The motor system consists of upper motor neurons originating in the motor cortex and lower motor neurons originating in the spinal cord. The motor cortex is located in the frontal lobe and contains a somatotopic map called the motor homunculus. Primary motor areas like the primary motor cortex directly control muscle contraction, while secondary areas like the premotor cortex plan movements. Upper motor neurons descend through tracts like the corticospinal tract to synapse on lower motor neurons, which innervate muscles. Damage to different parts of the motor system can cause muscle weakness or changes in tone like spasticity or flaccidity.

1. Motor System
• Starts at the motor cortex
• Motor cortex is located at the frontal lobe
– precentral cortex

5. Motor homunculus
First discovered
by
Penfield

6. Brodmann areas Primary motor cortex
Area 4

7. 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

8. 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

9. 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

10. 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)

11. Medulla
internal capsule
Upper
motor
neuron
Lower
motor
neuron
anterior horn cell

12. 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)

14. 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

15. Complex nature of Cortical
Control of Movement
8.15

16. idea
•premotor area
•supplementary
motor area
(SMA)
•Prefrontal
cortex (PFC)
Primary
motor cortex
movement
basal ganglia
cerebellum cerebellum
plan execute
memory, emotions

17. Motor system
• Consists of
– Upper motor neuron
– Lower motor neuron

18. Lower motor neuron
• consists of mainly
• alpha motor neuron
– and also gamma motor neuron
alpha motor neuron
gamma motor neuron

19. alpha motor neuron
gamma motor neuron
corticospinal tract
Arrangement at the
anterior horn cell

20. 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

21. 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”

22. 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

23. 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

24. Extrapyramidal tracts
• starts at the brain stem
• descends down either ipsilaterally or
contralaterally
• ends at the anterior horn cell
• modifies the motor functions

25. Extrapyramidal tracts
• there are 4 tracts
– reticulospinal tracts
– vestibulospinal tracts
– rubrospinal tracts
– tectospinal tracts

26. 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

27. midbrain
pons
medulla
spinal cord
reticulospinal tract

28. • 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

29. 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

30. 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

31. 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

32. midbrain
pons
medulla
spinal cord
vestibulospinal tract
mainly extensors

33. • 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

34. 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

35. midbrain
pons
medulla
spinal cord
rubrospinal tract
mainly flexors

36. 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

37. midbrain
pons
medulla
spinal cord
tectospinal tract
cervical segments

38. 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

39. Upper
motor
neuron
Lower
motor
neuron
extrapyramidal tracts
pyramidal tracts
alpha motor neurone
gamma motor neurone

40. 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

41. 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

42. 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

43. • 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

44. 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)

45. Main abnormalities
• Muscle Weakness / paralysis
– Reduced muscle power
• Flaccidity
– Reduced muscle tone
• Spasticity
– Increased muscle tone

46. • Lower motor neuron lesion causes
– flaccid paralysis (flaccid weakness)
• Upper motor neuron lesion causes
– spastic paralysis (spastic weakness)

47. 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