Part 4
Motor Functions of the Nervous System
I Motor Unit and Final Common Pathway
1. Motor Unit
•Every striated
muscle has
encapsulated
muscle fibers
scattered
throughout the
muscle called
muscle spindles.
•Extrafusal and
intrafusal fibers
The extrafusal muscle
fibers are innervated
by Alpha motor
neuron
The intrafusal muscle
fibers are innervated
by Gamma motor
neurons
Motor units
A motor unit is a single motor neuron (a
motor) and all (extrafusal) muscle fibers it
innervates
 Motor units are the physiological functional
unit in muscle (not the cell)

 All cells in motor unit contract synchronously
Motor units and innervation ratio
Innervation ratio
Fibers per motor
neuron
Extraocular muscle 3:1
Gastrocnemius 2000:1
Purves Fig. 16.4
•The muscle cells of
a motor unit are not
grouped, but are
interspersed among
cells from other
motor units
•The coordinated
movement needs
the activation of
several motors
organization of motor subsystems
Overview - organization of
motor systems
Motor Cortex
Brain Stem
Spinal Cord
a-motor
neuron
Final common
pathway
Skeletal muscle
Final common path - a-motor neuron
(-)
(+)
muscle
fibers
Transmitter?
Schwann
cells
motor nerve
fiber
(-)
(+)
axon
hillock
Receptors?
acetylcholine
esterase
NM junction
Final Common Pathway,
a motor pathway consisting of the motor
neurons by which nerve impulses from many
central sources pass to a muscle in the
periphery.
II Motor Functions of the Spinal
Cord – Spinal Reflex
Spinal Reflexes
 Somatic
reflexes mediated by the spinal cord
are called spinal reflexes
 These reflexes may occur without the
involvement of higher brain centers
 Additionally, the brain can facilitate or inhibit
them
1. Stretch Reflex
(1) Anatomy of Muscle Spindle

The muscle spindles detect
change in the length of the
muscle
-- stretch receptors that report
the stretching of the muscle to
the spine.

Each spindle consists of 310 intrafusal muscle fibers
enclosed in a connective
tissue capsule
 These fibers are less than
one quarter of the size of
extrafusal muscle fibers
(effector fibers)
Anatomy of Muscle Spindle

The central region of
the intrafusal fibers
which lack
myofilaments are
noncontractile,
 serving as the
receptive surface of
the spindle (sensory
receptor)
Anatomy of Muscle Spindle


Intrafusal fibers are wrapped by two types of afferent
endings that send sensory inputs to the CNS
Primary sensory endings
– Type Ia fibers
– Innervate the center of the spindle

Secondary sensory endings
– Type II fibers
– Associated with the ends of the spindle
Components of muscle spindle
Dynamic
intrafusal
fiber
Static intrafusal
fibers
Static
intrafusal
fibers
Afferent
axons
Ia
II
} Primary
ending
Secondary
}
ending
Anatomy of Muscle Spindle

Primary sensory endings
– Type Ia fibers

Stimulated by both the rate and amount of stretch
Anatomy of Muscle Spindle

Secondary sensory endings
– Type II fibers

stimulated only by degree of stretch
Anatomy of Muscle Spindle

The contractile region of the intrafusal muscle fibers are
limited to their ends as only these areas contain actin and
myosin filaments
 These regions are innervated by gamma () efferent fibers
Muscle stretch
reflex
(2) Muscle stretch reflex
Definition: Whenever a muscle is stretched, excitation of
the spindles causes reflexive contraction of the same
muscle from which the signal originated and also of
closely allied synergistic muscle.
The basic circuit: Spindle Proprioceptor nerve fiber
dorsal root of the spinal cord synapses with anterior
motor neurons
a -motor N. F. the same M. from
whence the M. spindle fiber originated.
Circuit of the Strength Reflex
Muscle
spindle
Dorsal root
Muscle fiber
Tendon
Ventral root
a-mn
The Stretch Reflex

Exciting a muscle spindle
occurs in two ways
– Applying a force that
lengthens the entire muscle
– Activating the  motor
neurons that stimulate the
distal ends of the intrafusal
fibers to contact,

thus stretching the mid-portion
of the spindle (internal stretch)
The Stretch Reflex

Whatever the
stimulus, when the
spindles are activated
 their associated
sensory neurons
transmit impulses at a
higher frequency to
the spinal cord
The Stretch Reflex

At spinal cord sensory neurons synapse directly
(mono- synaptically) with the a motor neurons
which rapidly excite the extrafusal muscle fibers of
stretched muscle
The Stretch Reflex

