2. Motor Functions
• 1. Voluntary motor functions
– Voluntary movement
• 2. Involuntary motor 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
Inverse stretch reflex
33. motor neuron
Undue stretch
Golgi tendon organ
muscle
relaxation
Ib afferent nerve
inhibitory
interneuron
Inverse stretch reflex
51. 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
57. Motor System
• Starts at the motor cortex
• Motor cortex is located at the frontal lobe
– precentral cortex
63. Motor cortex
• different areas of the body are
represented in different cortical areas in
the motor cortex
• Motor homunculus
– somatotopic representation
– not proportionate to structures but
proportionate to function
– distorted map
– upside down map
64. Motor cortical areas
• primary motor cortex (MI)
– precentral gyrus
• Movements are executed
• secondary motor cortex (MII)
– premotor cortex
– supplementary motor area (SMA)
• Movements are planned together with cerebellum, basal
ganglia and other cortical areas
65. Primary motor cortex
• Corticospinal tract (pyramidal tract) originates
from the primary motor cortex
• Corticobulbar tract also originates from the
motor cortex and supplies brainstem and the
cranial nerves
• Cell bodies of the corticospinal tracts are
called Betz cells (large pyramidal shaped
cells)
• Corticospinal tract descends down the
internal capsule
66. Course of the corticospinal tract
• Descends through
– internal capsule
– at the medulla
• cross over to the other side
• uncrossed tracts
– descends down as the corticospinal tract
– ends in each anterior horn cell
– synapse at the anterior horn cell (directly or through
interneurons)
68. Primary and secondary cortical
areas
• Primary areas are primarily connected with the
peripheral organs/structures
– Primary motor cortex (area 4)
• Secondary areas are inter-connected to each
other by cortico-cortical pathways and perform
complex processing
– Premotor cortex (area 6)
– Supplementary motor area (superomedial part of
area 6)
69.
70. Functional role of primary and
secondary motor areas
• SMA (Supplementary motor area)
assembles global instructions for
movements
• It issues these instructions to the
Premotor cortex (PMC)
• Premotor cortex works out the
details of smaller components
• And then activates specific Primary
motor cortex (MI)
• Primary motor cortex through
Corticospinal tracts (CST) activate
specific motor units
SMA
PMC
MI
CST
Motor units
76. alpha motor neuron
• this is also called the final common pathway
• Contraction of the muscle occurs through this
whether
– voluntary contraction through corticospinal tract
or
– involuntary contraction through gamma motor
neuron - stretch reflex - Ia afferent
77. motor unit
• muscle contraction occurs in terms of motor units
rather than by single muscle fibres
• a motor unit is defined as
– anterior horn cell
– motor neuron
– muscle fibres supplied by the neuron
• Muscle power/strength is obtained by the principle of
“Recruitment of motor units”
78. motor unit
• Innervation ratio
– motor neuron:number of muscle
fibres
• in eye muscles
– 1:23 offers a fine degree of
control
• in calf muscles
– 1:1000 more strength
79. Upper motor neuron
• Consists of
– Corticospinal tract (pyramidal tract)
– Extrapyramidal tracts
• Start from the brainstem
• Ipsilateral/contralateral
• Cortical pathways can excite/inhibit these tracts
• Modify the movement that is initiated by the CST
• Influence (+/-) gamma motor neuron, stretch reflex, muscle tone
• Important for postural control
• Cerebellar and basal ganglia influence on the lower motor neuron will
be through extrapyramidal tracts
80. Extrapyramidal tracts
• starts at the brain stem
• descends down either ipsilaterally or
contralaterally
• ends at the anterior horn cell
• modifies the motor functions
81. Extrapyramidal tracts
• there are 4 tracts
– reticulospinal tracts
– vestibulospinal tracts
– rubrospinal tracts
– tectospinal tracts
82. reticulospinal tract
• relay station for descending motor impulses
except pyramidal tracts
• receives & modifies motor commands to the
proximal & axial muscles
• maintain normal postural tone
• excitatory to alpha & gamma motorneurons
• end on interneurons too
