8.5: Cerebellum and Basal Ganglia
Constitutes 10% of the mass of the brain and contains over 50% of the neurons
o Organized systematically: lobes, columns, and layers.
Receives info from primary and secondary motor cortex
o Info about descending motor signals from brain stem motor nuclei
o Feed back from motor responses via the somatosensory and vestibular
Believed to play a large role in learning of sequences of movements
o Especially when timing is crucial
Consequences of diffuse cerebellar damage for motor function:
o Loss of ability to control precisely the direction, force, velocity, and
amplitude of movements
o Loss of ability to adapt patterns of motor output to changing conditions
and great difficulty learning new sequences.
o Difficult to maintain steady postures; leads to tremors
o Severe disturbances to balance, gait, speech, and the control of eye
Some believe the cerebellum can do more…
o fMRI’s have shown activity in the cerebellum during non-motor tasks
o Most believe that the cerebellum participates in the fine-tuning and
learning of sequences in a cognitive fashion as well as a motor one.
Less neurons than the cerebellum but in one sense considered more complex.
o Organization: complex heterogeneous collection of interconnected
Anatomy suggests modulatory function (like cerebellum)
No fibers contributed to descending motor pathways
Part of neural loop where messages are passed from:
o Various cortical areas > basal ganglia > back via the thalamus > various
areas of motor cortex.
Function has evolved:
o Modulation of motor output as well as involved in a variety of cognitive
o B.G. shown to participate in the learning of correct habitual responses to
8.6: Descending Motor Pathways
Neural signals conducted from the primary motor cortex to the motor neurons
of the spinal cord over four different pathways.
To work well: good communication from cortex > the spinal motor circuits >
Dorsolateral Corticospinal Tract & Dorsolateral Corticorubrospinal Tract Dorsolateral Corticospinal Tract: The motor tract that leaves the primary motor
cortex, descends to the medullary pyramids, decussates, and then descends in
the contralateral Dorsolateral spinal white matter.
o Majority of neurons are large pyramidal cells of the primary motor
cortex – called Betz Cells; axons terminate in lower spinal cord,
projecting to muscles of the legs (voluntary movement).
o Direct path
o Most axons synapse on small interneurons of the spinal gray matter
then motor neurons of distal muscles of wrists, hands, fingers, and toes.
o Primates and few other mammals (ex. Hamsters & raccoons) are the
only ones with neurons directly synapsing on digit motor neurons.
Dorsolateral Corticorubrospinal Tract: The descending motor tract that
synapses in the red nucleus of the midbrain, decussates, and descends through
medulla and terminates in either the nuclei of the cranial nerves controlling
facial muscles, the rest in the dorsolateral spinal white matter.
o Indirect path
o Axons synapse on interneurons then motor neurons projecting on the
distal muscles of the arms and legs.
See Fig. 8.7 on p. 200 for diagrams.
Ventromedial Corticospinal Tract & Ventromedial Cortico-brainstem-spinal
Ventromedial Corticospinal Tract: The direct ventromedial motor pathway,
which descends ipsilaterally from the primary motor cortex directly into the
ventromedial areas of the spinal white matter.
o Each axon branches diffusely and innervates the interneuron circuits in
many spinal segments on both side of the gray matter.
Ventromedial Cortico-Brainstem-Spinal Tract: The indirect ventromedial motor
pathway, which descends bilaterally from the primary motor cortex to several
interconnected brainstem motor structures and then descends in the
ventromedial portions of the spinal cord.
o Each side carries signals from both hemispheres and each neuron
synapses on the interneurons of many spinal cord segments that control
the proximal muscles of the trunk and limbs.
o Four major brainstem structures involved: Tectum, vestibular nucleus,
reticular formation, and the motor nucleus of the cranial nerves that
control the muscles of the face.
See Fig. 8.8 on p. 200 for diagrams.
Comparison of the Two Dorsolateral Motor Pathways and the Two Ventromedial
Dorsolateral tracts differ from the two ventromedial tracts in that:
o VM tracts are much more diffuse – affect both sides of the spinal gray
matter. DL only affects the contralateral side and sometimes directly on
a motor neuron
o VM tract neurons project to proximal muscles on the trunk and limbs
(ex. Shoulder muscles). DL tract neurons project to distal muscles (ex.
Because all originate in the cerebral cortex they are assumed to mediate
voluntary movement. Routes and destinations suggest different functions.
Lawrence & Kuypers (1968): o Cut through left & right DL Corticospinal tracts of monkey subjects in
the medullary pyramids, just above the decussation of the tracts.
Caused impaired ability to use their limbs for activities other
than standing, walking and climbing. For ex. Reaching was weak
and poorly directed.
Never regained ability to move fingers independently or to
release objects from their grasp.
o These monkeys were divided equally and additional cuts were made to
the DL corticorubrospinal tract of one group and another group had
both their VM tracts cut.
First group showed same results as original cuts, however, when
sitting their arms hung limply by their sides.
Second group showed severe postural abnormalities: difficulty
walking and sitting, usually needed to cling to bars of cage for
support when sitting/standing, any disturbance/noise would
cause a fall, inability to control shoulders.
o Therefore: VM controls posture & whole body movements, DL controls
movement of limbs.
8.7: Sensorimotor Spinal Circuits
Lowest level of the sensorimotor hierarchy – spinal circuits and the muscles
Spinal circuits are not just pathways, they have complexities and processes
independent of the brain.
Motor Units: A single motor neuron and all of the skeletal muscle fibers that are
innervated by it.
o Smallest units of motor activity
o When a neuron fires, all the muscle fibers of its unit contract together.
Tendons attach bones to groups of muscle fibers.
Motor End-Plate: The receptive area on a muscle fiber at a neuromuscular
o Acetylcholine stimulates this and causes the muscle fibers to contract.
Motor Pool: All of the motor neurons that innervate the fibers of a given muscle.
Generally two types of muscle fibers: fast (contracting and relaxing) and slow
o Fast can generate great force, but tire quickly from poor vascularization
(pale colour); vice versa for slow (more red).
Flexors: Muscles that act