Anatomy Module 3: Muscular System

myology

the study of the muscular system

functions of muscle

1. Motion — enables movement of joints for ambulation, reaching, and activities of daily living

2. Protection — muscles overlie (are superficial to) bone, providing a cushioning effect

3. Circulation support — skeletal muscle contraction helps pump venous blood back to the heart against gravity (no separate "pump" exists for venous return)

   • Example: the soleus muscle (calf) is nicknamed the "peripheral heart" because its contraction is essential for venous return from the lower leg

   • Clinical relevance: bedridden patients are instructed to perform ankle pumps to promote circulation, since standing/walking muscle activity isn't available to assist venous return

4. Insulation / thermoregulation

   • Goosebumps = muscular contraction around hair follicles (attempt to retain heat)

   • Shivering = muscle activity generating heat energy

   • (Note: the integumentary system is the primary insulator, but muscle contributes significantly)

5. Pumping blood (cardiac muscle) — the heart itself is composed entirely of cardiac muscle

6. Vascular/visceral wall function (smooth muscle) — forms the walls of arteries/veins and the GI tract (small/large intestine)

   • Enables vasoconstriction/vasodilation of blood vessels

   • Enables peristalsis (propulsion of food/waste through the GI tract)

sympathetic nervous system

("fight-or-flight"): constricts blood vessels to GI tract, redirects blood to skeletal muscle/extremities for action

• This constriction/dilation is only possible because vessel walls are muscular (smooth muscle) — connective tissue like ligaments or cartilage cannot actively constrict/dilate

parasympathetic nervous system

("rest-and-digest"): dilates central/GI blood vessels, constricts peripheral vessels, prioritizing digestion\

• This constriction/dilation is only possible because vessel walls are muscular (smooth muscle) — connective tissue like ligaments or cartilage cannot actively constrict/dilate

skeletal muscle

Fibers arranged in parallel

voluntary control

Biceps, triceps, quadriceps, hamstrings, etc.

fatigable

uses aerobic glycolysis (oxygen/hemoglobin dependent) energy pathway

cardiac muscle

Network/ridge-like (branching) arrangement

involuntary control

heart only

not fatigable

oxygen-dependent energy pathway but does not fatigue

smooth muscle

Fusiform (spindle-shaped) cells

involuntary control

Blood vessel walls, GI tract (small/large intestine)

not fatigable

oxygen-dependent energy pathway but does not fatigue

shared properties of all muscle tissue

1. Tone — maintains a baseline resting length/structure even when relaxed

   • Controlled by the central nervous system

   • Hypertonicity/spasticity — abnormally increased tone (seen after stroke, in spastic cerebral palsy, etc.)

2. Extensibility — ability to stretch (like a rubber band); contrasted with rigid tissues like cartilage/ligament, which would tear/break if stretched similarly

3. (Passive) Elastic recoil — ability to passively return to resting length after being stretched (distinct from active contraction)

4. Contractility — ability to actively contract (e.g., biceps flexing the elbow; heart contracting to pump blood)

tone

1. maintains a baseline resting length/structure even when relaxed

   • Controlled by the central nervous system

   • Hypertonicity/spasticity — abnormally increased tone (seen after stroke, in spastic cerebral palsy, etc.)

extensibility

ability to stretch (like a rubber band); contrasted with rigid tissues like cartilage/ligament, which would tear/break if stretched similarly

(passive) elastic recoil

ability to passively return to resting length after being stretched (distinct from active contraction)

contractility

ability to actively contract (e.g., biceps flexing the elbow; heart contracting to pump blood)

Layers Superficial to Deep (Skin → Muscle)

1. Epidermis (skin)

2. Dermis (skin)

3. Superficial fascia — yellowish, adipose (fatty) tissue layer

4. Deep fascia — denser connective tissue layer

5. Muscle

myofibril/muscle fiber

Smallest unit of the muscular system

fascicle

Bundle of muscle fibers

covered by perimysium

endomysium

Innermost layer; surrounds individual muscle fibers

perimysium

Surrounds bundle of 10-100 fibers called fascicles

muscle

a bundle of fascicles

covered by epimysium

epimysium

Outer connective tissue layer; surrounds entire muscle

tendon

how muscle connects to bone which anchors into periosteum

deep fascia in muscles

covers individual muscles and separates groups of muscles into compartments

• Each compartment contains its own muscles plus the blood vessels and nerves that supply them

• Example — thigh cross-section (3 compartments):

  • Anterior compartment — quadriceps

  • Posterior compartment — hamstrings

  • Medial compartment — adductors

compartment syndrome

• During exercise, muscles hypertrophy (temporarily swell with increased blood flow) within their fixed-volume fascial compartment

• Because the compartment is a closed space, swelling muscle → increased intra-compartmental pressure → compression of blood vessels and nerves within that compartment

• Symptoms can include tingling (e.g., in the toes during prolonged running)

• Treatment for severe/acute compartment syndrome: fasciotomy — surgically opening the fascia to relieve pressure

• Will be revisited in detail when covering the lower leg (shin splints, compartment syndrome)

myofascial release

a treatment technique targeting adhesions that form between muscle and fascia

neuromuscular junction

the connection point between a nerve ending and a single muscle fiber

motor unit

 one nerve fiber + all the muscle fibers it innervates (the smallest functional unit of contraction)

Each muscle (e.g., biceps) contains thousands of muscle fibers organized into many motor units

Action Potential & the All-or-None Law

• Nerve impulse → creates a potential difference (imbalance of positive/negative ions) across the muscle cell membrane

• This potential difference triggers an influx of ions (sodium, calcium, potassium) into the muscle cell, initiating contraction

• Action potential = the threshold of potential difference required to trigger a muscle contraction

