THE INITIATION OF MOVEMENT BY PRIMARY MOTOR CORTEX
SMA is heavily interconnected with M1, cortical area 4. Stimulation intensities that are unable to
evoke movement in other cortical areas are still effective in evoking movement when applied to
Thus area 4 has dense, strong synaptic connections with the motor neurons and the
spinal interneurons that drive them
Focal electrical stimulation of area 4 evokes the contraction of small groups of muscles and the
somatic musculature is mapped systematically. The ribbon of cortex that stretches the full length
of the precentral gyrus called the motor strip.
The Input-Output Organization of M1
The pathway by which motor cortex activates lower motor neurons originates in cortical layer V
which has a population of pyramidal neurons. The layer V pyramidal cells in M1 receive their
inputs primarily from two sources:
Other cortical areas which originate in areas adjacent to area 4
o Area 6 and areas and 3, 1, and 2 (see Fig. 14.7)
o Arises mainly from another part of the ventral lateral nucleus, VLc, which relays
information from the cerebellum
Besides projecting directly to the spinal cord, layer V pyramidal cells also send axon collaterals
to many subcortical sites involved in sensorimotor processing (i.e. brain stem)
The Coding of Movement in M1
It was previously though that the motor cortex consisted of a detailed mapping of the individual
muscles, such that the activity of a single pyramidal cell would lead to activity in a single motor
More recently, it is believed that individual pyramidal cells can drive numerous motor neuron
pools from a group of different muscles involved in moving a limb toward a desired goal.
Recordings from M1 neuron in behaving animals have revealed that a burst of activity occurs
immediately before and during a voluntary movement and this appears to encode two aspects
of movement: force and direction
Apostolos Georgopoulos used monkeys and trained them to move a joystick toward a small light
whose position varied randomly around a circle. The M1 cells fired most vigorously during
movement in one direction but also discharged during movements that varied ±45° from the
Thus, Georgopoulos hypothesized that movement direction was encoded instead by the
collective activity of a population of neurons.
To test this, Georgopolous recorded from more than 200 different neurons in M1 and for each
cell, they constructed a directional tuning curve (see Fig 14.13b).
The activity of each cell was represented as a direction vector pointing in the direction
that was best for that cell. The length of a vector represented how active that cell had been during a particular
movement (see Fig. 14.4).
The vectors were plotted together for each direction of movement, and then averaged to
yield a population vector.
Result: There was a strong correlation between this average vector, representing the activity of
the entire population of M1 cells, and the actual direction of movement (Fig. 14.15).
1. Much of the motor cortex is active for every movement
2. The activity of each cell represents a single “vote” for a pa