---

Muscles of the Shoulder Region, Rotator Cuff, and Intermuscular Spaces

---

Overview

1. The shoulder (scapular) region - including the superficial layer and the posterior scapular region
2. The glenohumeral joint - stabilized critically by the rotator cuff

---

Part 1: Superficial Muscles of the Shoulder

Trapezius

Deltoid

Key point: The trapezius attaches the scapula and clavicle to the trunk, while the deltoid attaches the scapula and clavicle to the humerus. Together, they provide the characteristic contour of the shoulder. Both attach to opposing surfaces and margins of the spine of scapula, acromion, and clavicle.

---

Muscles Deep to Trapezius (Scapulothoracic Muscles)

• Origin: Transverse processes of C1-C2, posterior tubercles of C3-C4 transverse processes
• Insertion: Posterior surface of medial border of scapula (superior angle to root of spine)
• Innervation: Dorsal scapular nerve + direct branches from C3, C4
• Action: Elevates the scapula

• Origin: Lower ligamentum nuchae, spinous processes CVII and TI
• Insertion: Smooth triangular area at root of spine of scapula (posterior surface)
• Innervation: Dorsal scapular nerve (C5)
• Action: Retracts and elevates the scapula

• Origin: Spinous processes TII-TV + supraspinous ligaments
• Insertion: Posterior surface of medial border of scapula (root of spine to inferior angle)
• Innervation: Dorsal scapular nerve (C4, C5)
• Action: Retracts and elevates the scapula

---

Part 2: The Posterior Scapular Region

1. Supraspinatus
2. Infraspinatus
3. Teres minor
4. Teres major

---

Part 3: The Rotator Cuff (SITS Muscles)

Individual Muscle Details

1. Supraspinatus

• Origin: Medial 2/3 of supraspinous fossa + deep fascia covering the muscle
• Insertion: Most superior facet of greater tubercle of humerus
• Innervation: Suprascapular nerve (C6) - passes through suprascapular foramen
• Action: Initiates abduction of glenohumeral joint (first 15°); stabilizes head of humerus in glenoid cavity

Clinical note: The supraspinatus tendon passes beneath the acromion and acromioclavicular ligament through a fixed-dimension space. This makes it the most commonly impinged and torn rotator cuff tendon. Blood supply is relatively poor, making it susceptible to degenerative change, calcium deposition, and partial/full-thickness tears. - Gray's Anatomy for Students, p.831

2. Infraspinatus

• Origin: Medial 2/3 of infraspinous fossa + deep fascia
• Insertion: Middle facet on posterior surface of greater tubercle of humerus
• Innervation: Suprascapular nerve (C6) - passes through greater scapular (spinoglenoid) notch
• Action: Lateral rotation of arm at glenohumeral joint; stabilizes joint

3. Teres Minor

• Origin: Upper 2/3 of a narrow strip on the posterior surface of the lateral border of scapula
• Insertion: Lowest (inferior) facet of greater tubercle of humerus
• Innervation: Axillary nerve (C5, C6)
• Action: Lateral rotation of arm; stabilizes glenohumeral joint

4. Subscapularis (the "S" on the anterior side)

• Origin: Medial 2/3 of subscapular fossa (anterior/costal surface of scapula)
• Insertion: Lesser tubercle of humerus (and capsule of glenohumeral joint)
• Innervation: Upper and lower subscapular nerves (C5, C6)
• Action: Medial rotation of arm; stabilizes glenohumeral joint
• This muscle is located anterior to the shoulder joint, forming much of the posterior wall of the axilla

Key concept: The four rotator cuff tendons blend with and reinforce the glenohumeral joint capsule. Contribution to stability occurs through: (1) joint compression, (2) barrier effect (the tendons physically block displacement), and (3) dynamic ligament-like effect. - Rockwood and Green's Fractures in Adults

---

Part 4: Teres Major (not a rotator cuff muscle)

• Origin: Oval area on posterior surface of inferior angle of scapula
• Insertion: Medial lip of the intertubercular sulcus (bicipital groove) of humerus
• Innervation: Lower subscapular nerve (C5, C6)
• Action: Medial rotation and extension of arm; adducts the arm ("little lat")

---

Part 5: Intermuscular Spaces

1. Quadrangular Space (Quadrilateral Space)

• Superior: Inferior margin of teres minor
• Inferior: Superior margin of teres major
• Medial: Lateral margin of long head of triceps brachii
• Lateral: Surgical neck of humerus

• Axillary nerve (posterior cord, C5, C6)
• Posterior circumflex humeral artery (and vein)

Clinical significance: Quadrilateral space syndrome - compression of the axillary nerve and posterior circumflex humeral artery within this space, most commonly by a fibrous band between teres major and minor. Presents with pain and paresthesia in the shoulder, worsened by abduction and external rotation.

