Properties of the motor unit

Question 1

The motor system

Answer 1

Consituted from tissues that work together to:

• set muscle tone
• bring about voluntary movements

Question 2

The motor unit

Answer 2

The smallest functional unit of the motor system, defined as a somatic efferent (motor neurone) plus all the muscle fibres it supplies.

Question 3

Components of the motor unit

Answer 3

• 1 alpha-motoneurone
• all extrafusal muscle fibres it supplies

The number of muscle fibres can range from very few (10 in extra-ocular muscles) to many (1000 in quadriceps).

Question 4

Structural components of the motor system

Answer 4

• Neuronal; motoneurones
• Muscular; skeletal muscles
• Interface; neuromuscular junction

Question 5

Skeletal muscles

Answer 5

Responsible for voluntary movement under the control of the somatic nervous system. Important for movement, posture, stability of joints and heat generation.

Question 6

Organisation of skeletal muscles

Answer 6

A single skeletal muscle cell is an elongated, multinucleate fibre called a myocyte. These fibres are arranged in bundles called fascicles. Fasciculi collect to form muscles.

Question 7

Connective tissues within muscles

Answer 7

Endomysium- envelops a myocyte
Perimysium- envelops a fascicle
Epimysium- envelops all fascicles and the neurovascular bundle

Question 8

Tendons

Answer 8

Tough bands of fibrous connective tissue that form a point of pull (confluence of contraction) between myocytes of a muscle.

Question 9

Fascicular architecture

Answer 9

• convergent muscles
• strap muscles
• circular muscles
• fusiform muscles
• pennated muscles (unipennate, bipennate, multipennate)

Question 10

Myofibrils

Answer 10

Contractile fibres that appear as alternating dark and light bands of tissue within tubular skeletal muscles.

Question 11

Lower motoneurone

Answer 11

A somatic motor efferent, whose cell body is found in either the spinal motor nucleus or cranial nerve motor nucleus. Its axon directly supplies skeletal muscle.

Question 12

Characteristics of the motor unit

Answer 12

• each muscle cell receives motor supply from only 1 motoneurone
• muscle cells of a motor unit are randomly distributed throughout the thickness of a muscle
• the muscle fibres of a motor unit have the same physiological and histochemical profile

Question 13

The arrangement of the motor unit allows for…

Answer 13

• uniform development of force throughout the muscle
• the nervous system to regulate the rate and speed of contraction
• muscle contraction to be distributed through the muscle

Question 14

3 classes of muscle fibres

Answer 14

• Slow twitch (type S/I)
• Intermediate twitch (type FR/IIA)
• Fast twitch (type FF/IIB)