Motor system2 pathways
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The motor system begins with the motor cortex located in the frontal lobe. Within the motor cortex, different areas control different parts of the body in a somatotopic map called the motor homunculus. The primary motor cortex contains Betz cells whose axons form the corticospinal tract which descends through the internal capsule and brainstem to synapse on alpha motor neurons in the spinal cord. Secondary motor areas like the premotor and supplementary motor areas help plan and coordinate movements that are executed through the primary motor cortex and corticospinal tract. Both upper motor neurons from the cortex and lower motor neurons initiate muscle contraction through motor units consisting of an alpha motor neuron and innervated muscle fibers.
Motor system2 pathways
- 1. Motor System Starts at the motor cortex Motor cortex is located at the frontal lobe precentral cortex
- 6. Motor homunculus First discovered by Penfield
- 8. Motor cortex different areas of the body are represented in different cortical areas in the motor cortex Motor homunculus somatotopic representation not proportionate distorted map upside down map
- 9. Motor cortical areas primary motor cortex (MI) precentral gyrus secondary motor cortex (MII) premotor cortex supplementary motor area
- 10. primary motor cortex Functions Corticospinal tracts (pyramidal tracts) starts here Cell bodies are located in the cortical area (large cell bodies are known as Betz cells) Corticospinal tract descends down
- 11. Course of the corticospinal tract Descends through internal capsule at the medulla cross over to the other side descends down as the corticospinal tract ends in each anterior horn cell synapse at the anterior horn cell
- 12. Medulla motor cortex internal capsule Upper motor neuron Lower motor neuron anterior horn cell
- 13. Functional role of primary and secondary motor areas SMA (Supplementary Motor Area) assembles global instructions for movements It issues these instructions to the PreMotor Area. PreMotor Cortex (PMC) 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
- 14. Motor system Consists of Upper motor neuron Lower motor neuron
- 15. Lower motor neuron consists of mainly alpha motor neuron and also gamma motor neuron alpha motor neuron gamma motor neuron
- 16. alpha motor neuron gamma motor neuron corticospinal tract Arrangement at the anterior horn cell
- 17. 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
- 18. 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 neurone muscle fibres supplied by the neuron
- 19. 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
- 20. Upper motor neuron Consists of Corticospinal tract (pyramidal tract) Extrapyramidal tracts
- 21. Extrapyramidal tracts starts at the brain stem descends down either ipsilaterally or contralaterally ends at the anterior horn cell modifies the motor functions
- 22. Extrapyramidal tracts there are 4 tracts reticulospinal tracts vestibulospinal tracts rubrospinal tracts tectospinal tracts
- 23. 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
- 24. midbrain pons medulla spinal cord reticulospinal tract
- 25. pontine reticular formation medial reticulospinal tracts controls proximal muscles (axial), excitatory to flexor medullary reticular formation lateral reticulospinal tracts (also medial) excitatory or inhibitory to axial muscles
- 26. 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
- 27. midbrain pons medulla spinal cord vestibulospinal tract mainly extensors
- 28. 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
- 29. 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
- 30. midbrain pons medulla spinal cord rubrospinal tract mainly flexors
- 31. tectospinal tract tectospinal tract originates from the tectum of the midbrain ends on interneurons mainly contralateral supplies cervical segments only
- 32. midbrain pons medulla spinal cord tectospinal tract cervical segments
- 33. 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
- 34. Renshaw cells Renshaw cells are inhibitory interneurons found in the spinal cord They receive excitatory collateral from the alpha motor neuron’s axon as they emerge from the motor root (they are "kept informed" of how vigorously that neuron is firing) They send their own inhibitory axon to synapse with the cell body of the initial alpha neuron In this way, Renshaw cell inhibition represents a negative feedback mechanism A Renshaw cell may be supplied by more than one alpha motor neuron collaterals and it may synapse on multiple motor neurons
- 35. Upper motor neuron Lower motor neuron extrapyramidal tracts pyramidal tracts alpha motor neurone gamma motor neurone
- 36. 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
- 37. 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
- 38. 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
- 39. 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
- 40. Clinical situations Muscle power Normal Reduced (muscle weakness) muscle paralysis muscle paresis Muscle tone Normal Reduced Hypotonia (Flaccidity) Increased Hypertonia (Spasticity)
- 41. Main abnormalities Muscle Weakness / paralysis Reduced muscle power Flaccidity Reduced muscle tone Spasticity Increased muscle tone
- 42. Lower motor neuron lesion causes flaccid paralysis Upper motor neuron lesion causes spastic paralysis
- 43. Lower motor neuron lesion muscle weakness flaccid paralysis muscle wasting (disuse atrophy) reduced muscle tone (hypotonia) reflexes: reduced or absent spontaneous muscle contractions (fasciculations) plantar reflex: flexor superficial abdominal reflexes: present
- 44. Upper motor neuron lesion muscle weakness spastic paralysis increased muscle tone (hypertonia) reflexes: exaggerated 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 bee seen Passive stretch causing initial incerased resistance which is released later
- 45. tendon jerks (reflexes) reflex level biceps jerk C 5 6 triceps jerk C 7 8 knee jerk L 3 4 ankle jerk S 1 2
- 46. 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 (becuase of immature corticospinal tract)
- 48. Site of lesions Cortex Internal capsule Brain stem Spinal cord Anterior horn cell Motor nerve Neuromuscular junction Muscle
- 49. Site of lesions monoplegia only 1 limb is affected either UL or LL, lower motor neuron lesion hemiplegia on 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