Free Receptors MCQs with Answers

11. The myelin sheath around peripheral nerve fibres is produced by

• A. astrocytes
• B. microglia
• C. ependymal cells
• D. Schwann cells

Explanation: Schwann cells wrap themselves repeatedly around the axon, their membranes forming the insulating myelin. In the central nervous system the same job is done by oligodendrocytes, a distinction examiners like to test.

12. The gaps between successive Schwann cells along a myelinated axon are called

• A. nodes of Ranvier
• B. synaptic clefts
• C. dendritic spines
• D. neurofibril nodes

Explanation: At the nodes the axon membrane is exposed and rich in ion channels, so the action potential jumps from node to node in saltatory conduction. This jumping is what makes myelinated fibres conduct many times faster than unmyelinated ones.

13. The minimum strength of stimulus needed to generate an action potential in a neuron is called the

• A. refractory value
• B. threshold
• C. resting potential
• D. summit potential

Explanation: A stimulus below threshold opens too few sodium channels to trigger the positive feedback of depolarisation, so nothing happens; at threshold, a full impulse fires. This is the basis of the all or none behaviour of the neuron.

14. In a single neuron, an impulse normally travels in the direction

• A. axon to dendrite to cell body
• B. cell body to dendrite to axon
• C. dendrite to cell body to axon
• D. axon to cell body to dendrite

Explanation: Dendrites receive signals and conduct them to the cell body, and the axon carries the impulse away to the terminals. The synapse then enforces the same one way flow onto the next cell.

15. Which of the following acts as an inhibitory neurotransmitter in the brain?

• A. Acetylcholine
• B. Noradrenaline
• C. Dopamine
• D. GABA

Explanation: GABA, gamma aminobutyric acid, hyperpolarises the postsynaptic membrane and calms neural activity, and many sedatives enhance its action. Acetylcholine and noradrenaline are generally excitatory at the synapses where they are best known.

16. A dog that has learned to salivate at the sound of a bell whenever food is presented shows a

• A. conditioned reflex
• B. unconditioned reflex
• C. voluntary action
• D. hormonal response

Explanation: Pavlov paired the neutral bell with food until the bell alone triggered salivation, building a reflex through experience in the cerebral cortex. Salivating to food itself is inborn and unconditioned, which is the contrast the experiment relies on.

17. Blinking of the eyes when dust touches the cornea is an example of a

• A. spinal reflex
• B. cranial reflex
• C. conditioned reflex
• D. hormonal reflex

Explanation: The reflex arc passes through the brain stem rather than the spinal cord, so it is classed as a cranial reflex. Spinal reflexes, such as knee jerk, are integrated in the spinal cord itself.

18. In a reflex arc, the motor neuron ends by stimulating an

• A. interneuron
• B. sensory receptor
• C. effector such as a muscle or gland
• D. association area

Explanation: The effector, a muscle or gland, carries out the response, completing the receptor to effector pathway. Receptors start the arc and interneurons link the two, so only the effector belongs at the motor end.

19. In a spinal reflex arc, the cell bodies of the interneurons that link sensory and motor neurons lie in the

• A. dorsal root ganglion
• B. white matter of the spinal cord
• C. cerebral cortex
• D. grey matter of the spinal cord

Explanation: Interneurons sit in the grey matter of the cord, where they synapse with the incoming sensory fibre and the outgoing motor neuron. The dorsal root ganglion holds only the cell bodies of sensory neurons, a frequent mix up.

20. The sudden constriction of the pupil when bright light falls on the eye is an example of

• A. a reflex action
• B. a conditioned response
• C. a voluntary movement
• D. a hormonal effect

Explanation: The pupil reflex is automatic, rapid and involuntary, integrated in the brain stem, meeting every criterion of a reflex. It happens even in sleep, which shows that no conscious control or learning is involved.