Motor Cortex
The motor cortex is the pre-central gyrus (Boardman’s area 4) and is organized somatotopically (i.e. hand is next to wrist is next to arm…etc.)
Areas requiring fine motor control (hand) are overrepresented in the cortex. Representation of body within cortex is called motor homunculus.
M1 is part of precentral gyrus which needs the least amount of current to elicit movements.
Two types of cells exist in the motor cortex: Pyramidal cells have a cell body in the shape of a pyramid with the apex pointing to the surface of the brain; contains one apical dendrite that runs to outermost layer of the cortex. They are projection neurons which carry output form a cortical area.
Non-pyramidal cells are involved in receiving input to the cortex and in local processing.
Layer 4 of the precentral gyrus contains non-pyramidal cells and receives most of the sensory input from the thalamus. Layer 5 contains the most pyramidal cells and gives rise to the axons which descend to the basal ganglia. Large pyramidal cellsà Betz cells
All corticospinal cell bodies lie in layer 5 of motor cortex. Most axons in corticospinal tract project directly into the spinal cord.
The motor cortex is subdivided into the primary motor cortex, premotor cortex, and supplementary motor cortex all receiving inputs from subcortical structures.
Inputs to the vetrolateral nucleus come from both the cerebellum and basal ganglia but inputs are segregated. Cerebellum influences premotor; basal ganglia the primary and supplementary.
Individual muscles are represented in multiple locations in the primary motor cortex. Outputs from disparate parts of the cortex therefore converge on a common set of motor neurons.
The primary motor cortex is involved in the initiation and triggering of a movement. Modulation of the activity occurs before the contraction of the muscle.
Primary cortex motor neurons encode the DIRECTION of movement as well. Movement in a particular direction is determined by a population of several neurons. Encodes both direction and force.
Population coding implies that various neurons are attuned to cause movement in a specific direction. The sum of the vectors of these related neurons cause the movement in the desired direction.
Premotor cortex concerned with excitation and strategy. Located anterior to the primary motor cortex. Receives heavy sensory input from the basal ganglia-à planning of complex motor strategy. Preparation of movement as well.
Supplementary motor cortex is involved in Programming complex sequences of movement. Coordinates bilateral movements. Mental rehearsal.
Tags: basal ganlia, cerebellum, cortex, corticospinal, motor homunculus, muscles, neurons, Non-pyramidal cells, Pyramidal cells, recentral gyrus
