2. Motor Functions
• 1. Voluntary Functions
– Voluntary movement
• 2. Involuntary Functions
– Reflexes
3. What is a reflex?
• Response to a stimulus
• Involuntary, without significant
involvement of the brain
• Stimulus Response
Task:
Write down 3 reflexes
4. What is a reflex?
Stimulus
Effector organ
Response
Central
connections
Efferent nerve
Afferent nerveReceptor
Higher centre
control
5. Stretch reflex
• This is a basic reflex present in the
spinal cord
• Stimulus: muscle stretch
• Response: contraction of the muscle
• Receptors: stretch receptors located
in the muscle spindle
6.
7. skeletal muscle
• two types of muscle fibres
– extrafusal
• normally contracting fibres
– Intrafusal
• fibres present inside the muscle spindle
• lie parallel to extrafusal fibres
• either end of the fibre contractile
• central part contains
stretch receptors
10. Nerve supply
Sensory to intrafusal fibre:
Ia afferent
II afferent
Motor:
to extrafusal fibre
A motor neuron
to intrafusal fibre
A motor neuron
11. Ia afferent nerve
motor neuron
one
synapse
muscle
stretchmuscle
contraction
Stretch reflex
12. • When a muscle is stretched
• stretch receptors in the intrafusal fibres
are stimulated
• via type Ia afferent impulse is transmitted
to the spinal cord
• motor neuron is stimulated
• muscle is contracted
• Monosynaptic
• Neurotransmitter is glutamate
15. – nuclear bag fibre
• primary (Ia) afferent
– supplies annulospiral ending in the centre
– provide information on muscle length and velocity
(phasic response) fast stretch reflex
– nuclear chain fibre
• primary (Ia) and secondary (II) afferent
– supplies flower spray ending
– monitor the length of the muscle (tonic response) –
slow stretch reflex
Two types of intrafusal fibres
16. Ia afferent fibre
II afferent fibre
nuclear bag fibre
nuclear chain fibre
motor
neuron
motor
neuron
18. • Phasic stretch reflex
– Stretching the quadriceps muscle quickly (e.g. by tapping
the patellar tendon) evokes a discharge in the primary
afferent (Ia) fibres
– These form monosynaptic excitatory connections with motor
neurons supplying physiological extensors of the knee,
which contract briefly
• Tonic stretch reflex
– Passive bending of the joint elicits a discharge from the
group II afferents that increases the tone of physiological
extensor (antigravity) muscles
– Tonic stretch reflex is important for maintaining erect body
posture
19. motor neuron
• cell body is located in the anterior
horn
• motor neuron travels through the
motor nerve
• supplies the intrafusal fibres
(contractile elements at either end)
21. • When motor neuron is active
– extrafusal fibres are contracted
– muscle contracts
• when motor neuron is active
– intrafusal fibres are contracted
– stretch receptors are stimulated
– stretch reflex is activated
– impulses will travel through Ia
afferents
– alpha motor neuron is activated
– muscle contracts
28. motor neuron activity
• active all the time - mild contraction
• Maintain the sensitivity of the muscle
spindle to stretch
• modified by the descending pathways
• descending excitatory and inhibitory
influences
• sum effect is generally inhibitory in nature
29. Alpha gamma co-activation
• gamma motoneurons are activated in parallel
with alpha motoneurons to maintain the firing
of spindle afferents when the extrafusal
muscles shorten
• Activity from brain centres often causes
simultaneous contraction of both extra- and
intrafusal fibres, thereby ensuring that the
spindle is sensitive to stretch at all muscle
lengths
30.
31. Inverse stretch reflex
• When the muscle is strongly
stretched -> muscle is relaxed
• Golgi tendon organs are stimulated
• Via type Ib afferents impulse is
transmitted to the spinal cord
• inhibitory interneuron is stimulated
• motor neuron is inhibited
• muscle is relaxed
32. motor neuron
Undue stretch
Golgi tendon organ
muscle
relaxation
Ib afferent nerve
inhibitory
interneuron
33. motor neuron
Undue stretch
Golgi tendon organ
muscle
relaxation
Ib afferent nerve
inhibitory
interneuron
52. Primitive reflexes
• These are reflexes present in
newborn babies but disappear as the
child develops
• They were evolutionarily primitive in
origin
• In adults these reflexes are inhibited
by the higher centres
54. Babinski sign
• when outer border of the sole of the foot is
scratched
• upward movement of big toe (dorsiflexion)
• fanning out of other toes
• also called extensor plantar reflex
• feature of
• upper motor neuron lesion
• seen in infants during 1st year of life (because of
immature corticospinal tract)
56. Clinical Importance of reflexes
(tendon jerks)
• Locate a lesion in the motor system
• To differentiate upper motor neuron
lesion from a lower motor neuron
lesion