2. Triangular Space (Medial Triangular Space)

• Lateral (and inferior): Medial margin of long head of triceps brachii
• Superior: Inferior margin of teres minor
• Inferior: Superior margin of teres major

• Circumflex scapular artery (and vein) - branch of subscapular artery
• This space communicates between the axilla and posterior scapular region

3. Triangular Interval (Lower Triangular Space / Lateral Triangular Space)

• Medial: Lateral margin of long head of triceps brachii
• Superior: Inferior margin of teres major
• Lateral: Shaft of humerus

• Radial nerve
• Profunda brachii artery (deep artery of arm) and associated veins

Because this space lies below the inferior margin of teres major (which marks the inferior boundary of the axilla), the triangular interval serves as a passageway between the anterior and posterior compartments of the arm and between the posterior arm and axilla.

---

Summary Table: Intermuscular Spaces

---

Summary: Rotator Cuff Muscles at a Glance

---

Key MS-1 Exam Points

1. Rotator cuff = SITS (Supraspinatus, Infraspinatus, Teres minor, Subscapularis)
2. All four insert at or near the tubercles of the humerus and blend with the joint capsule
3. Supraspinatus is the most commonly injured - passes through the tight subacromial space
4. All three posterior rotator cuff muscles insert on the greater tubercle (sup to inf: Supraspinatus > Infraspinatus > Teres minor); subscapularis inserts on the lesser tubercle
5. Quadrangular space carries the axillary nerve and posterior circumflex humeral artery
6. Triangular space carries the circumflex scapular artery
7. Triangular interval carries the radial nerve and profunda brachii artery
8. The long head of triceps is the key muscle that separates the triangular space (medial to it) from the triangular interval (lateral to it)
9. The axillary nerve wraps around the surgical neck of humerus - at risk in proximal humeral fractures and shoulder dislocations
10. Suprascapular nerve passes under the superior transverse scapular ligament (artery passes over it - "army over the bridge") to reach supraspinatus, then through spinoglenoid notch to infraspinatus

---

Nerve Fibres: Properties, Classification, Degeneration, Regeneration, Neuropathy, and Synapse

---

Part 1: Nerve Fibres - Structure and Properties

Basic Structure of a Peripheral Nerve Fibre

• Axon - the conducting process of the neuron
• Myelin sheath (in myelinated fibres) - formed by Schwann cells in the PNS (oligodendrocytes in CNS)
• Nodes of Ranvier - gaps in the myelin sheath at 1-2 mm intervals
• Endoneurium - connective tissue surrounding each fibre
• Perineurium - surrounds a fascicle (bundle of fibres)
• Epineurium - surrounds the entire nerve

Key Properties of Nerve Fibres

---

Part 2: Classification of Nerve Fibres

System 1: Erlanger-Gasser Classification (A, B, C) - used for all fibres

Key Rule: Larger diameter = faster conduction velocity = less sensitive to local anaesthetics (compared to same type). Exception: small unmyelinated C fibres are relatively resistant to local anaesthetics compared to larger myelinated fibres.

System 2: Numerical Classification (I, II, III, IV) - used for sensory fibres only

Mnemonic for speed order: Aα > Aβ > Aγ > Aδ > B > C

---

Part 3: Nerve Degeneration

1. Segmental (Focal) Demyelination

• Occurs with mild compressive or traction force
• The axon is intact - only the myelin sheath of one or more internodes is damaged
• Segments distal and proximal to injury are not affected
• Result: widened node of Ranvier → slowing of conduction velocity across the segment
• May cause asynchronous conduction → paresthesia, loss of vibration sense, reduced reflexes
• More severe compression → conduction block → weakness or sensory loss
• Recovery: Full remyelination by Schwann cell division within weeks to months; new sheath is thinner with more internodes

2. Wallerian Degeneration

• Day 1-2: Disruption of retrograde and anterograde axonal flow; influx of Ca²+ and Na+ through damaged axonal membrane activates proteolytic cascades
• Day 3: Schwann cells retract from nodes of Ranvier; activated Schwann cells and macrophages begin digesting myelin
• ~1 week: Complete degeneration of the distal axon and its myelin ("myelin debris") - the entire process takes approximately 1 week
• Denervated muscle begins to atrophy

• Limited degree of axon breakdown up to the first node of Ranvier
• The cell body undergoes chromatolysis: dissolution and dispersal of Nissl substance (rough ER), eccentric displacement of nucleus, increased protein synthesis
• Chromatolysis represents a switch from axon maintenance to axon regeneration mode

3. Axonal Degeneration (Dying-Back Neuropathy)

• More characteristic of metabolic and toxic disorders (e.g., diabetes mellitus, renal failure, alcoholism)
• Degeneration begins at the distal end of the axon and progresses proximally ("dying back")
• Produces a length-dependent pattern: symptoms start in the feet and ascend (stocking-glove distribution)
• This is the basis of most peripheral neuropathies

