Reflexes

Question 1

What is a reflex ?

Answer 1

A rapid, automatic, involuntary, inbuilt, hardwired and unconscious response

Question 2

What is the reflex pathway ?

Answer 2

Sensory receptor > afferent pathway > integrating centre > efferent pathway > effector organ

Question 3

Summarise the reflex arc

Answer 3

A stimulus is detected by a receptor (specialised cell or dendrites of sensory neurone)
This activates an afferent/sensory neurone leading to the propagation of action potentials along the axon of sensory neurones
A motor neurone is then activated and the axon of the stimulated neurone carries action potentials to an effector organ
The effector organ responds as a result of neurotransmitter being released by the motoneuron

Question 4

Do reflex responses require conscious perception ?

Answer 4

They are not dependent on conscious perception but can be inhibited by conscious control

Question 5

Give some examples of receptors in the skin :

Answer 5

• Meissner’s corpuscle (rapidly adapting - touch and pressure)
• Merkle’s corpuscle (slowly adapting - touch and pressure)
• free nerve ending (slowly adapting)
• Pacinian corpuscle (rapidly adapting - vibration and deep pressure)
• Ruffini corpuscle (slowly adapting - skin stretch)

Question 6

What do mechanoreceptors detect ?

Answer 6

Touch and pressure

Question 7

What is a receptive field ?

Answer 7

A receptive field is when each sensory neurone responds to stimulus within a specific region of the skin

Question 8

What does a small receptive field indicate ?

Answer 8

That there is a high receptor density and so greater acuity

Question 9

How do you find the receptive field size ?

Answer 9

Adjust the distance between the tips and determine the minimum distance at which 2 points can be felt

Question 10

What do sensory receptors do ?

Answer 10

They transduce stimuli into generator potentials and therefore action potentials

Question 11

What are the properties of sensory receptors ?

Answer 11

They have :
• high specificity - respond more readily to one particular stimulus
• high sensitivity - extremely sensitive to a specific stimulus

Question 12

What do olfactory receptors respond to ?

Answer 12

They respond to a few odour molecules in the air

Question 13

What do (dark adapted) rod photoreceptors respond to ?

Answer 13

They can respond to a single photon

Question 14

Where are effectors found ?

Answer 14

They are found in muscles (skeletal, smooth and cardiac) and glands (endocrine and exocrine)

Question 15

What is a stimulus ?

Answer 15

A stimulus causes local depolarisation in nerve endings and then action potentials are propagated along sensory axons

Question 16

Give some examples of types of reflexes :

Answer 16

1) spinal reflex e.g. withdrawal reflex
2) cranial reflex e.g. pupil constriction
3) innate and conditioned reflexes
4) somatic reflexes e.g. withdrawal reflexes
5) autonomic reflexes
6) monosynaptic and polysynaptic reflexes

Question 17

What is the difference between innate and conditioned synapses ?

Answer 17

Innate synapses are natural whereas conditioned synapses need to be learned

Question 18

Where are autonomic reflexes carried out ?

Answer 18

Carried out by the autonomic nervous system to smooth muscle, cardiac muscle or glands

Question 19

Why are monosynaptic reflexes quicker than polysynaptic reflexes ?

Answer 19

Because they involve less neurones

Question 20

What is the complexity of reflexes dependent on ?

Answer 20

Dependent on the number of synapses involved

Question 21

What is a monosynaptic reflex ?

Answer 21

It is a rapid response to a specific stimulus. The sensory neurone synapses directly on to a motoneurone.

Question 22

What is a polysynaptic reflex ?

Answer 22

There is a longer delay between the stimulus and response. There is atleast 1 interneurone between sensory and motoneuron.

Question 23

What type of reflex is the knee jerk stretch reflex ?

Answer 23

It is a monosynaptic spinal reflex

Question 24

Summarise the method of the knee jerk stretch reflex

Answer 24

1) The patellar tendon is tapped with a reflex hammer
2) The spindle receptors in the quadriceps muscle are stretched
3) Distortion of receptors stimulates sensory neurones that extend to the spinal cord
4) The sensory neurone synapses with the motoneuron in the spinal cord that controls motor units in stretched muscle
5) The knee extends in a brief kick
6) The collaterals of the afferent fibres make synaptic contact with inhibitory interneurones which in turn inhibit the antagonistic flexor muscles of the knee joint

Question 25

Summarise the withdrawal reflex

Answer 25

• the stimulated afferent neurone stimulates excitatory interneurones which stimulate the biceps
• contraction of the biceps causes flexion of the elbow joint which leads to withdrawal of the hand from the stimulus
• the afferent neurone also stimulates inhibitory interneurones which inhibits efferent interneurones supplying the triceps to prevent them from contracting
• this is known as reciprocal innervation

Question 26

What type of reflex is the flexor/crossed extensor reflex ?

Answer 26

A polysynaptic spinal reflex

Question 27

Give some examples of neonate reflexes

Answer 27

Rooting reflex - happens when the corners of the baby’s mouth is stroked or touched
Grasp reflex - happens when you put something in a baby’s hand
Moro reflex - also called the startle reflex

Question 28

Summarise muscle spindle reflexes

Answer 28

Muscle spindles are bundles of small specialised intrafusal muscle fibres which are innervated by both sensory and motor neurones.
The afferent neurones are activated by stretch and they detect changes in the length of muscle fibres which cause contraction of that fibre.
It is a negative feedback loop.