Week 2 - Brachial plexus, Axilla and Skeletal Muscle

Question 1

What is the brachial plexus?

Answer 1

A network of nerve fibres that supplies the skin and musculature of the upper limb
- Begins in the root of the neck, passes through the actual and enters the upper arm

Question 2

What is the structure of the brachial plexus?

Answer 2

Consists of:
Roots: C5, C6, C7, C8 T1
Trunks:
- Superior: C5, C6
- Middle: C7
- Inferior: C8, T1
Divisions: anterior/posterior
Cords:
- Lateral: anterior superior + middle anterior
- Posterior: all posterior cords
- Medial: inferior anterior
Branches:
- Musculocutaneous (lateral)
- Axillary (posterior)
- Median (lateral + medial)
- Radial (posterior)
- Ulnar (medial)
There are also a number of smaller branches

Question 3

What are the functions of the musculocutaneous nerve?

Answer 3

Motor functions: innervates the brachialis, biceps brachii and coracobrachialis muscles
Sensory functions: innervates the lateral forearm

Question 4

What are the functions of the axillary nerve?

Answer 4

Motor: innervates the deltoid, teres minor and the long head of the triceps brachii
Sensory: innervates the inferior region of the deltoid (regimental badge area)

Question 5

What are the functions of the median nerve?

Answer 5

Motor: innervates most of the flexor muscles in the forearm, the thenar muscles and the 2 lateral lumbrical muscles
Sensory: innervates the lateral palm, the radial and ulnar halves of the thumb, index finger and middle finger, and the radial half of the ring finger

Question 6

What are the functions of the radial nerve?

Answer 6

Motor: innervates the triceps brachii and the extensor muscles in the posterior forearm
Sensory: innervates the posterior forearm. the posterior arm and the posterior, lateral hand

Question 7

What are the functions of the ulnar nerve?

Answer 7

Motor: innervates the muscles of the hand, flexor carpi ulnas and medial half of the flexor digitorum profundus
Sensory: innervates the anterior and posterior surfaces of the medial 1 and a half fingers and associated palm area

Question 8

What is the axilla?

Answer 8

An area that lies underneath the glenohumeral joint, at the junction of the upper limb and the thorax
- It is a passage by which the structures can enter and leave the upper limb

Question 9

What are the contents of the axilla?

Answer 9

• Axillary artery: main artery supplying the upper limb. Has 3 parts: 1 medial to the pectoralis minor, 1 posterior to the pec minor and 1 lateral to the pec minor (the medial and posterior parts travel in the axilla)
• Axillary vein: the main vein draining the upper limb, its 2 largest tributaries are the cephalic and basilic veins
• Brachial plexus
• Biceps brachii and coracobrachialis (the muscle tendons move through the axilla, where they attach to the coracoid process)
• Axillary lymph nodes (filter lymph that has drained from the upper limb and pectoral region)

Question 10

What passageways exit the axilla?

Answer 10

• Main route: immediately laterally and inferiorly, into the upper limb
• Via the quadrangular space: a gap in the posterior arm and shoulder area (structures passing through: axillary nerve, posterior circumflex humeral artery)
• The clavipectoral triangle: an opening in the anterior wall of the axilla, bounded by the pec major, deltoid and clavicle (structures passing through: cephalic vein enters, medial and lateral pectoral nerves leave)

Question 11

What are the different arrangements of skeletal muscles?

Answer 11

• Circular (e.g. orbicularis oris)
• Convergent (e.g. pectoralis major)
• Parallel (e.g. sartorius)
• Unipennate (e.g. extensor digitorum longus)
• Multipennate (e.g. deltoid)
• Fusiform (e.g. biceps brachii)
• Bipennate (e.g. rectus femoris)

Question 12

Describe the structure of skeletal muscle

Answer 12

• Can be striated
• Provides movement, posture, stability of joints and heat
• Has different arrangements
• Muscle: muscle fibre (wrapped by endomysium) < fascicle (wrapped by perimysium) < muscle (wrapped by epimysium)

Question 13

What are the different levers that muscle can be?

Answer 13

• First class lever: very efficient, can be found in the brain (force is at opposite end to load)
• Second class lever: reasonably efficient, can be found in lower leg (force is close to load, but load is closer to fulcrum)
• Third class lever: very inefficient, most muscles use it (force is close to load, but force is closer to fulcrum)

Question 14

What different muscle groups are there?

Answer 14

• Agonists: prime movers; main muscles responsible for a particular movement
• Antagonists: oppose prime movers
• Synergists: assist prime movers; neutralise extra motion to keep the force acting in 1 direction
• Fixators: stabilises action of prime movere

Question 15

What is compartment syndrome?

