1. Part 4
Motor Functions of the Nervous System
I Motor Unit and Final Common Pathway

3. 1. Motor Unit

4. Every striated muscle has encapsulated muscle fibers scattered throughout the muscle called muscle spindles.
Extrafusal and intrafusal fibers

5. The extrafusal muscle fibers are innervated by Alpha motor neuron
The intrafusal muscle fibers are innervated by Gamma motor neurons

6. 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

7. Motor units and innervation ratio
Fibers per motor neuron
Extraocular muscle 3:1
Gastrocnemius 2000:1
Purves Fig. 16.4

8. 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

9. organization of motor subsystems

11. Overview - organization of motor systems
Motor Cortex
Brain Stem
Spinal Cord
-motor neuron
Final common pathway
Skeletal muscle

12. Final common path - -motor neuron
(-)
muscle
fibers
(+)
(-)
axon
hillock
motor nerve
fiber
NM junction
Schwann
cells
Receptors?
acetylcholine
esterase
Transmitter?

13. 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.

14. II Motor Functions of the Spinal Cord – Spinal Reflex

15. 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

16. 1. Stretch Reflex

17. (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 3-10 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)

18. 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)

19. 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

20. Components of muscle spindle
Dynamic intrafusal fiber
Static intrafusal fibers
Static intrafusal fibers
Primary
ending } Ia
Afferent
axons II
Secondary
ending }

21. Anatomy of Muscle Spindle
Primary sensory endings
Type Ia fibers
Stimulated by both the rate and amount of stretch

22. Anatomy of Muscle Spindle
Secondary sensory endings
Type II fibers
stimulated only by degree of stretch

23. 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

24. Muscle stretch reflex

25. (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  -motor N. F. the same M. from whence the M. spindle fiber originated.

26. Circuit of the Strength Reflex
Muscle spindle
Dorsal root
Muscle fiber
Tendon
Ventral root
-mn

27. 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)

28. 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

30. The Stretch Reflex
At spinal cord sensory neurons synapse directly (mono- synaptically) with the  motor neurons which rapidly excite the extrafusal muscle fibers of stretched muscle

31. The Stretch Reflex
The reflexive muscle contraction that follows (an example of serial processing) resists further stretching of the muscle

32. The Stretch Reflex
Branches of the afferent fibers also synapse with inter- neurons that inhibit motor neurons controlling the antagonistic muscles

33. 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

34. 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;

35. 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.

36. 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

37. (3) Gamma impact on afferent response

39. Muscle spindle: motor innervation
Gamma motoneurons:
Innervate the poles of the fibers.

40. Activation of the g-loop
WHAT IS THE g-LOOP?
Descending influence (UMN)
Muscle spindle 1a a g
Activation of the g-loop
results in increased
muscle tone
MUSCLE

43. 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.

45. 2. The Deep Tendon Reflex
(1) Structure and Innervation of Golgi Organ

46. Golgi tendon organ: structure
Located in the muscle tendon junction.
Connective tissue encapsulating collagen fibers and nerve endings.
Attached to muscle fibers and several MUs.
Ib afferent fiber.
sensitive to tension

48. (2) Golgi tendon organ: response properties
Less frequent than muscle spindle.

49. Golgi tendon organ: response properties (cont)
Sensitive to the change of tension caused by the passive stretch or active contraction

50. (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

52. The Deep Tendon Reflex
Upon reaching the spinal cord, informa- tion 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)

53. 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

54. Compare spindle and golgi

55. Compare spindle and golgi

56. 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

57. 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

58. 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

59. 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)

60. (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）,

61. (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.

62. III. Role of the brain stem:
Support of the Body Against Gravity – Roles of the Reticular and Vestibular nuclei

64. 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.

65. 4. Reticular inhibitory area; 5. Reticular facilitated area;
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)
motor cortex;
2. Basal ganglia;
3. Cerebellum;
4. Reticular inhibitory area;
5. Reticular facilitated area;
6. Vestibular nuclei.

67. (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.
motor cortex;
2. Basal ganglia;
3. Cerebellum;
4. Reticular inhibitory area;
5. Reticular facilitated area;
6. Vestibular nuclei.

68. 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

69. 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.

70. 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.

71. 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

72. (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.

73. 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.

74. 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

75. 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).