The reflexive muscle contraction that follows (an example
of serial processing) resists further stretching of the muscle
The Stretch Reflex

Branches of the afferent fibers also synapse with interneurons that inhibit motor neurons controlling the
antagonistic muscles
•Inhibition of the antagonistic muscles is called reciprocal
inhibition
•In essence, the stretch stimulus causes the antagonists to relax
so that they cannot resist the shortening of the “stretched”
muscle caused by the main reflex arc
The types of the Stretch Flex
1) Tendon reflex
(dynamic stretch reflex)
 Caused by rapid stretch of the
muscle, as knee-jerk reflex;
 Transmitted from the IA
sensory ending of the M. S.
 Causes an instantaneous,
strong reflexive contraction of
the same muscle;
 Opposing sudden changes in
length of the M.;
A monosynaptic pathway
 being over within 0.7 ms;
The types of the Stretch Flex
2) Muscle tonus (static stretch reflex):
 Caused by a weaker and continues stretch of the muscle,
 Transmitted from the IA and II sensory ending of the M. S.
 Multiple synaptic pathway, continues for a prolonged period.
 Non-synchronized contraction,
 M. C. for at least many seconds or minutes, maintaining the
posture of the body.
The Stretch Reflex

The stretch reflex is most important in large
extensor muscles which sustain upright
posture
 Contractions of the postural muscles of the
spine are almost continuously regulated by
stretch reflexes
(3) Gamma impact on afferent
response
Muscle spindle: motor
innervation

Gamma motoneurons:
– Innervate the poles of the fibers.
WHAT IS THE -LOOP?
Descending influence (UMN)
Muscle spindle
1a

Activation of the -loop
results in increased
muscle tone
a
MUSCLE
Functional significance of
gamma impact on spindle activity

The tension of intrafusal fibers is maintained during
active contraction by gamma activity.
 The system is informed about very small changes in
muscle length.
2. The Deep Tendon Reflex
(1) Structure and Innervation of Golgi
Organ
Golgi tendon organ: structure





Located in the
muscle tendon
junction.
Connective tissue
encapsulating
collagen fibers and
nerve endings.
Attached to 10-20
muscle fibers and
several MUs.
Ib afferent fiber.
sensitive to tension
(2) Golgi tendon organ:
response properties

Less frequent than muscle spindle.
Golgi tendon organ: response
properties (cont)

Sensitive to the
change of tension
caused by the
passive stretch or
active contraction
(3) The Deep Tendon Reflex

When muscle
tension increases
moderately during
muscle contraction
or passive
stretching,
 GTO receptors are
activated and
afferent impulses are
transmitted to the
spinal cord
The Deep Tendon Reflex

Upon reaching the
spinal cord, information is sent to the
cerebellum, where it is
used to adjust muscle
tension
 Simultaneously, motor
neurons in the spinal
cord supplying the
contracting muscle are
inhibited and
antagonistic muscle
are activated
(activation)
The Deep Tendon Reflex

Deep tendon reflexes cause muscle relaxation and
lengthening in response to the muscle’s contraction
 This effect is opposite of those elicited by stretch
reflexes
 Golgi tendon organs help ensure smooth onset and
termination of muscle contraction
 Particularly important in activities involving rapid
switching between flexion and extension such as in
running
Compare spindle and golgi
Compare spindle and golgi
3. The Crossed
Extensor Reflex




The reflex occur when you
step on a sharp object
There is a rapid lifting of
the affected foot (ipsilateral
withdrawal reflex ),
while the contralateral
response activates the
extensor muscles of the
opposite leg (contralateral
extensor reflex)
support the weight shifted
to it
4. Superficial Reflexes

