8.2 PRIMARY MOTOR CORTEX M1 /4 Brodman´s Area

PRIMARY MOTOR CORTEX M1 /4 Brodman´s Area → The primary motor cortex (M1) is organized in a

• somatotopic map

Upper motor neurons in M1 can control 

• multiple lower motor neurons in the spinal cord that innervate different muscles.

 activation of an upper motor neuron can

• excite or inhibit different neurons at once

 primary motor cortex is responsible for 

• movements patterns

Stimulation of upper motor neurons in monkeys can lead to

• complex motions like bringing the hand to the mouth or moving into a defensive position

Motor control modulation

• basal ganglia

• cerebellum

 basal ganglia & cerebellum form

• Specific neural loops with the motor areas of the cortex.

• They do not directly command movement, but help regulate its timing, precision, selection, & learning.

Basal ganglia

basal ganglia nucleis are composed of

• caudate & putamen (which both make up the striatum), as well as the globus pallidus.

Basal ganglia is closely associated with

• ventral thalamus

• dopaminergic neurons in the substantia nigra

Basal ganglia influence movement by

• regulating the activity of upper motor neuron circuits.

Basal ganglia is essential for

• The initiation & ending of movement sequences

• Adjustment of the force of movement (strength)

Basal ganglia inputs

Striatum

striatum

• input zone of the basal ganglia.

• Its neurons receive projections from several regions of the cerebral cortex & the substantia nigra.

The primary output of the GABAergic neurons in the striatum is to the

•  globus pallidus

Basal ganglia projections 

• outputs

main source of output from the basal ganglia to other parts of the brain is

• globus pallidus:

globus pallidus:

• Many of its projections reach the motor cortex via a relay in the thalamus

• Also projects to the subthalamic nucleus

• This way, the loop circuitry of the basal ganglia is complete.

 efferent cells of the globus pallidus are

 GABAergic & tonically active

main output of the basal ganglia is an

• inhibition to the motor cortex, preventing unwanted movement

The basal ganglia behave as a

• “movement filter”, preventing the initiation of unwanted motor plans and allowing only desired movements.

BASAL GANGLIA: The gatekeepers of movement

•  connections between cortex, basal ganglia, & substantia nigra forming two pathways

connections between cortex, basal ganglia, & substantia nigra form two pathways →

• DIRECT PATHWAY

• INDIRECT PATHWAY

DIRECT PATHWAY:

• Facilitates the initiation of a movement

1. The cerebral cortex sends a signal to the striatum to start movement.

2. Dopamine from the substantia nigra binds to D1 (excitatory) receptors.

3. The internal segment of the globus pallidus (Gi) is inhibited.

The inhibition of the globus pallidus frees the

• upper motor neurons from the tonic inhibition, facilitating voluntary movement.

If an inhibitory pathways is blocked

• it allows excitation

When the striatum neurons become active

• they remove the break over the upper motor neurons, & the movement can be initiated.

INDIRECT PATHWAY:

• antagonizes (inhibits) the activity of the direct pathway

antagonizes (inhibits) the activity of the direct pathway to →

• prevent activation of unwanted movement programs

in indirect pathways the Striatum

1. Inhibits the external segment of the Globus Pallidus (GPe).

2. Decreased GPe activity disinhibits the Subthalamic Nucleus (which is normally inhibited).

3. The STN excites the Globus Pallidus internal (GPi).

Result from indirect pathways

•  Increased inhibition over the thalamus & the motor cortex, preventing the movement.

Activation of the Indirect Pathway detailed version

1. When the indirect pathway is activated by the cortex, the striatum neurons project inhibitory signals to the external segment of the globus pallidus (GPe).

2. This inhibition of the GPe neurons results in disinhibition of the excitatory neurons in the subthalamic nucleus.

3. The excitatory output from the subthalamic nucleus to the GP internal segment increases inhibition of the thalamus, leading to decreased thalamic output to the cortex.

Motor effect of dopamine

• release of dopamine from the substantia nigra pars compacta is associated with the facilitation of movements.

striatum neurons that activate the two pathways have different dopamine receptors

• Direct pathway → D1 dopamine receptors (excitatory)

• Indirect pathway → rich in D2 dopamine receptors (inhibitory)

the release of dopamine from the substantia nigra stimulates 

• direct pathway & inhibits the indirect pathway

This is why dopamine depletion increases

• difficulty in initiating movement & dopamine excess can cause involuntary movements.

Dopamine and the basal ganglia → pathways

• The nigrostriatal pathway

Drug-induced Parkinsonism

• Side effect of drugs, such as antipsychotics (D2 inhibitors).

Basal ganglia lesions

• Co-activation of both pathways is important for the smooth initiation & execution of new motor actions.

HYPOKINETIC DISORDERS

• Difficulty in the expression of movement (more tonic inhibition

• Parkinson

HYPERKINETIC DISORDERS

• Expression of unwanted movement (less tonic inhibition)

• Huntington

• Tourette's Syndrome