76. Activation of the g-loop
The extensor rigidity is g-loop dependent
section the dorsal roots interrupts the g-loop, and the rigidity is relieved. This is g-rigidity.
THE g-LOOP?
Descending influence (UMN)
Muscle spindle 1a a g
Activation of the g-loop
results in increased
muscle tone
MUSCLE

77. IV. The cerebellum and its motor functions

78. Cerebellar Input/Output Circuit

79. Based on cerebral intent and external conditions
The cerebellum tracks and modifies millisecond-to-millisecond muscle contractions,
to produce smooth, reproducible movements

80. Without normal cerebellar function, movements appear jerky and uncontrolled

81. Functional Divisions-cerebellum
Vestibulocerebellum (flocculonodular lobe)

82. The vestibulocerebellum
input-vestibular nuclei
output-vestibular nuclei

83. 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.

85. Spinocerebellum (vermis & intermediate)

86. Spinocerebellum (vermis & intermediate)
input-periphery & spinal cord:
output-cortex

87. 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.

88. Cerebrocerebellum (lateral zone)
input-pontine N.
output-pre & motor cortex

89. Cerebrocerebellum (lateral zone)
 Receives all its input from the motor cortex, adjacent pre-motor 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)

90. Vestibulocerebellum (flocculonodular lobe)
Balance and body equilibrium
Spinocerebellum (vermis & intermediate)
Rectify voluntary movement
Cerebrocerebellum (lateral zone)
Plan voluntary movement

91. V The motor functions of basal ganglia

93. Components of Basal Ganglia
Caudate
Putamen
GPe
1. Corpus Striatum
Striatum Caudate Nucleus & Putamen
Pallidum Globus Pallidus (GP)
GPi

94. Components of Basal Ganglia
2. Substantia Nigra
Pars Compacta (SNc)
Pars Reticulata (SNr)
STN
3. Subthalamic Nucleus (STN)
SN (r & c)

95. 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

96. somatosensory cortices
cortex to basal ganglia to thalamus to cortex
somatosensory cortices
Thalamus
Putamen
GPe
GPi
STN
SNc
motor cortices
excitation
inhibition D1 D2
direct
indirect
D1 & D2 Dopamine receptors
GPe/i: Globus pallidus internal/external
STN: Subthalamus Nucleus
SNc: Pars Compacta (part of substantia Nigra)

97. somatosensory cortices
Direct Pathway:
Disinhibition of the thalamus facilitates cortically mediated behaviors
somatosensory cortices
Thalamus
Putamen
GPe
GPi
STN
SNc
motor cortices
excitation
inhibition
D1 & D2 Dopamine receptors D1 D2
direct
indirect
GPe/i: Globus pallidus internal/external
STN: Subthalamus Nucleus
SNc: Pars Compacta (part of substantia nigra))

98. somatosensory cortices
Indirect pathway:
Inhibition of the thalamus inhibits cortically mediated behaviors
somatosensory cortices
Thalamus
Putamen
GPe
GPi
STN
SNc
motor cortices
excitation
inhibition D1 D2
direct
indirect
D1 & D2 Dopamine receptors
GPe/i: Globus pallidus internal/external
STN: Subthalamus Nucleus
SNc: Pars Compacta (part of substantia nigra)

99. Medical Remarks

100. somatosensory cortices
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
Thalamus
Putamen
GPe
GPi
STN
SNc
motor cortices
excitation
inhibition D1 D2
D1 & D2 Dopamine receptors
direct
indirect
GPe/i: Globus pallidus internal/external
STN: Subthalamus Nucleus
SNc: Pars Compacta (part of substantia nigra)

101. Disease of mesostriatal
Parkinson’s Disease PD
Disease of mesostriatal
dopaminergic system
normal
Muhammad Ali in Alanta Olympic

102. Parkinson’s Disease Substantia Nigra, Clinical Feature (1)
Pars Compacta (SNc)
DOPAminergic Neuron
Clinical Feature (1)
Slowness of Movement
- Difficulty in Initiation and Cessation
of Movement

103. Parkinson’s Disease Clinical Feature (2) Resting Tremor
Parkinsonian Posture
Rigidity-Cogwheel Rigidity

104. somatosensory cortices
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
Thalamus
Putamen
GPe
GPi
STN
SNc
motor cortices
excitation
inhibition D1 D2
direct
indirect
D1 & D2 Dopamine receptors
GPe/i: Globus pallidus internal/external
STN: Subthalamus Nucleus
SNc: Pars Compacta (part of substantia nigra)

105. 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

106. Principal Pathologic Lesion:
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

107. HEMIBALLISM Clinical Feature Lesion: Subthalamic Nucleus
- 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
surgery
Lesion: Subthalamic Nucleus

108. VI Control of muscle function by the motor cortex
Two principal components
Primary Motor Cortex
Premotor Areas

110. The primary motor cortex
The topographical representations of the different muscle areas of the body in the primary motor cortex

111. 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.

112. Projects directly Projects indirectly
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

113. 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

114. 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

115. Premotor area composed of supplementary motor area and lateral Premotor area

116. 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