(a) Motor neurones and sensory neurones have different roles in nervous coordination. Contrast the structure and function of sensory neurones and motor neurones.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

1. [1] (a) motor transmits impulses from CNS to effector and sensory transmits impulses from, receptors / (named) sense organs, to CNS ;
2. [1] (a) motor: cell body at end of, neurone / axon and sensory: cell body, in middle of/part-way along, neurone OR cell body not at end of, neurone / axon ;
3. [1] (a) motor: cell body in, CNS / brain / spinal cord and sensory: cell body in (dorsal root) ganglion ;
4. [1] (a) motor: long axon OR axon and sensory: short axon OR axon and dendron ;
5. [1] (a) motor: dendrites attached to cell body and sensory: dendrites attached to dendron ;
6. [3] (b) any three from: 1 no myelin slow and myelin fast(er) / myelin increases speed ; 2 no myelin = 0.5–2 m s¯¹ and with myelin > 5 / up to 120 m s¯¹ ; 3 as axon diameter increases speed increases ; 4 diameter (+ units once) and speed (+ units once) for two neurones ;
7. [3] (c) A - any one reason from: 1 Na+/ sodium, channels already open OR Na+ has already entered neurone OR no more Na+ channels to open OR less Na+ outside to diffuse in OR less steep Na+ concentration gradient ; B – any one reason from: 2 sodium channels are, inactive / unresponsive OR potassium channels are open OR membrane is, impermeable / less permeable, to Na+ OR membrane is more permeable to K+ ; C - any one reason from: 3 harder to reach threshold OR potassium channels are (still) open OR sodium-potassium pumps need to restore the resting potential; 4 hyperpolarisation at C ;