Answer 15

A build up of pressure within a limb, due to the inextensible connective tissue layer, that affect nerve.
- May be due to a burst blood vessel

Question 16

What are the different types of muscular contraction?

Answer 16

• Isotonic contraction

- Isometric contraction

Question 17

How does isotonic contraction occur?

Answer 17

• The muscle changes length and moves the load
• Constant tension, variable muscle length
• Can be:
• – Concentric: muscle shortens (e.g. lifting a load with the arm)
• – Eccentric: the muscle exerts a force while being extended (can cause delayed-onset muscle soreness, e.g. walking downhill)

Question 18

How does isometric contraction occur?

Answer 18

• Constant length, variable tension

- E.g. hand grip (uses forearm muscles)

Question 19

How does contraction occur at the neuromuscular junction?

Answer 19

• An action potential arrives, opening the voltage-gated Ca2+ channels and Ca2+ floods in
• This causes vesicles containing acetylcholine to move to and fuse with the plasma membrane
• ACh is released by exocytosis and binds to nicotinic receptors on the skeletal muscle membrane
• These receptors are ligand-gated Na+ channels, so ACh binding causes them to open and Na+ moves into the skeletal muscle cell
• It stimulates the sarcoplasmic reticulum to release Ca2+ which is used for muscle contraction
• Acetylcholinesterase breaks down ACh after contraction

Question 20

How is muscle relaxed?

Answer 20

• Ca2+ is pumped back into the sarcoplasmic reticulum via Ca2+ pumps
• Some Ca2+ can bind to calmodulin

Question 21

Why is energy required for muscular contraction?

Answer 21

Continuous muscular contraction requires ATP to detach myosin heads from actin

Question 22

What are the sources of energy for muscular contraction?

Answer 22

ATP:
- Short term stores of ATP in muscle fibre
- Creatine phosphate (enzyme = creatine kinase)
- Glycolysis
- Oxidative phosphorylation
Anaerobic glycolysis:
- Pyruvate –> lactate (enzyme = lactate dehydrogenase)

Question 23

What is contracture?

Answer 23

A state of continuous contraction that occurs when ATP is depleted (rigor mortis)

Question 24

What is peripheral fatigue?

Answer 24

• Depletion of muscle glycogen stores
• Occurs within 1 minute if blood flow is interrupted (intermittent claudication: caused by a lack of blood flow to the working muscle)

Question 25

What is muscle tone?

Answer 25

The baseline tone present in muscles at rest

Question 26

What is muscle tone due to?

Answer 26

• Motor neurone activity

- Muscle elasticity

Question 27

How is muscle tone controlled?

Answer 27

Via motor control centres in the brain

- Afferent fibre signals originating in the muscle

Question 28

What is hypotonia?

Answer 28

Low muscle tone
- May be due to primary degeneration of the muscles, cerebral/spinal neural shock, lesions of the cerebellum, lesion of lower motor neurones

Question 29

What is a motor unit?

Answer 29

A motor neurone and the muscle fibres it innervates

• Usually innervates several
• The fewer it innervates, the more sensitive the muscle will be

Question 30

What is cross-talk?

Answer 30

When signalling molecules communicate between nerve and muscle
- Atrophy of nerve/muscle can lead to atrophy of the corresponding muscle/neurone

Question 31

What is spatial summation?

Answer 31

The more neurones activated, the more muscle fibres recruited to develop more force

Question 32

What is temporal summation?

Answer 32

Increased frequency of action potentials to muscle fibres causes summation/tetanus

• Tetanus = painful muscle spasms and muscle stiffness
• Can also be caused by clostridium tetani (bacteria)

Question 33

What is axillary clearance?

Answer 33

Removal of all or most lymph nodes from under the arm

• Surgical operation
• Gives a risk of long term swelling of the arm (lymphoedema)
• Reasons for removing them: to check whether any cancer cells have spread to the lymph glands. If any cancer cells have spread to the lymph nodes, the operation aims to ensure that this is removed

Question 34

What is the effect of long thoracic nerve damage?

Answer 34

• If it is not functioning, then the affected scapula will lift off the chest when force is applied to that shoulder unit (a winged scapula)
• This is due to weakness in the affected serrates anterior muscle

Question 35

What is lymphangitis?

Answer 35

Inflammation of the lymphatic channels

• Occurs when viruses and bacteria infect the channels of the lymphatic system, typically through a cut/wound that has become infected
• There will often be tender red streaks going from the wound towards the nearest lymph glands
• Symptoms = fever, chills and a general sense of illness