MOTOR CORTEX


PRIMARY MOTOR CORTEX

  • Largest of all motor areas
  • Lies in the precentral Gyrus –> Area 4 of Brodmann extending from medial surface to lateral sulcus.
  • low electrical –> Specific & repeatable movement.
  • HOMONCULUS (little man) –> Drawn by Penfield & Rasmussen 1950.
    • Body is upside down
    • legs most medially
    • pharynx lateral most.
    • SIZE OF REPRESENTATION – More for body parts for fine & skilled movement (Thumb, Fingers, Face, Pharynx, Vocal cords etc), Less for trunk muscles.
  • Lacks LAYER 4 (Granules). So, called AGRANULAR CORTEX.
  • Cells are arranged in Columns –> cells from several columns project to same muscles.
  • about 30% of Corticospinal Tract fibers take origin
  • provide most refined degree of motor control
  • Areas enlarge with practice & learning —- CORTICAL PLASTICITY
  • LESION — Paresis/paralysis of opposite side
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SUPPLEMENTARY MOTOR AREA

  • Medial portion of Area 6 (Anterior & Medial to M1, largely on medial surface of hemisphere)
  • High electrical stimulation–> Head & Eye movement, vocalization & complex postural movement.
  • Somatotopic arrangement like M1 —Face, Anterior. Legs, Posterior.
  • Bilaterally connected through Corpus callosum
  • Subdivided into 2 areas
  • FUNCTION :
    • Planning & integration of complex (fixations)attitudinal movements.
    • more active in bi-manual tasks
    • learning of skilled acts.
  • LESION –> Transient speech deficit (Aphasia) — typically disappears after several weeks. Slowness in generating repetitive movement. Retards the movement of opposite limb.

PRE-MOTOR AREA (PMC)

  • High stimulation – Proximal musculature
  • minor contribution to Cortico-spinal Tract.
  • Major contribution to Extrapyramidal Tracts.
  • FUNCTIONS:
    • Responsible for patterning & posturing
  • LESION:
    • Weakness of opposite shoulder/ hip muscle
    • limb movement slower
    • Inability in simultaneous coordinated movement of both limbs
  • 2 Subdivisions : DORSAL & VENTRAL

CINGULATE MOTOR AREA (CMA) within Cingulate Sulcus

CMA- r –> Rostral

CMA – d –> Dorsal

CMA – v –> Ventral

3 CMA, each with somatotopic map, contribute to CST. High stimulation similar to motor cortex stimulation.

  • FUNCTION :Preparation & execution of movement.

OTHER IMPORTANT AREAS

  • FRONTAL EYE FIELD — Controls eye movement
  • POSTERIOR PARIETAL CORTEX — (5,7)
    • Lesion : No motor weakness, but sensory & motor neglect in opposite hemifield —> AMORPHOSYNTHESIS
  • BROCA’S AREA — Motor speech area, Area 44.
    • Lesion : Motor aphasia
  • SUPPRESSOR AREAS (4s, 2s, 8s, 19s, 24s) — Inhibition of strech reflex


HISTOLOGICAL STRUCTURE OF MOTOR CORTEX

3 Types of cells are present – 1. PYRAMIDAL CELLS 2.STELLATE/GRANULE CELL 3. FUSIFORM CELLS

6 layers or laminae are present–>

  • most afferent fibers from the specific nuclei of thalamus make synapses in the laminae I to VI
  • Afferent projections from non-specific thalamic nuclei & from ascendind reticular system terminate in al laminae of cortex
  • Laminae II & IV are concerned with sensorial modalities.
  • Laminae III to V are meant for somatomotor or visceromotor activities.
  • Laminae I to VI are engaged for integration of association of sensorimotor behaviour.

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