Lecture 16 (The Motor Unit)

Question 1

Decision -> … -> Skeletal muscle activation

Answer 1

Motor planning areas
Control circuits
Descending motor pathways -> lower motor neurons
Spinal interneurons
Lower motor neurons

Question 2

Lower motor neurons (LMN)

Answer 2

1. Directly innervate skeletal muscle fibers
2. Cell body located in brain stem or spinal cord
3. Two types: alpha (large cell bodies and large myelinated axons) or gamma (medium-sized myelinated axons)

Question 3

Upper motor neurons (UMN)

Answer 3

1. Synapse on LMNs and interneurons in brain stem or spinal cord
2. Cell body located in cerebral cortex and brain stem

Question 4

Descending pathways

Answer 4

• neurons whose axons travel in the descending pathways are UMN
• classified as postural / gross movement pathway and fine movement pathway

Question 5

Alpha-motor neurons

Answer 5

1. Branch into numerous terminals as approach muscle
2. Each terminal ends near a single muscle fiber at a NMJ
3. Release neurotransmitter (ACh) depolarises membrane
4. Causes muscle fibers to contract

Question 6

Descending Motor Pathways

Answer 6

Monitored and adjusted by the basal nuclei and cerebellum
1. Corticospinal
2. Medial
3. Lateral

Question 7

Corticospinal pathway

Answer 7

Voluntary control of skeletal muscle
Originates in layer V of the primary motor cortex
1. Corticonuclear tracts
2. Lateral corticospinal tracts
3. Anterior corticospinal tracts

Question 8

Medial pathway

Answer 8

Control gross movements of trunk and proximal limb muscles

Question 9

Lateral pathway

Answer 9

Control distal limb muscle that perform more precise movements

Question 10

Corticonuclear tracts

Answer 10

Provide conscious control over skeletal muscles that move eye, jaw, face and some muscles of neck and pharynx

Question 11

UMN lesions

Answer 11

1. Abnormal cutaneous reflexes
2. Abnormal timing of muscle activation
3. Paresis
4. Hypertonia
5. Clasp-knife response

Question 12

LMN lesions

Answer 12

Decreases or prevents muscle contraction
1. Loss of reflexes
2. Atrophy
3. Hypotonia (flaccid paralysis)
4. Fibrillations

Question 13

Control circuits

Answer 13

1. Basal nuclei and cerebellum
2. Adjust activity in the descending pathways
3. Results in excitation or inhibition of LMNs

Question 14

Basal nuclei

Answer 14

Involved in control of posture and movement
1. Striatum
2. Globus pallidus
3. Subthalamic nucleus
4. Substantia nigra
5. Amygdala

Question 15

Pathology of basal nuclei

Answer 15

Dysfunction results in movement disorders
1. hypokinetic disorders (PD)
2. hyperkinetic disorders (Huntington’s Disease, subtypes of Cerebral Palsy)

Question 16

Parkinson’s disease

Answer 16

1. most common basal nuclear disorder
2. muscular rigidity, shuffling gait, droopy posture, muscular tremors, mask-like facial expression
3. interferes with voluntary and automatic movements
4. pathology: death of dopamine producing cells in Substantia nigra (pars compacta)

Question 17

Cerebellar Disease

Answer 17

1. Ataxia
2. Dysarthria
3. Intention tremor
4. Past pointing

Question 18

Ataxia

Answer 18

Lack of coordination

Question 19

Dysarthria

Answer 19

Slurred, poorly articulated speech

Question 20

Intention tremor

Answer 20

Shaking of the limb during voluntary movement

Question 21

Past pointing

Answer 21

Inability to accurately perform repetitious tasks