Neurones table

Here is the complete comparison of sensory and motor neurons, compiled into a single, comprehensive table tailored to the OCR A-Level Biology specification.

Feature / Category	Sensory Neuron	Motor Neuron
Main Function	Carries impulses from receptors to the CNS.	Carries impulses from the CNS to effectors.
Direction of Impulse	Towards the central nervous system (CNS).	Away from the central nervous system (CNS).
Cell Body Position	Located on a side branch (dorsal root ganglion).	Located at the terminal end (inside the CNS).
Dendron Structure	One long dendron present.	No dendron; multiple short dendrites instead.
Axon Length	Relatively short axon.	Very long axon.
Connection (Start)	Connects to specialised sensory receptors.	Connects to relay neurons inside the CNS.
Connection (End)	Connects to relay neurons in the CNS.	Connects to effectors (muscles or glands).
Synapse Type at End	Standard central nervous system synapse.	Neuromuscular junction (if controlling muscle).
Myelination	Typically myelinated by Schwann cells.	Typically myelinated by Schwann cells.
Action Potentials	Uses sodium-potassium pumps for resting potential.	Uses sodium-potassium pumps for resting potential.
Node of Ranvier Presence	Present along the axon and long dendron.	Present along the long axon.
Presence in PNS	Fully a part of the Peripheral Nervous System.	Axon is in PNS; cell body is in CNS.

Here is an exam-style question tailored to the OCR A-Level Biology specification, focusing on the reflex arc, followed by the official mark scheme and tips.

The Exam Question

Figure 1.1 shows a simplified diagram of a reflex arc involved in withdrawing a hand from a hot object. [1]

       [ Receptor in Skin ] 
                │
                ▼ (Neuron X)
         [ Spinal Cord ] ──► [ Neuron Y ] ──► [ Neuron Z ]
                                                   │
                                                   ▼
                                           [ Biceps Muscle ]

(a) Identify the type of neurons labeled X, Y, and Z. (3 marks)

X: ____________________________
Y: ____________________________
Z: ____________________________

(b) Describe the path taken by the electrical impulse from the moment the hot object is touched until the arm is withdrawn. (4 marks)

(c) Explain why the impulse can only travel in one direction across the synapses in this reflex arc. (3 marks) [2]

Mark Scheme & Revision Guide

Part (a)

X: Sensory neuron (1)
Y: Relay neuron / Intermediate neuron (1)
Z: Motor neuron (1)

Part (b) (Maximum 4 marks)

1: Receptor detects the stimulus (heat/pain) and generates an action potential. (1)
2: Impulse travels along the sensory neuron (X) into the spinal cord / CNS via the dorsal root. (1)
3: Chemical transmitter diffuses across the synapse to the relay neuron (Y). (1)
4: Impulse passes to the motor neuron (Z) via a second synapse and leaves via the ventral root. (1)
5: Impulse reaches the effector (biceps muscle), causing it to contract and pull the hand away. (1)

Part (c) (Maximum 3 marks)

1: Neurotransmitter receptors are only located on the postsynaptic membrane. (1)
2: Neurotransmitter vesicles are only stored in the presynaptic knob. (1)
3: Diffusion gradient of neurotransmitters only moves from presynaptic to postsynaptic side. (1)

Would you like me to break down the marking points for any of these questions, or should we try another question on action potentials and myelination?