Reflexes (Physiology)

Question 1

List the features of a spinal reflex and draw the components of a typical reflex arc

Answer 1

• Sensory receptor
• Sensory nerve (afferent)
• Motor nerve (efferent)

Question 2

Describe the muscle spindle and stretch reflex

Answer 2

Muscle spindle

• Receptor for the stretch reflex.
• Intrafusal fibres (muscle spindle) arranged parallel with extrafusal fibres (rest of main muscle which contracts).
• Has contractile regions at each end which are stimulated to contract by gamma motor neurones. This maintains tension and sensitivity of the intrafusal fibres.

Stretch reflex

• Simplest reflex in the body as it involves a monosynaptic reflex arc.
• Important in controlling the resting muscle tone, especially the extensor muscles of the lower limb which resist gravity.
• When the muscle spindle becomes activated when the muscle increases in length, this leads to action potentials being generated in the associated sensory nerve.
• These action potentials are conducted to the spinal cord where there is a synapse with the alpha motor neurone to the stretched muscle.
• An excitatory neurotransmitter is released producing an action potential in the axon of the motor neurone which conducts it back to the neuromuscular junction.
• This results in contraction of the muscle.
• Stretch reflexes often depend on reciprocal inhibition, a reflex that prevents muscles from working against each other by inhibiting antagonists.
• The sensory nerve has branches in the spinal cord which synapse with interneurones (Renshaw cells).
• These interneurones synapse with alpha motor neurones which supply muscles antagonistic in action to the stretched muscle.
• The interneurones are inhibitory so that contraction in these antagonists is inhibited. This reduces the degree of stretch in the muscle from which the reflex originates.

Question 3

Describe the Golgi tendon reflex

Answer 3

• Golgi tendon organs are located in a tendon near its junction with a muscle.
• It consists of an encapsulated bundle of small, loose collagen fibres and one or more nerve fibres that penetrate the capsule and end in flattened leaflike processes between the collagen fibres.
• Increasing muscle tension activates these receptors and the sensory afferent is connected by an inhibitory interneurone within the spinal cord to the alpha motor neurone supplying the same muscle.
• The sensory afferent also synapses with an excitatory interneurone which excites antagonist muscle.
• This reflex is protects by reducing muscle tension before permanent damage to the muscle or tendon results.

Question 4

Describe flexor and crossed extensor reflexes

Answer 4

• The flexor reflex is a protective polysynaptic spinal reflex which enables quick contraction of flexor muscles and relaxation of extensors, resulting in the withdrawal of a limb from an injurious stimulus.
• The crossed extensor reflex involves the contraction of extensors and relaxation of flexors in the limb opposite from the one that is withdrawn.
• This allows you to push away from harmful stimuli.
• The reflex involves interneurones crossing the midline of the spinal cord.
• The reflex can be modified consciously.

Question 5

Give an example of how a spinal reflex may be modified by higher centres

Answer 5

• The pain reflex is a good example of how conscious control can modify reflex actions.
• If you pick up a hot dish the reflex response is to drop it, flex you arm, and push back.
• If the dish contains your dinner, and you have no other prospect of a meal, you can consciously inhibit the reflex, allowing you to safely transport the dish to the table.