1. Section 6 Control of posture and movements

2. 1. Cellular mechanism of motor control
Section Outline
The motor neuron and motor unit
Stretch reflex

3. 1.1 Motor neuron and motor unit
Motoneuron:
α motoneuron: Innervates extrafusal muscle fiber. Final common path
γ motoneuron: Innervates intrafusal muscle fiber.
Motor unit: a single α motoneuron and the muscle fibers it innervates.

4. Motor Pool
The motor pool is the total population of neurons innervating a particular muscle.

5. Size Principle
When a contraction occurs, small motor units fire first, as the strength of contraction increases, larger units are recruited, the orderly recruitment of motoneurons is referred to as size principle

6. 1.2 Stretch reflex
Stretch reflex: Stretching or elongating a muscle causes it to contract within a short time period.
Tendon reflex
Muscle tonus

7. Tendon reflex A phasic part, exemplified by tendon jerks
Sensor: muscle spindle
Monosynaptic reflex

8. Muscle tone A tonic part Sensor: muscle spindle Multisynaptic reflex
Important for maintaining posture

9. Nuclear bag fiber and nuclear chain fiber
Nuclear bag fiber: dynamic response
Nuclear chain fiber: static response

10. Dynamic response and static response

11. Role of -motoneurons Stimulation of -efferent causes
the contractile ends
of intrafusal fiber
to shorten, stretches
NBF/NCF, initiating impulses in Ia afferents,
this in turn leads to excitation of -neurons.
Increased -efferent discharge increases spindle sensitivity.

12. Inverse stretch reflex
Inverse stretch reflex: The active contraction of a muscle also causes reflex inhibition of the contraction. This response is called the inverse myotatic reflex.

13. Inverse stretch reflex
Sensor:
Golgi tendon organ
Afferent fiber: Ib
Inhibitory interneuron

14. Review Cellular mechanism of motor control Motor neurons: α γ
Motor unit
Motor pool
Size principle
Stretch reflex: tendon reflex, muscle tonus
Muscle spindle: nuclear bag, nuclear chain
Inverse stretch reflex: tendon organ

15. 2. Control of movement at the spinal cord level
Spinal shock: In all vertebrates, transection of the spinal cord is followed by a period of spinal shock during which all spinal reflex responses are profoundly depressed.

16. Mechanism of spinal shock
The initial shock is considered to be due to the sudden withdrawal of tonic facilitatory influence from the brain

17. Posture regulating mechanisms at the spinal cord level
Flexion-withdrawal reflex
Crossed-extension reflex

18. Flexion-withdrawal reflex
Flexion-withdrawal reflex: The limb is quickly withdrawn from a painful stimulus, usually by simultaneous contraction of all the flexor muscles in the limb

19. Flexion-withdrawal reflex
Polysynaptic reflex
Consists of a contraction of flexors and a relaxation of extensors in the stimulated limb
Protective reflex

20. Crossed-extension reflex
Crossed-extension reflex: An opposite effect in the contralateral limb, that is, excitation of extensor motor neurons and inhibition of flexor motor neurons

21. 3. Posture control by the brain stem
Section Outline
Mechanism of decerebrate rigidity
Facilitatory and inhibitory area of muscle tonus
α-rigidity and γ-rigidity

22. 3.2 Posture reflex pathways
Lateral motor pathways:
voluntary movement
Mediate motor pathways:
posture reflex
Vestibulospinal tracts
Reticulospinal tracts
Medial
Lateral

23. 4. Cortical control of voluntary movement

24. 4. Cortical control of voluntary movement
Section Outline
Cortical motor area
Primary motor cortex
Secondary motor cortex
Lateral motor pathways
Corticospinal tract
Corticobulbar tract
Rubrospinal tract

25. 4.1 Cortical motor area Brodmann 4 area: Primary motor cortex, MI
Execution of movement
Brodmann 6 area: Secondary motor cortex, MII
Planning movement
Supplementary motor area, SMA
Complex motor sequence
Premotor area, PM
Targeted movement

26. 4.1 Cortical motor area Brodmann 4 area: Primary motor cortex, MI
Execution of movement
Brodmann 6 area: Secondary motor cortex, MII
Planning movement
Supplementary motor area, SMA
Complex motor sequence
Premotor area, PM
Targeted movement