Superficial reflexes are elicited by gentle
cutaneous stimulation
 These reflexes are dependent upon
functional upper motor pathways and spinal
cord reflex arcs
 Babinski reflex
Babinski reflex - an UMN sign
 Adult
response - plantar flexion of the big toe and adduction
of the smaller toes
 Pathological (Infant) response - dorsoflexion (extension) of
the big toe and fanning of the other toes
 Indicative of upper motor neuron damage
5. Spinal cord transection and spinal shock
(1) Concept: When the spinal cord is suddenly
transected in the upper neck, essentially all cord
functions, including the cord reflexes, immediately
become depressed to the point of total silence. (spinal
animal)
(2) During spinal shock:
complete loss of all reflexes,
no tone, paralysis,
complete anaesthesia,
no peristalsis, bladder and rectal reflexes absent (no
defecation and micturition )
no sweating
arterial blood Pressure decrease（40mmHg）,
(3) the reason: The normal activity of the spinal
cord neurons depends to a great extent on
continual tonic excitation from higher centers (the
reticulospinal-, vestibulospinal- corticospinal
tracts).
(4) The recovery of spinal neurons excitability.
III. Role of the brain stem:
Support of the Body Against Gravity –
Roles of the Reticular and Vestibular
nuclei
Facilitated and inhibitory area
Areas in the cat brain where stimulation produces facilitation (+) or
inhibition (-) of stretch reflexes. 1. motor cortex; 2. Basal ganglia; 3.
Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6.
Vestibular nuclei.
1. Facilitated area—roles of the reticular and vestibular
nuclei.：
(1) The pontine reticular nuclei
 Located slightly posteriorly and laterally in the pons and extending
to the mesencephalon,
 Transmit excitatory signals downward into the cord (the pontine
reticulospinal tract)
1. motor cortex;
2. Basal ganglia;
3. Cerebellum;
4. Reticular
inhibitory area;
5. Reticular
facilitated area;
6. Vestibular nuclei.
(2) The vestibular nuclei
 selectively control the excitatory signals to the different
antigravity M. to maintain equilibrium in response to signals
from the vestibular apparatus.
1. motor cortex;
2. Basal ganglia;
3. Cerebellum;
4. Reticular
inhibitory area;
5. Reticular
facilitated area;
6. Vestibular
nuclei.
MOTOR TRACTS & LOWER MOTOR NEURON
MOTOR CORTEX
MIDBRAIN &
RED NUCLEUS
(Rubrospinal Tract)
UPPER MOTOR NEURON
(Corticospinal Tracts)
VESTIBULAR NUCLEI
(Vestibulospinal Tract)
PONS & MEDULLA
RETICULAR FORMATION
(Reticulospinal Tracts)
LOWER (ALPHA) MOTOR NEURON
THE FINAL COMMON PATHWAY
SKELETAL
MUSCLE
Properties of the Facilitated Area
 Terminate on the motor neurons that exciting antigravity M. of the
body (the M. of vertebral column and the extensor M. of the limbs).
 Have a high degree of natural (spontaneous) excitability.
 Receive especially strong excitatory signals from vestibular nuclei and
the deep nuclei of the cerebellum.
 Cause powerful excitation of the antigravity M throughout the body
(facilitate a standing position), supporting the body against gravity.
1. motor cortex; 2. Basal
ganglia; 3. Cerebellum; 4.
Reticular inhibitory area;
5. Reticular facilitated
area; 6. Vestibular nuclei.
2. Inhibitory area –medullary reticular system
(1) Extend the entire extent to the medulla, lying ventrally and
medially near the middle.
(2) Transmit inhibitory signals to the same antigravity anterior
motor neurons (medullary reticulospinal tract).
1. motor cortex;
2. Basal ganglia;
3. Cerebellum;
4. Reticular
inhibitory area;
5. Reticular
facilitated area;
6. Vestibular
nuclei.
MOTOR TRACTS & LOWER MOTOR NEURON
MOTOR CORTEX
MIDBRAIN &
RED NUCLEUS
(Rubrospinal Tract)
UPPER MOTOR NEURON
(Corticospinal Tracts)
VESTIBULAR NUCLEI
(Vestibulospinal Tract)
PONS & MEDULLA
RETICULAR FORMATION
(Reticulospinal Tracts)
LOWER (ALPHA) MOTOR NEURON
THE FINAL COMMON PATHWAY
SKELETAL
MUSCLE
(3) Receive collaterals from the corticospinal tract; the rubrospinal
tracts; and other motor pathways.
These collaterals activate the medullary reticular inhibitory system
to balance the excitatory signals from the P.R.S.,
so that under normal conditions, the body M. are normally tense.
1. motor cortex;
2. Basal ganglia;
3. Cerebellum;
4. Reticular
inhibitory area;
5. Reticular
facilitated area;
6. Vestibular
nuclei.
Areas in the cat brain where stimulation produces facilitation
(+) or inhibition (-) of stretch reflexes. 1. motor cortex; 2.
Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5.
Reticular facilitated area; 6. Vestibular nuclei.
Decerebrate Rigidity
• Decerebrate Rigidity: transection of the brainstem at
midbrain level (above vestibular nuclei and below red
nucleus)
• Symptoms include:
– extensor rigidity or posturing in both upper and lower
limbs
•Results from:
–loss of input from inhibitory medullary RF (activity of
this center is dependent on input from higher centers).
–active facilitation from pontine RF (intrinsically active,
and receives afferent input from spinal cord).
•The extensor rigidity is -loop dependent
–section the dorsal roots interrupts the -loop, and the
rigidity is relieved. This is -rigidity.
THE -LOOP?
Descending influence (UMN)
Muscle spindle
1a

