1. The Motor System
Chapter 8

2. Overview The motor system is divided into 4 components:
Direct activation pathway
Control circuits
Indirect activation pathway
Final common pathway

3. Direct activation pathway
Largest, best-defined motor pathway
It is a “single neuron” pathway, extending from the cerebral cortex to the spinal cord (corticospinal tract)
Major function: to effect voluntary activity, particularly skilled movements under conscious control

4. Direct activation pathway
Origins:
Primary motor cortex
Premotor cortex
Supplementary motor cortex
Major neurotransmitters: glutamate (excitatory) and GABA (inhibitory)
Damage results in weakness, with loss of voluntary movements especially fine, skilled movements, but preservation of other forms of movements including segments reflexes

5. Control Circuits
2 parallel pathways, the cerebellar and basal ganglia, control and modify motor activity
They both receive input from several motor and sensory cortical areas and project back to the cerebral cortex via the thalamus. They integrate and modulate motor activity primarily through the cerebral cortex and direct activation pathways.

6. Control Circuits
They also send information to the brainstem and the indirect activation pathways.
Basal ganglia
Concerned with learned, automatic behavior and with preparing and maintaining the background support, or posture, needed for voluntary motor activity. (Neurotransmitters: dopamine and GABA)
Cerebellum
Accomplishes the coordination and correction of movement errors of muscles during active movements. (Neurotransmitters: glutamate and GABA)

7. Indirect activation pathways
The older, more diffuse motor pathways, sometimes called extrapyramidal pathways
These pathways mediate the enormous number of automatic activities involved in normal motor function. For example, the maintenance of erect posture when sitting or standing requires the coordinated contraction of many muscles.

8. Indirect activation pathways
This coordination is under subconscious control and is mediated by the indirect activation pathways which are:
Reticulospinal
Vestibulospinal
Rubrospinal
Diseases affecting these pathways are manifested with abnormal muscle tone and reflexes.

9. Final common pathway
The somatic (skeletal) muscles that perform the voluntary movements are all under direct control of lower motor neurons and contract only in response to activation by these neurons.
These neurons are located in the ventral horn of the spinal cord and brainstem.
A lower motor neuron and the muscle fibers under its control constitute a motor unit.
The final common pathway consists of many motor units through which all activities in the motor system must act.
Neurotransmitter: acetylcholine

10. Direct Activation Pathway Anatomy

11. Motor homonculus

12. Internal capsule
Corticobulbar fibers occupy a more anterior location in the posterior limb than the corticospinal

13. Clinical Correlations
Knowledge of the clinical manifestations of the diseases affecting the direct activation pathway comes from experimental studies and from observation of clinical disorders.
Disturbances of the corticospinal system may be irritative (positive) or paralytic (negative).

14. Clinical Correlations: irritative vs. paralytic
Irritative (seizures)
Jacksonian seizures – focal motor seizures starting from the distal upper extremity spreading proximally
Paralytic
Weakness – fine movements, skilled movements (distal muscles); no atrophy; presence of Babinski sign and primitive reflexes (upper motor neuron manifestations)

15. Control Circuits BASAL GANGLIA Anatomy and Physiology
learned, automatic behavior
preparing and maintaining the background support or posture needed for voluntary motor activity
Anatomy and Physiology
caudate nucleus, lentiform nucleus (putamen, globus pallidus), substantia nigra, subthalamic nucleus
striatum=caudate nucleus + putamen
striatum neurotransmitters: acetylcholine, dopamine (produced in the substantia nigra and transported to striatum via the nigrostriatal pathway)

16. Control Circuits: Basal Ganglia
Clinical Correlations:
excess discharge  slowing (hypokinesia)
lack of discharge  hyperactivity (involuntary movements)
Involuntary movements
athetosis
chorea
tremors
dystonia
ballismus

17. Control Circuits: Basal Ganglia
Clinical Correlation:
Speech
hypokinetic – quiet, weak, monotonous voice, poor articulation, short rushes of words, breathy quality
hyperkinetic – bursts of loudness, elevation of pitch, voice stoppages, distortion of vowels, disintegration of articulation

18. Control Circuits CEREBELLUM
coordination and correction of movement errors of muscles during active movements
neurotransmitters: glutamate, GABA

19. Control Circuits: Cerebellum
Functional subdivisions
Vestibulocerebellum
Flocculonodular lobe connected with the vestibular nuclei through the inferior cerebellar peduncle
Controls and coordinates axial musculature and movements of the head and eyes

20. Control Circuits: Cerebellum
Functional subdivisions
Spinocerebellum
Made up of vermis and paravermis
Its primary input is from dorsal spinocerebellar tract (via the inferior cerebellar peduncle) and the ventral spinocerebellar tract (via the superior cerebellar peduncle) which provide information about motor performance and motor neuron excitability, respectively.

21. Control Circuits: Cerebellum
Functional subdivisions
Cerebrocerebellum
Made up of the lateral portions of the cerebellar hemispheres and receives inputs from pontine nuclei via the middle cerebellar peduncle
The output is from the dentate nucleus through the superior cerebellar peduncle
This subdivision is important in initiation of movement and in coordination of muscles (specifying direction, pattern, and intensity of movement)

22. Cerebellum: Clinical Correlations
Flocculonodular Syndrome
Occurs with midline tumors in children
Produces truncal ataxia, with unsteadiness and falling and disturbances with gait and standing; with nystagmus

23. Cerebellum: Clinical Correlations
Anterior Lobe Syndrome
Commonly due to degenerative diseases of the adults
Produces severe gait ataxia (broad-based) without ataxia of the upper limbs, and mild ataxia of the legs
Cerebellar Hemisphere Disease
Produces limb ataxia with hypotonia, intention tremors, dyssynegria and dysmetria ipsilateral to the lesion

24. Cerebellum: Clinical Correlations
Disorders of speech also occur in cerebellar disease, particularly when it involves the vermis.
These disorders are manifested as irregularities of pitch, loudness, and rhythm and are known as ataxic dysarthria.
Many cerebellar diseases can be reversed if they are identified early and treated appropriately.
Cerebellar degeneration produced by vitamin E deficiency
Small cell lung carcinoma and ovarian cancers produce antibodies to Purkinje cells than cause cerebellar degeneration.

25. Indirect Activation Pathway
MEDIAL
PATHWAY
LATERAL PATHWAY
Vestibulospinal tract
Rubrospinal tract
Proximal > distal
Muscles
Distal > proximal
Extensors > flexors
Flexors > extensors
Posture and balance
Fractionated movements
Controls posture and whole limb movements
Function
Independent flexor (especially arm) movements

26. Final Common Pathway
Motor unit – a lower motor neuron (ventral horn cell/CN nucleus) + muscle fibers under its control
alpha motor neuron
axon (nerve)
synapse
muscle fibers
Final common pathway – many motor units through which all activities in the motor system must act