27. 4.2 Lateral motor pathways
Corticospinal tract
Lateral corticospinal tract
Anterior corticospinal tract
Corticobulbar tract
Rubrospinal tract

28. 5.Function of the basal ganglia
Outline
Functional circuitry of the basal ganglia
Direct pathways
Indirect pathways
Dopamine modulation
Diseases of the basal ganglia
Parkinson’s disease
Huntington’s disease

29. 5.1 Functional circuitry of the basal ganglia
Neostriatum
Globus pallidus
Subtantia nigra
Subthalamic nucleus
Caudate nucleus
Putaman
External globus pallidus
Internal globus pallidus
Substantia nigra pars reticulata
Substantia nigra pars compacta

30. Basal ganglia

31. Basal ganglia circuit Direct pathway Increase the
Indirect pathway D2 D1
Direct pathway
Increase the
excitability of cortex
Indirect pathway
Decrease the

32. Basal ganglia disorders
Parkinson’s disease
Huntington’s disease

33. Clinical Characteristics
James Parkinson, 1917 “ ...involuntary tremulous motion, with lessened muscular power, in parts not in action and even when supported; with a propensity to bend the trunk forwards, and to pass from a walking to a funning pace, the senses and the intellects uninjured.” • rhythmic tremor at rest • rigidity with “cog-wheel characteristic” • akinesia

34. Neural mechanism of parkinsonism
Normal
Parkinson’s disease (PD)

35. Pathology of Parkinson’s disease
Loss of dopaminergic neurons in the substantia nigra lead to decreased activity in the direct pathway and increased activity in the indirect pathway.
As a result, thalamic input to the motor area of the cortex is reduced and the patient exhibits rigidity and bradykinesia.

36. L-dopa
L-dopa taken up by dopaminergic neurons in the substantia nigra and converted to dopamine by L-Amino Acid Decarboxylase (LAAD)
As the disease progresses, more dopaminergic neurons are lost and the effectiveness of L-dopa as a replacement therapy declines.

37. Huntington’s disease Huntington’s chorea Onset: 30-45 years
Excessive movements
Rapid, jerky, involuntary
Mild twitching  larger, more generalized
Progressive dementia
Other characteristics
Grimace
Dancing gait

38. Huntington’s disease
Normal
Huntington’s disease (HD)

39. Cause & Treatment of HD Genetic: autosomal dominant
Abnormal protein builds up & kills brain cells especially in basal ganglia and cortex
Surgery

40. Surgery for Huntington’s disease
Huntington’s disease (HD)

41. 6. Cerebellar function Functional divisions Vestibulocerebellum
Spinocerebellum
Cerebrocerebellum
Cytoarchitecture of the cerebellar cortex
Outline

42. Functional divisions

43. Functional divisions Vestibulocerebellum:
The oldest part of the cerebellum
Receives input from the vestibular system
Output goes to the vestibular nuclei
Functions to control equilibrium and eye movements

44. Functional divisions Spinocerebellum:
Consists of vermis and intermediate zones
Receives proprioceptive input from the body as well as a copy of the “motor plan” from the motor cortex
By comparing plan with performance, it smoothes and coordinates movements that are ongoing

45. Functional divisions Cerebrocerebellum
Occupies the lateral aspects of the cerebellar hemispheres
Input comes from the cerebral cortex
Output is directed to the dentate nuclei
Interact with the motor cortex in planning and programming movements

46. The cerebellar cortex is organized into three layers and contains five types of neurons
Purkinje cell
Granule cell
Basket cell
Stellate cell
Golgi cell

47. The cerebellar cortex is organized into three layers and contains five types of neurons
Purkinje cell
The biggest neurons
GABAergic
Extensive dendritic arbors that extend throughout the molecular layer
Receive input from the parallel fibers
The only output from the cerebellum

48. The cerebellar cortex is organized into three layers and contains five types of neurons
Granule cells
Receive input form the mossy fibers
Innervate Purkinje cells
Send an axon to the molecular layer, they are called parallel fibers

49. The cerebellar cortex is organized into three layers and contains five types of neurons
Basket cells
Located in the molecular layer
Receive input from the parallel fibers
The axons from a basket around the cell body and axon hillock of Purkinje cell they innervate