Activation of the -loop
results in increased
muscle tone
a
MUSCLE
IV. The cerebellum and its motor functions
Cerebellar Input/Output Circuit
Based on cerebral intent and
external conditions
The cerebellum tracks and modifies
millisecond-to-millisecond muscle
contractions,
to produce smooth, reproducible
movements
Without normal cerebellar function,
movements appear jerky and uncontrolled
Functional Divisions-cerebellum
• Vestibulocerebellum (flocculonodular lobe)
The vestibulocerebellum
input-vestibular nuclei
output-vestibular nuclei
The vestibulocerebellum
Function:
The control of the equilibrium and postural movements.
Especially important in controlling the balance between
agonist and antagonist M. contractions of the spine, hips,
and shoulders during rapid changes in body positions.
Method
Calculate the rates and direction where the different parts of
body will be during the next few ms.
The results of these calculations are the key to the
brains’s progression to the next sequential movement.
•Spinocerebellum (vermis & intermediate)
•Spinocerebellum (vermis & intermediate)
–input-periphery & spinal cord:
–output-cortex
•Spinocerebellum (vermis & intermediate)
Functions:
-- Provide the circuitry for coordinating mainly the movements of
the distal portions of the limbs, especially the hands and fingers
-- Compared the “intentions ” from the motor cortex and red
nucleus, with the “performance” from the peripheral parts of the
limbs,
--Send corrective output signals to the motor neurons in the anterior
horn of spinal cord that control the distal parts of the limbs (hands
and fingers)
--Provides smooth, coordinate movements of the agonist and
antagonist M. of the distal limbs for the performance of acute
purposeful patterned movements.
•Cerebrocerebellum (lateral zone)
input-pontine N.
output-pre & motor cortex
• Cerebrocerebellum (lateral zone)
 Receives
all its input from the motor cortex, adjacent premotor and somatic sensory cortices of the brain. Transmits its
output information back to the brain.
Functions in a “feedback” manner with all of the cortical
sensory-motor system to plan sequential voluntary body and
limb movements,
Planning these as much as tenths of a second in advance of
the actual movements (mental rehearsal of complex motor
actions)
•Vestibulocerebellum (flocculonodular lobe)
Balance and body equilibrium
•Spinocerebellum (vermis & intermediate)
Rectify voluntary movement
•Cerebrocerebellum (lateral zone)
Plan voluntary movement
V The motor functions of basal ganglia
Components of Basal Ganglia
Caudate
Putamen
GPe
GPi
1. Corpus Striatum
Striatum ----- Caudate Nucleus & Putamen
Pallidum ----- Globus Pallidus (GP)
Components of Basal Ganglia
2. Substantia Nigra
Pars Compacta (SNc)
Pars Reticulata (SNr)
STN
3. Subthalamic Nucleus (STN)
SN (r & c)
Basal Ganglia
Connections
•Circuit of connections
–cortex to basal ganglia to
thalamus to cortex
–Helps to program
automatic movement
sequences (walking and arm
swinging or laughing at a
joke)
•Output from basal ganglia
to reticular formation
–reduces muscle tone
–damage produces
rigidity of Parkinson’s
disease
cortex to basal ganglia to thalamus to cortex
somatosensory
cortices
motor cortices
excitation
D1
Putamen
inhibition
D1 & D2
GPe
Dopamine
receptors
D2
GPi
Thalamus
STN
GPe/i: Globus pallidus
internal/external
STN: Subthalamus
Nucleus
SNc: Pars Compacta
SNc
(part of substantia Nigra)
• Direct Pathway:
– Disinhibition of the thalamus facilitates cortically mediated
behaviors
somatosensory
cortices
motor cortices
excitation
inhibition
D1
Putamen
D1 & D2
Dopamine
receptors
GPe
D2
GPi
Thalamus
GPe/i: Globus pallidus
internal/external
STN: Subthalamus Nucleus
STN
SNc: Pars Compacta (part
of substantia nigra))
SNc
•Indirect pathway:
–Inhibition of the thalamus inhibits cortically mediated behaviors
somatosensory
cortices
motor cortices
excitation
D1
Putamen
inhibition
D1 & D2
GPe
Dopamine
receptors
D2
GPi
Thalamus
STN
SNc
GPe/i: Globus pallidus
internal/external
STN: Subthalamus Nucleus
SNc: Pars Compacta (part
of substantia nigra)
Medical Remarks
