THE MOTOR SYSTEM

Ahmed A. E. Eljack 1

Objectives:

By the end of this session you should be able to:

■ Discuss basic concepts about reflexes.

■ Describe the structure and function of the muscle spindle.

■ Discuss the stretch, Golgi tendon, and withdrawal reflexes.

■ Describe the structure and function of the corticospinal tract.

■ Describe the role of cerebellum and basal ganglia in the motor system.

■ Mention important clinical considerations about the motor system.

Ahmed A. E. Eljack 2

Types of Nerve Fibers:

■ Either myelinated or unmyelinated.

■ Classified numerically into: Ia, Ib, II, III, and

IV (the only unmyelinated nerve fiber).

■ Classified alphabetically into: Aα, Aβ, Aδ,

and C (the only unmyelinated nerve fiber).

Ahmed A. E. Eljack 3

Concepts about Reflexes:

■ Integrated mainly in the grey matter of the

spinal cord.

■ There are two types of the anterior somatic

motor neurons:

– Alpha motor neurons.

– Gamma motor neurons.

■ The role of interneurons.

Ahmed A. E. Eljack 4

Concepts about Reflexes:

■ Two types of muscle sensory receptors:

– Muscle spindles (throughout the belly – detect

muscle length).

– Golgi tendon organs (in the tendons – detect

tendon tension).

■ The reflex arc contains:

– Sensory receptor.

– Sensory afferent nerves.

– Interneurons.

– Motor neurons.Ahmed A. E. Eljack 5

Ahmed A. E. Eljack 6

Muscle Spindle:

■ Consists of tiny intrafusal muscle fibers.

■ Muscle spindle fibers are divided into two

types:

– Nuclear bag muscle fibers.

– Nuclear chain muscle fibers.

Ahmed A. E. Eljack 7

Ahmed A. E. Eljack 8

Stretch Reflex:

■ The simplest reflex (only one synapse).

■ The mechanism:

– Stretching and activation of the muscle

spindle.

– A sensory branch synapses with the anterior

motor neurons.

– The motor neurons innervate the muscle(s).

■ The knee jerk reflex.

Ahmed A. E. Eljack 9

Golgi Tendon Reflex:

■ The Golgi tendon organ.

■ It’s a disynaptic reflex.

■ Considered a protective reflex.

■ The mechanism:

– Muscle contraction increases the tendon tension

activating the Golgi tendon organ.

– Sensory fibers synapse on inhibitory interneurons.

– Inhibitory interneurons inhibit the α motor neurons.

Ahmed A. E. Eljack 10

Withdrawal Reflex:

■ Polysynaptic reflex.

■ The mechanism:

– painful stimuli activate many afferents which synapse on

multiple interneurons and then motor neurons.

– Contraction of the flexors and relaxation of the extensors

on the ipsilateral side.

– Contraction of the extensors and relaxation of the flexors

on the contralateral side.

– There could be an afterdischarge.

Ahmed A. E. Eljack 11

The Corticospinal Tract:

■ The position of the motor cortex.

■ Divisions of the motor cortex:

– Primary motor cortex.

– Premotor area.

– Supplementary motor area.

■ From the motor cortex to the spinal cord (directly or indirectly).

■ The role of the extrapyramidal system.

Ahmed A. E. Eljack 12

The Corticospinal Tract:

■ Sensory and voluntary signals to the motor cortex from:

– The somatosensory cortex.

– Different parts of the frontal lobe.

– Visual and auditory cortices.

– The contralateral cerebral hemisphere (by way of corpus

callosum).

– The ventrobasal complex of the thalamus.

– The ventrolateral, ventroanterior, and intralaminar nuclei

of the thalamus.

Ahmed A. E. Eljack 13

Ahmed A. E. Eljack 14

Ahmed A. E. Eljack 15

Ahmed A. E. Eljack 16

The Role of Cerebellum:

■ The main role of cerebellum is the

correction of body movements.

■ It receives continuous sensory signals.

■ Motor learning!

Ahmed A. E. Eljack 17

The Role of the Basal Ganglia:

■ Most of their fibers go to the cerebral cortex

and also received from the cerebral cortex.

■ The main role is the initiation and

programming movements

■ The relationship between the striatum and

the thalamus.

■ The important role of dopamine.

Ahmed A. E. Eljack 18

Clinical Considerations:

■ Cerebellar impairment:

– Hypotonia.

– Ataxia:

■ Intention tremors.

■ Dysdiadochokinesia.

■ Dysmetria.

– Nystagmus.

– Dysarthria.

Ahmed A. E. Eljack 19

Clinical Considerations:

■ Upper motor neuron lesions.

■ Lower motor neuron lesions.

■ Parkinson's disease.

■ Huntington’s chorea.

Ahmed A. E. Eljack 20

Further Reading:

■ Barret, K.E, Barman, S.M, Boitano, S, Brooks, H.L. Ganong's

Review of Medical Physiology. (24th ed.). : McGraw-Hill; 2012.

■ Costanzo, L.S. Physiology. (5th

ed.). Philadelphia: Saunders; 2014

■ Hall, J.E. Guyton and Hall Textbook of Medical

Physiology. (12th ed.). Philadelphia: Saunders; 2011.

■ Eljack, A. A. E. Eljack’s Lecture Notes in Neuroscience.

Khartoum; 2015

■ Siegel, A, Sapru, H.N. Essential Neuroscience. (3rd

ed.). Philadelphia: Lippincott Williams & Wilkins; 2015.

Ahmed A. E. Eljack 21