Control of Muscle Tone

1. Control of Muscle Tone By Dr D Fisher

3. Cerebellar ‘Awareness' • After MS stimulation (stretch) APs are conducted along the afferent fiber (Ia) • It enters into the spinal cord and divides into several collaterals. • Some of these collaterals synapse on the cell bodies of neurons which ascend to the cerebellum (anterior and posterior spinocerebellar tracts). • Thus, at all times the cerebellum is aware of the state of stretch in muscles, in other words the TONE of muscles.

4. Cerebellar ‘Awareness'

5. Coactivation of Gamma Efferents • Whenever a motor command descends from the motor cortex and synapses on neural cell bodies which innervate muscles, collaterals from these descending fibers also synapse on the corresponding cell bodies (gamma efferents) which innervates the ends of the intrafusal muscle fibers. • This is important so that as the extrafusal muscle fibers contract and shorten, the intrafusal also shorten and remain taunt.

6. Coactivation of Gamma Efferents • This enables the MS to always respond to stretch even immediately after contraction of a muscle. • In other words the coactivation of gamma efferents avoids 'silent periods' which would occur if the intrafusal muscle fibers did not contract simultaneously with the extrafusal muscle fibers.

7. Coactivation of Gamma Efferents

8. The Golgi Tendon Organ • The Golgi tendon organ is a receptor found within the tendons of muscle. • It detects tension >100 grams in the tendon. • It is innervated with a 1b afferent fiber.

10. The Golgi Tendon Organ

12. Summary • Golgi tendon organs detects tension in the tendon. • Afferent neurons conduct action potentials to the spinal cord. • Afferent neurons synapse with inhibitory (inter) association neurons (secretes GABA) which in turn synapse with alpha motor neurons. • Inhibition of the alpha motor neurons causes muscle relaxation, relieving the tension in the muscle.

13. Control of Skeletal Muscle • The prefrontal area has association areas for the motivation and foresight to plan and initiate movements. • In the premotor area motor functions are organized before they are initiated in the Motor Cortex. • The motor cortex (primary motor cortex) is found on the precentral gyrus.

15. Homunculus

16. Homunculus • Definition: It is the literal representation of connected parts of the human body on the surface of the brain. • The surface of the postcentral gyrus provides the literal somatosensory homunculus, • while the surface of the precentral gyrus provides the literal motor homunculus. • Please note that the sensory homunculus is almost a mirror image of the motor homunculus. 

17. The Somatic Motor System The Somatic motor system (SMS) can be divided roughly into 3 components: • The Pyramidal System • Extrapyramidal System • Cerebellar System

18. The Pyramidal System (direct system) • Involved with fine, discrete and precise voluntary control of movement. • The command arises from the precentral gyrus (where the somatic motor cortex resides). • Fibers from the cortex descend to the spinal cord where they synapse on the anterior horn motor neurons (alpha and gamma).

19. Descending Spinal Pathways • Direct • Control muscle tone and conscious skilled movements • Direct synapse of upper motor neurons of cerebral cortex with lower motor neurons in brainstem or spinal cord • Tracts • Corticospinal • Lateral • Anterior corticobulbar

20. The Extrapyramidal System (indirect system) • Its main function is to provide course background voluntary movement. • Arises from various sites on the cerebral cortex. • Interconnected with the pyramidal system. • The EPS is directly connected to the Basal Ganglia

21. Descending Spinal Pathways • Indirect • Synapse in some intermediate nucleus rather than directly with lower motor neurons • EPS Tracts • Rubrospinal • Vestibulospinal • Reticulospinal • Testospinal tract

22. The Cerebellar Component • Function: To enable smooth coordinated movement • plays an important role in the maintenance of posture and equilibrium. • Sends fibers via the cerebellar-spinal tract to modulate the activity of the alpha (lower motor neurons) as well as the gamma motor neurons.

23. The Cerebellar Functions • The cerebellum receives information from a wide array of senses: • Pressure and touch • Positional receptors: Spindle and Golgi tendon organs • Eyes and ears • A copy of the command signal is sent from the cerebral cortex to the cerebellum, where it acts as a comparator.

24. Cerebellar Comparator Function

26. The Basal Ganglia What is the Basal Ganglia? • Paired masses of gray matter in each cerebral hemisphere • Largest Nuclei – Corpus Striatum: • Caudate nucleus • Lentiform nucleus: • Putamen and globus pallidus • Claustrum and amygdaloid • Substantia nigra, subthalamic nucleus and red nucleus.

27. Neural Connections of the Basal Ganglia •  The basal ganglia are interconnected by many fibers. •  They have fibers connected to the: • Cerebral cortex (ascending fibers) • Thalamus and Hypothalamus • Descending fibers to Gamma motor fibers (sp.Cord)

28. Clinical Manifestation • Abnormal body movements: • Tremor (uncontrollable shaking) • Involuntary movements of the skeletal muscles • Paralysis – Akinesis: (destruction of the caudate (most affected site in strokes) results in paralysis in the opposite side of the body). • Globus pallidus: mostly concerned with muscle tone for specific body movements. • Lesion in the subthalamic nucleus – hemiballisms, jerky movements, spontaneous movements of the arms (affects the extremities – legs and arms)

30. Other Clinical Manifestations Parkinsonism: • Concentration of dopamine in certain of the basal ganglia is reduced. Treatment involves giving the patient L-dopa which passes the B-B Barrier. Alzheimers: • Related to the degeneration of substantia innominata (basal ganglia nuclei)