50. The cerebellar cortex is organized into three layers and contains five types of neurons
Stellate cells
Located in the molecular layer but more superficial
Golgi cells
Located in granular layer
Axons project to the dendrites of the granule cell

51. Main inputs to the cerebellar cortex
Climbing fibers
Mossy fibers
Both are excitatory

52. Climbing fibers Come form a single source, the inferior olivary nuclei
Project to the primary dendrites of Purkinje cell, around which it entwines like a climbing plant

53. Mossy fibers
Provide direct proprioceptive input from all parts of the body plus input from the cerebral cortex
They end on the dendrites of granule cells in complex synaptic groupings called glomeruli

54. Summary Cellular mechanism of motor control Motor neurons: α γ
Motor unit
Motor pool
Size principle
Stretch reflex: tendon reflex, muscle tonus
Muscle spindle: nuclear bag, nuclear chain
Inverse stretch reflex: tendon organ

55. Summary Control of movement at the spinal cord Spinal shock
Flexor withdrawal reflex
Crossed extension reflex
Posture control by the brain stem
Decerebrate rigidity
Cortical control voluntary movement

56. Summary Control of movement in the basal ganglia Direct pathway
Indirect pathway
Parkinson’s disease
Huntington’s disease
Control movement in the cerebellum
Functional division
Five types of neurons
Two main inputs to the cerebellum

58. 1.1 Motor neuron and motor unit
Motoneuron
α motoneuron: Innervates extrafusal muscle fiber. Final common path.
γ motoneuron: Innervates intrafusal muscle fiber.
β motoneuron: Innervates both
Motor unit: a single motoneuron and the muscle fibers it innervates.

59. Motor Pool & Recruitment
The motor pool is the total population of neurons innervating a particular muscle,e.g.,25,000 muscle fibers in cat soleus are supplied by 100 -motoneurons, single contraction of whole muscle can be graded in 100 steps by recruitment of motor units.

60. 3.1 Mechanism of decerebrate rigidity
Decerebrate rigidity: In experimental animals in which the hindbrain and spinal cord are isolated from the rest of brain by transection of the brain stem at the superior border of the pons, the most prominent finding is marked spasticity of the body musculature. The resulting pattern of spasticity is called

61. Mechanism of decerebrate rigidity
Decerebrate rigidity: is found to be spasticity due to diffuse facilitation of stretch reflexes.
The facilitation is due to two factors
Increased general excitability of the motor neuron pool
Increase in the rate of discharge in the γ-efferent neurons.

62. Decorticate rigidity
The more common pattern of extensor rigidity in the legs and moderate flexion in the arms is actually decorticate rigidity due to lesions of the cerebral cortex, with most of the brain stem intact.

63. 3.3 Posture control by the brain stem
Attitudinal reflex
Tonic labyrinthine reflex
Tonic neck reflex
Righting reflex

64. Motor Unit
The number of muscle fibers in a motor unit ranges from a few, muscle of eye for fine control, to thousands of muscle fibers, large proximal muscle of limbs.
Smoothness and precision of movement are brought about by varying the number and timing of motor unit brought into play.

65. Mechanism of spinal shock
The acute loss and eventual overactivity of all of these reflexes results from the lack of influence of the neural tracts that descend from higher motor control centers to the motor neurons and associated interneuron pools.

66. Main inputs to the cerebellar cortex
Climbing fibers
Mossy fibers
Both are excitatory

67. Climbing fibers Come form a single source, the inferior olivary nuclei
Project to the primary dendrites of Purkinje cell, around which it entwines like a climbing plant

68. Mossy fibers
Provide direct proprioceptive input from all parts of the body plus input from the cerebral cortex
They end on the dendrites of granule cells in complex synaptic groupings called glomeruli

69. Mechanisms contributing to the recovery
Denervation supersensitivity
Increased numbers of postsynaptic receptors
Sprouting of afferent terminals

70. Functional divisions Spinocerebellum:
Consists of vermis and intermediate zones
Carrying somatosensory information terminate in the vermis and intermediated zones
Output from the vermis is directed to the fastigial nuclei—controls antigravity muscles in posture and locomotion
Output goes to the interposed nuclei and from there to the red nucleus—act on stretch reflexes and other somatosensory reflexes