---

Part 4: Nerve Regeneration

After Segmental Demyelination (Grade I - Neurapraxia)

• Schwann cell divides and initiates remyelination
• Recovery within weeks to a few months
• New myelin is thinner with more internodes per original internode

After Wallerian Degeneration (Grade II-V)

• Intact adjacent axons send sprouts to reinnervate denervated muscle/skin
• Sprouts arise from nodes of Ranvier (nodal sprouts) or nerve terminals (terminal sprouts) as early as 4 days after injury
• Enlarges surviving motor units
• Clinical recovery: 3-6 months

• Regenerating sprouts grow from the proximal stump guided by Schwann cell tubes (bands of Büngner) - Schwann cells proliferate within the endoneurial tube and align to form a scaffold
• Upregulation of c-Jun protein in Schwann cells switches them from myelination to repair mode
• Growth rate: approximately 1-3 mm/day (rule of thumb: 1 inch/month)
• New myelination occurs once the axon reaches its target
• Regeneration requires endoneurial tube continuity; neurotrophins (NGF, BDNF) guide the growing cone
• If the nerve gap is too large or connective tissue is disrupted, a neuroma forms

Rate of Regeneration and Prognosis

---

Part 5: Neuropathy

Definition

• Mononeuropathy - single nerve affected (e.g., carpal tunnel = median nerve)
• Mononeuritis multiplex - multiple individual nerves (e.g., vasculitis, diabetes)
• Polyneuropathy - diffuse, usually symmetric (most common)

• Large fibre neuropathy - loss of vibration, proprioception, deep tendon reflexes (e.g., B12 deficiency, CIDP)
• Small fibre neuropathy - pain, temperature, autonomic dysfunction (e.g., diabetes, HIV)

• Demyelinating - conduction velocity slowed (e.g., Guillain-Barré syndrome, CIDP, Charcot-Marie-Tooth type 1)
• Axonal - reduced amplitude on nerve conduction study (e.g., most toxic/metabolic neuropathies)

---

Part 6: The Synapse

Definition

Types of Synapses

• Adjacent cells connected by gap junctions (clusters of ion channels)
• Allow free movement of ions from one cell to another
• Bidirectional transmission (unlike chemical synapses)
• Useful for coordinating large groups of neurons (synchronous firing)
• Found in visceral smooth muscle, cardiac muscle, some hypothalamic neurons

Structure of the Chemical Synapse

• Separated from the postsynaptic membrane by the synaptic cleft: 200-300 Å wide
• Contains two key structures:
  • Synaptic vesicles - contain neurotransmitter
  • Mitochondria - provide ATP for neurotransmitter synthesis
• A single anterior motor neuron can receive 10,000 to 200,000 presynaptic terminals
• 80-95% are on dendrites, only 5-20% on the soma

Mechanism of Synaptic Transmission (Step by Step)

1. Action potential arrives at presynaptic terminal
2. Depolarization opens voltage-gated Ca²+ channels in presynaptic membrane
3. Ca²+ influx → triggers SNARE protein complex → vesicle fusion with presynaptic membrane
4. Exocytosis → neurotransmitter released into synaptic cleft
5. Neurotransmitter diffuses across the cleft (200-300 Å)
6. Binds to postsynaptic receptors (ionotropic or metabotropic)
7. Produces EPSP or IPSP depending on transmitter and receptor type

Neurotransmitters

• Acetylcholine (ACh) - NMJ, parasympathetic, basal forebrain
• Norepinephrine - sympathetic postganglionic, locus coeruleus
• Dopamine - substantia nigra, VTA (reward, motor control)
• Serotonin - raphe nuclei (sleep, mood)
• GABA - major inhibitory transmitter (CNS)
• Glycine - inhibitory (spinal cord)
• Glutamate - major excitatory transmitter (CNS)
• Histamine - hypothalamus (wakefulness)

Postsynaptic Receptors

Excitatory and Inhibitory Postsynaptic Potentials

• Produced by opening of Na+/K+ channels or closing of K+ channels
• Depolarizes the postsynaptic membrane (makes it less negative)
• Resting membrane potential of motor neuron soma = -65 mV (Guyton)
• If depolarization reaches threshold (~-45 mV), an action potential fires

• Produced by opening of Cl- channels (GABA-A) or K+ channels
• Hyperpolarizes the membrane (more negative) - moves it away from threshold
• Prevents action potential generation

Summation

"One-Way" Conduction at Chemical Synapses

Synaptic Fatigue and Facilitation

• Facilitation: Repeated stimulation → synapses become more capable of transmitting the same signal (basis of short-term memory)
• Synaptic fatigue: Depletion of neurotransmitter with rapid stimulation → declining EPSP

---

Summary: Key Exam Points (MS-1)