• this effect is inhibited by cerebral influence
• mainly ipsilateral
85. Reticular formation
• A set of network of interconnected
neurons located in the central
core of the brainstem
• It is made up of ascending and
descending fibers
• It plays a big role in filtering
incoming stimuli to discriminate
irrelevant background stimuli
• There are a large number of
neurons with great degree of
convergence and divergence
86. Functions
• Maintain consciousness, sleep and arousal
• Motor functions (postural and muscle tone
control)
– Reticulospinal pathways are part of the
extrapyramidal tracts
• Pain modulation (inhibition)
– Several nuclei (PAG, NRM) are part of the
descending pain modulatory (inhibitory) pathway
87. vestibular nuclei & tracts
• responsible for maintaining tone in antigravity
muscles & for coordinating the postural
adjustments in limbs & eyes
• connections with vestibular receptors (otolith
organs) & cerebellum
• mainly ipsilateral
• supplies extensors
89. • vestibulospinal tracts
– lateral vestibulospinal tract
– medial vestibulospinal tract
– excitatory to antigravity alpha motor neurons &
supplies interneurons too
– lateral tract
• excitation of extensor muscles & relaxation of flexor
muscles
– medial tract
• inhibition of neck & axial muscles
90. red nucleus
• present in the midbrain
• rubrospinal tract originates from the red nucleus
• ends on interneurons
• control the distal muscles of limbs
• excite limb flexors & inhibit extensors
• higher centre influence (cerebral cortex)
• mainly contralateral
• supplies flexors
• Functionally this tract is not important in human motor
system
92. tectospinal tract
• tectospinal tract originates from the tectum of
the midbrain
• ends on interneurons
• mainly contralateral
• supplies cervical segments only
• Functionally this tract is not important in human
motor system
94. inferior olivary nucleus
• present in the medulla
• function:
– motor coordination
• via projections to the cerebellum
• sole source of climbing fibres to the cerebellum
– motor learning
– Functionally this nucleus is not important in human
motor system
96. Clinical Importance of the motor system
examination
• Tests of motor function:
– Muscle power
• Ability to contract a group of muscles in order to make an
active movement
– Muscle tone
• Resistance against passive movement
97. Basis of tests
• Muscle power
– Test the integrity of motor cortex, corticospinal tract
and lower motor neuron
• Muscle tone
– Test the integrity of stretch reflex, gamma motor
neuron and the descending control of the stretch
reflex
98. Muscle tone
• Resistance against passive movement
– Gamma motor neuron activate the spindles
– Stretching the muscle will activate the stretch reflex
– Muscle will contract involuntarily
– Gamma activity is under higher centre inhibition
99. • There is a complex effect of corticospinal and extrapyramidal tracts on the alpha and
gamma motor neurons (in addition to the effect by muscle spindle)
• There are both excitatory and inhibitory effects
• Sum effect
– excitatory on alpha motor neuron
– Inhibitory on gamma motor neuron
Corticospinal
tract
Extrapyramidal
tracts
Alpha motor
neuron
Gamma
motor
neuron•Voluntary movement
•Muscle tone
Muscle spindle
100. Clinical situations
• Muscle power
– Normal
– Reduced (muscle weakness)
• Paralysis, paresis, plegia
• MRC grades
0 - no movement
1 - flicker is perceptible in the muscle
2 - movement only if gravity eliminated
3 - can move limb against gravity
4 - can move against gravity & some resistance exerted by examiner
5 - normal power
• Muscle tone
– Normal
– Reduced
• Hypotonia (Flaccidity)
– Increased
• Hypertonia (Spasticity)
101. Main abnormalities
• Muscle Weakness / paralysis
– Reduced muscle power
• Flaccidity
– Reduced muscle tone
• Spasticity
– Increased muscle tone
106. Upper motor neuron lesion
• muscle weakness
• spastic paralysis
• increased muscle tone (hypertonia)
• reflexes: exaggerated (hyperreflexia)
• Babinski sign: positive
• superficial abdominal reflexes: absent
• muscle wasting is very rare
• clonus can be seen:
– rhythmical series of contractions in response to sudden stretch
• clasp knife effect can be seen