• All-or-none law: all muscle fibers within a motor unit contract simultaneously, or none do — because they all receive the nerve impulse at the same time

action potential

the threshold of potential difference required to trigger a muscle contraction

all-or-none law

all muscle fibers within a motor unit contract simultaneously, or none do — because they all receive the nerve impulse at the same time

sarcomere

•  the repeating contractile unit of a muscle fiber, bounded by two "light lines"; many sarcomeres arranged in series make up one muscle fiber

• Within each sarcomere:

  • Actin filament — thin filament (protein)

  • Myosin filament — thick filament (protein), with a mobile "head"

  • Troponin — protein with affinity for calcium; normally bound to actin

  • Tropomyosin — winding, cord-like protein that holds troponin in place along the actin filament

actin filament

• thin filament (protein)

myosin filament

thick filament (protein), with a mobile "head"

troponin

protein with affinity for calcium; normally bound to actin

tropomyosin

winding, cord-like protein that holds troponin in place along the actin filament

contraction sequence

1. Nerve impulse → potential difference → calcium influx into the muscle cell

2. Troponin (bound to tropomyosin) has a strong affinity for calcium → releases its hold on actin and binds to calcium instead

3. With troponin/tropomyosin no longer blocking actin, the myosin heads bind to the now-exposed actin

4. Myosin heads "walk" along the actin filament, pulling the actin and myosin filaments to slide past each other

5. As filaments slide together, the sarcomere shortens → repeated across thousands of sarcomeres/fibers = whole-muscle contraction

6. Contraction continues as long as the nerve impulse (and calcium influx) continues

7. When the nerve signal stops: calcium is removed, troponin/tropomyosin re-bind to actin, and the muscle returns to its resting (relaxed) state

energy requirements for contraction

• Muscle contraction requires ATP (adenosine triphosphate), produced via cellular respiration (Krebs cycle) — not covered in detail in this course, as it belongs to physiology

• Hemoglobin (in blood) and myoglobin (in muscle) both bind oxygen for transport/storage — proteins are fundamental building blocks for these processes

fast twitch skeletal muscle fiber

short bursts of high-intensity power

Whitish (lower hemoglobin/myoglobin content)

fatigues quickly

Sprinting (100m dash), weightlifting

slow twitch skeletal muscle fiber

Sustained, endurance-based activity

Darker (higher hemoglobin/myoglobin content, more blood supply)

resists fatigue

Marathon/distance running

intermediate twitch skeletal muscle fiber

Combination of power + endurance

intermediate color

moderate fatigability

Brisk walking, general daily activity

Clinical/rehab application — Multiple Sclerosis (MS) example

• MS is a demyelinating disorder — nerves lose their myelin sheath (the "insulation" that allows efficient electrical signal transmission)

• Without myelin, nerves must expend more energy to transmit the same impulse → contributes to the hallmark symptom of fatigue in MS flare-ups

• Rehab approach: focus on energy conservation — train patients to recruit slow-twitch fibers more (lower energy cost, sustained output) rather than fast-twitch (high energy cost, quick fatigue)

biomechanics: muscles as levers

Muscle-joint systems function like seesaws/levers, involving:

• Fulcrum (F) — the pivot point (typically the joint)

• Effort (E) — the muscular force

• Resistance (R) — the weight/load being moved

• First-order arrangements can offer the most mechanical efficiency, but the human body cannot be redesigned to always favor this arrangement

• What can be adjusted clinically: the distance between resistance and fulcrum (or effort and fulcrum)

  • Example: External rotation strengthening — start with the arm/resistance close to the body (shorter lever arm = easier), then progress by moving the resistance farther from the body (longer lever arm = harder) as the patient improves

  • This principle is used to progress rehab exercises by manipulating leverage/distance, since the underlying tissue anatomy itself can't be changed

first-order level muscle lever

Fulcrum in the middle, effort and resistance on either side

Head/neck extension — posterior neck muscles (effort) pull the head up; the resistance (head's weight) is anterior to the spinal fulcrum

second-order level muscle lever

Resistance in the middle, fulcrum and effort on either end

Standing on tiptoes (calf raise) — fulcrum = ball of foot, effort = calf muscles, resistance = body weight (in the middle)

third-order level muscle lever

Effort in the middle, fulcrum and resistance on either end

Bicep curl — fulcrum = elbow joint, effort = biceps (middle), resistance = dumbbell (farthest from fulcrum)

agonist muscle

The primary muscle(s) actively producing a given motion

Biceps during elbow flexion

antagonist muscle

The opposing muscle group that must lengthen/relax (and may control the motion eccentrically) to allow the agonist's action

Triceps lengthening (controlling the descent) during a slow, controlled elbow extension after a bicep curl

synergist muscle

Muscles that assist/support a movement without directly producing the primary action themselves

Rotator cuff muscles (e.g., supraspinatus) and scapular stabilizers assisting shoulder abduction alongside the deltoid

stabilizer muscle

Muscles that stabilize a joint/region to allow the prime movers to act efficiently

(Scapular stabilizers during arm elevation)

Primary vs. Secondary (Synergistic) Muscle Actions

• Many muscles cross more than one joint and therefore have both a primary action and secondary/synergistic actions

• Example: Quadriceps group (4 muscles total)

  • Rectus femoris — a two-joint muscle: primary action = knee extension; secondary action = assists with hip flexion

  • Vastus muscles (3 of the 4 quadriceps muscles) — one-joint muscles: knee extension only

• Clinical relevance: When a muscle is weakened (e.g., due to nerve injury), both its primary action AND its secondary/synergistic actions will be affected — important for understanding the full functional impact of nerve injuries or muscle weakness