49. Reflexes

Question 1

What are reflexes?

Answer 1

Reflexes are neuronal pathways
that produce rapid, automatic and
predictable responses to a stimulus.

The basic components of the reflex
arc include a receptor, 
afferent sensory neurone, 
synapse, efferent motor
neurone and effector organ.

When we talk about reflexes
we typically think of somatic reflexes,

e.g. knee jerk stretch reflex,

which are important in the functioning of the skeletal
muscle system.

However, equally important are
the visceral reflexes, e.g.
pupillary light reflex,
which form the basis of the functioning of the autonomic nervous system.

Question 2

What is the Bell–Magendie law?

Answer 2

This law states that the
anterior spinal nerve roots
contain only motor fibres
and the

posterior nerve roots contain
only sensory fibres.

Question 3

Describe the physiology of the stretch reflex.

Answer 3

The stretch reflex (e.g. knee jerk stretch reflex)
is a monosynaptic reflex that
results in the contraction of a muscle
in response to its being stretched.

Muscle spindle receptors within the muscle
are stimulated when the muscle
fibres are stretched,

resulting in the generation of an electrical potential.

Provided this is of sufficient magnitude,
an action potential is generated
and propagates down afferent
sensory neurones (Ia and II)

to enter the grey matter of the spinal cord.

They synapse within the ventral horn
with efferent motor neurones (Aα),

which project back to the
extrafusal muscle fibres of
the stretched muscle and
cause it to contract.

The sensory neurones also
synapse with inhibitory inter-neurones,
which innervate the antagonistic
muscle group.

Hence when the stretched muscle contracts
during the reflex, its antagonistic muscles relax.

This is known as reciprocal innervation.

Stretch reflexes help maintain muscle tone,
aid posture and prevent injury by
opposing overstretching of muscles.

Question 4

What is the inverse stretch reflex?

Answer 4

This refers to the relaxation
of a muscle in response to a strong stretch.

The harder a muscle is stretched,
the more forcefully it will contract.

However, there comes a point when
muscle tension becomes so great
that the Golgi tendon organs detect

this and inhibit the activity of the efferent

Aα motor neurones via an inhibitory feedback mechanism.

This system is designed to
prevent muscle damage.

Question 5

Describe the physiology of the withdrawal (flexor) reflex.

Answer 5

This is an important reflex governing

the response to a painful stimulus

and is an example of a polysynaptic reflex.

Nociceptors are stimulated and
impulses are propagated 
along sensory
Aδ and C fibres into the grey matter
of the spinal cord. 

Here they synapse with

inter-neurones that extend to
several spinal cord segments.

These inter-neurones

activate various Aa motor neurones,

which culminate in the contraction
of the flexor muscles of the affected limb.

However, in order to maintain balance
during a sudden flexor withdrawal
of a lower limb,

the pain stimulus also activates a cross extensor
reflex, which results in the automatic extension of the contralateral
limb.

Question 6

How are nerve fibres classified?

Answer 6

Classification of nerve fibres has
become confusing with the use of

both alphabetical and numerical classification systems.

Originally, mammalian
nerves were classified alphabetically into A, B or C fibre types.

The A fibre type was then further subdivided
into a, b, g and d groups.

However, as
science evolved it became
apparent that the alphabetical classification
system was inadequate

because not all nerve fibre types
within the originally
assigned group were the same.

Therefore, a numerical system was
introduced to classify sensory neurones.

Question 7

How are nerve fibres classified?

Answer 7

Type diam velocity fxn

Aa 10–20 60–120 Motor
Ab 5–10 40–70 Touch Pressure
Ag 3–6 15–30 Motor to muscle spindles
Ad 2–5 10–30 Pain Temperature
B 1–3 3–15 Autonomic (pre-ganglionic)
C 0.5–1 0.5–2 Pain
Temperature
Ia
(Aa type)
12–20 72–120 Sensory from muscle spindle
(annulospiral)
Ib
(Aa type)
12–20 72–120 Sensory from Golgi tendon
II
(Ab type)
4–12 24–72 Sensory from muscle spindle
(flower-spray)
III
(Ad type)
1–4 6–24 Pain
Cold
IV
(C type)
0.5–1 0.5–2 Pain
Temperature