– passive stretch causing initial increased resistance which is released
later
• eg. Stroke
109. Babinski sign
• when outer border of the sole of the foot is
scratched
• upward movement of big toe
• fanning out of other toes
• feature of upper motor neuron lesion
• extensor plantar reflex
• seen in infants during 1st year of life (because
of immature corticospinal tract)
111. • Observation
• When the spinal cord is suddenly transected, essentially all
cord functions, including spinal cord reflexes, immediately
become depressed
• This is called “spinal shock”
• Period of spinal shock is about 2 weeks in humans
• It may vary depending on the level spinal cord injury
• Higher the animal in evolution greater is the spinal shock
period
Spinal cord transection and spinal shock
112. During spinal shock period
• complete loss of all reflexes
• no tone
• paralysis
• complete anaesthesia
• no peristalsis
• bladder and rectal reflexes absent
• no sweating
• arterial blood pressure decreases
113. Possible mechanism of spinal shock
• Normal activity of the spinal cord reflexes depends to a great
extent on continual tonic excitation from higher centers
(pyramidal and extrapyramidal tracts)
• Spinal shock may be due to the sudden cessation of tonic
bombardment of spinal cord interneuron pool by descending
influences
• During recovery from spinal shock, the excitability of spinal cord
reflexes increase due to the lack of descending inhibition and
possible denervation hypersensitivity
• After the spinal shock period typical upper motor neuron
features appear
114. after the spinal shock
• reflexes will reappear, mostly exaggerated
• bladder become reflex
• mass reflex will appear
– afferent stimuli irradiate to several reflex centres
– noxious stimulus causes: withdrawal response,
evacuation of bladder, rectum, sweating, pallor
115. Site of lesions
Cortex
Internal capsule
Brain stem
Spinal cord
Anterior horn cell
Motor nerve
Neuromuscular junction
Muscle
117. Site of lesions
monoplegia
only 1 limb is affected either UL or LL,
lower motor neuron lesion
hemiplegia
one half of the body including
UL and LL
lesion in the Internal capsule
paraplegia
both lower limbs
thoracic cord lesion
quadriplegia (tetraplegia)
all 4 limbs are affected
cervical cord or brain stem lesion
119. Stroke
• Cerebrovascular accident (CVA)
• A serious neurological disease
• Large number of deaths per year
• Cerebrovascular ischaemia causing
infarction or haemorrhage
• Sudden onset hemiplegia
• Hypertension, diabetes, obesity are
risk factors
135. Components of the motor system
•
Main motor system
– Primary motor area (MI)
– Corticospinal tract (corticobulbar tract)
– Lower motor neuron
– Peripheral nerve (cranial nerve)
– Neuromuscular junction
– Skeletal muscle
– Descending tracts (extrapyramidal tracts)
Secondary motor areas
– Premotor cortex
– Supplementary motor area (SMA)
Cerebellum
Basal ganglia
Vestibular system
140. Basal ganglia
• help cortex to execute patterns of
movements
• planning motor commands
• Purposeful movement
• Control of muscle tone and posture
141. Premotor cortex and SMA
• these are necessary to plan motor command
and also work as an intermediate centre
between cerebellum, basal ganglia and
motor cortex
• control complex movements, bimanual
tasks
142. Functional role of primary and
secondary motor areas
• SMA (Supplementary Motor Area)
initiate instructions for movements
• issues these instructions to the
PreMotor Area.
• PreMotor Cortex (PMC) works out
the details of smaller components
• activates specific Primary Motor
Cortex (MI)
• Primary Motor Cortex through
corticospinal tracts (CST) activate
specific motor units
SMA
PMC
MI
CST
Motor units
145. hierarchical motor control
a lesion at a lower level
muscles
lower motor neuron
upper motor neuron
primary motor area
premotor
area
SMA
cerebellum
basal
ganglia
x
eg. carpal tunnel syndrome
146. hierarchical motor control
a lesion at a higher level
muscles
lower motor neuron
upper motor neuron
primary motor area
premotor
area
SMA
cerebellum
basal
ganglia
x
eg. Parkinsonism
147. • But what finally drives us to
action???
•perhaps motivation
•motivation is controlled by limbic
system and hypothalamus