• Hypokinetic disorders result from overactivity in the indirect pathway.
example: Decreased level of dopamine supply in nigrostriatal
pathway results in akinesia, bradykinesia, and rigidity in Parkinson’s
disease (PD).
somatosensory
cortices
motor cortices
excitation
inhibition
Putamen
D1
D1 & D2
Dopamine
receptors
GPe
D2
GPi
GPe/i: Globus pallidus
internal/external
Thalamus
STN
SNc
STN: Subthalamus
Nucleus
SNc: Pars Compacta
(part of substantia
nigra)
Parkinson’s
Disease
PD
Disease of mesostriatal
dopaminergic system
Muhammad Ali in Alanta Olympic
normal
Parkinson’s Disease
Substantia Nigra,
Pars Compacta (SNc)
DOPAminergic Neuron
Clinical Feature (1)
Slowness of Movement
- Difficulty in Initiation and Cessation
of Movement
Parkinson’s Disease
Clinical Feature (2)
Resting Tremor
Parkinsonian Posture
Rigidity-Cogwheel Rigidity
•Hyperkinetic disorders result from underactivity in the indirect pathway.
example: Lesions of STN result in Ballism. Damage to the pathway
from Putamen to GPe results in Chorea, both of them are involuntary
limb movements.
somatosensory
cortices
motor cortices
excitation
D1
Putamen
inhibition
D1 & D2
GPe
Dopamine
receptors
D2
GPi
Thalamus
STN
GPe/i: Globus pallidus
internal/external
STN: Subthalamus
Nucleus
SNc: Pars Compacta
SNc
(part of substantia nigra)
SYDENHAM’S CHOREA
Clinical Feature
- Fine, disorganized , and
random movements of
extremities, face and
tongue
- Accompanied by
Muscular Hypotonia
- Typical exaggeration of
associated movements
during voluntary activity
- Usually recovers
spontaneously
in 1 to 4 months
Principal Pathologic Lesion: Corpus Striatum
HUNTINGTON’S CHOREA
Clinical Feature
- Predominantly autosomal dominantly
inherited chronic fatal disease
(Gene: chromosome 4)
- Insidious onset: Usually 40-50
- Choreic movements in onset
- Frequently associated with
emotional disturbances
- Ultimately, grotesque gait and sever
dysarthria, progressive dementia
ensues.
Principal Pathologic Lesion:
Corpus Striatum (esp. caudate nucleus)
and Cerebral Cortex
HEMIBALLISM
Clinical Feature
Lesion: Subthalamic
- Usually results from CVA
(Cerebrovascular Accident)
involving subthalamic nucleus
- sudden onset
- Violent, writhing, involuntary
movements of wide excursion
confined to one half of the body
- The movements are continuous
and often exhausting but cease
during sleep
- Sometimes fatal due to exhaustion
- Could be controlled by
phenothiazines and stereotaxic
Nucleus surgery
VI Control of muscle function by the
motor cortex
•Two principal
components
–Primary Motor
Cortex
–Premotor Areas
The primary motor cortex
The topographical representations of the different
muscle areas of the body in the primary motor cortex
Characteristics of the PMC:
1, It has predominant influence on
the opposite side of the body (except
some portions of the face)
2. It is organized in a homunculus pattern with inversed order
3. The degree of representation is proportional to the
discreteness (number of motor unit) of movement required of
the respective part of the body. (Face and fingers have large
representative)
4. Stimulation of a certain part of PMC can cause very specific
muscle contractions but not coordinate movement.
•Projects directly
–to the spinal cord to regulate movement
–Via the Corticospinal Tract
–The pyramidal system
•Projects indirectly
–Via the Brain stem to regulate movement
–extrapyramidal system
Descending Spinal Pathways
pyramidal system

Direct
 Control muscle tone
and conscious skilled
movements
 Direct synapse of
upper motor neurons
of cerebral cortex with
lower motor neurons
in brainstem or spinal
cord
Descending Spinal Pathways
extrapyramidal system




Indirect
coordination of head &
eye movements,
coordinated function of
trunk & extremity
musculature to
maintaining posture and
balance
Synapse in some
intermediate nucleus
rather than directly with
lower motor neurons
• Premotor area composed of supplementary motor
area and lateral Premotor area
Premotor Areas
•Receive information from parietal and prefrontal
areas
•Project to primary motor cortex and spinal cord
•For planning and coordination of complex planned
movements