form the sheath (1)
• acts as an electrical insulator / ions cannot cross it, except at nodes of Ranvier (1)
• so impulses are conducted faster (1)
• Max 3

• similarity: both have a cell body / axon / are myelinated / conduct action potentials (1)
• sensory carries impulses from receptor to CNS; motor carries impulses from CNS to effector (1)
• sensory cell body is in the dorsal root ganglion / outside the spinal cord; motor cell body is in the grey matter of the CNS (1)
• sensory has a long dendron (and short axon); motor has many short dendrites and a long axon (1)
• Max 4. Accept any other valid difference of structure or position

• pain / temperature receptor in the skin detects the stimulus and generates an impulse (1)
• impulse travels along a sensory neurone to the spinal cord, entering through the dorsal root (1)
• sensory neurone synapses with a relay neurone in the grey matter (1)
• relay neurone synapses with a motor neurone (1)
• motor neurone carries the impulse out through the ventral root to the effector / muscle (1)
• muscle (in arm) contracts and hand is withdrawn (1)
• Max 5

• reflex arc is short / has few synapses, so the response is fast (1)
• the reflex does not involve the brain / conscious thought, so the response occurs before the brain is aware (1)
• white matter (axons) carries impulses up the spinal cord to the brain, so pain is felt later (1)

• central nervous system, made up of the brain and spinal cord (1)
• peripheral nervous system, made up of the neurones / nerves outside the brain and spinal cord (1)
• receptor detects a stimulus / converts energy of stimulus into an electrical impulse (1)
• effector (muscle or gland) responds / contracts or secretes (1)