Structure and function of muscles

Functions of the muscles

Movement, stability, control of body openings and passages, heat production (thermogenesis), hormone secretion, glycemic control

Muscular system

Organ system composed of the skeletal muscles, specialized mainly for maintaining postural support and producing movements of the bones

Myology

Study of the skeletal muscles

How many muscles in the human muscular system?

About 600

The three kinds of muscle tissue

Skeletal, cardiac, smooth

• muscle means “little mouse”

What are all three types of muscles specialized to do?

Convert chemical energy in ATP into the mechanical energy of motion

What do skeletal muscle contain?

Muscular tissue, connective tissue, nerves, blood vessels

Endomysium (deepest)

Think sleeve of loose connective tissue around each muscle fiber

• allows room for blood capillaries and nerve fibers to reach each muscle fiber; provides extracelluar chemical environment for muscle fibers and nerve ending

• excitation of muscle fiber is based on exchange of calcium, sodium, and potassium ions between endomysial tissue fluid and nerve and muscle fibers

Perimysium (above endomysium)

Thicker layer of connective tissue that wraps muscle fibers together in bundles fascicles

• carries larger nerves, blood vessels, and stretch receptors (muscle spindles)

Fascicles

Bundles of muscle or nerve fibers wrapped in connective tissue

• 20 to 60 muscle fibers

• visible to naked eye as parallel strands (grain in a cut of meat, beef separates along fascicles)

• multiple fascicles bound together constitute a muscle or nerve as a whole

Epimysium (most superficial)

Fibrous sheath surrounding entire muscle

• blends with fascia; inner surface issues projections between fascicles to form the perimysium

Fascia

Sheet of connective tissue that separates neighboring muscles/muscle groups from each other and from the subcutaneous tissue

• muscles are group in compartments separated from each other by fasciae

Connective tissue of a muscle

How is the strength and direction of how a muscle pulls determined?

By the orientation of the fascicles defined by the perimysium

Fusiform muscles

Thick in the middle with fascicles converging on each tapered end; relatively strong type of muscle

ex. biceps brachii of the arm and gastrocnemius of the calf

Muscle strength is proportional to?

The diameter of a muscle at its thickest point

Parallel muscles

Uniform width and parallel fascicles; few muscle fibers, produces less force than fusiform

• may be elongated straps (ex. rectus abdominis of the abdomin, sartorius of the thigh, zygomaticus major of the face)

• may be more squarish; called quadrilateral (four sided, ex. masseter of the jaw)

• can span long distances (from hip to knee); shorten more than other muscle types

Triangular (convergent muscles)

Fan-shaped—broad at one end with fascicles converging on the other, narrower end; relatively strong because they contain a large number of fibers in the wider part of the muscle

ex. pectoralis major in the chest and temporalis on the side of the head

Pennate muscles (3 types)

Feather-shaped; fascicles insert obliquely on a tendon that runs the length of the muscle

• generates more force because they fit more muscle fibers into a given length of muscle

Unipennate

All fascicles approach the tendon from one side

ex. palmar interosseous muscles of the hand, semimembranosus of the thigh

Bipennate

Fascicles approach the tendon from both sides

ex. rectus femoris of the thigh

Multipennate

Shaped like a bunch of feathers with their quills converging on a single point

ex. deltoid of the shoulder

Circular muscles (sphincters)

Fascicles form rings around certain body openings and passages; when they contract, they constrict the opening and tend to prevent the passage of material through it

ex. orbicularis oculi of the eyelids, external urethra and anal sphincters

Classification of muscles according to fascicle orientation

Muscle compartments

Fasciae package groups of functionally related muscles; stands between the muscles and the overlying hypodermis and skin

• also contains the nerves and blood vessels that supply the muscle group

• occurs in thoracic and abdominal walls, pelvic floor, limbs

Intermuscular septa

Extra thick fasciae between compartments

Muscle compartments

Compartment syndrome

Dangerous and painful condition where high pressure builds up inside an enclosed group of muscles, nerves, and blood vessels (a muscle compartment)

• fascia cannot stretch to make room for the bleeding of severe swelling inside the space

• rising pressure squeezes blood vessels, cutting off the flow of fresh blood, oxygen, and nutrients to the muscles and nerves

• severe cases may lead to amputation

Ischemia

Poor blood flow

Myoglobinuria

Presence of myoglobin in the urine; gives it a dark color, key sign of compartment syndrome/other degenerative muscle disorders

Direct (fleshy) attachment to bone

Little separation between muscle and bone

• at microscopic level, the muscle fibers stop slightly short of the bone and the gap between muscle and bone is spanned by collagen fibers

ex. brachialis and lateral head of the triceps brachii

Indirect attachment to bone

Fibrous cord called a tendon connects muscle to bone

ex. the two ends of the biceps brachii

Tendon

Collagenous band or cord associated with a muscle, usually attaching it to a bone and transferring muscular tension to it

Aponeurosis

Tendon is a broad, flat sheet

• used to refer to the tendon located beneath the scalp, now refers to similar tendons associated with certain abdominal, lumbar, hand, and foot muscles

Retinaculum

Connective tissue band that tendons from separate muscles pass under

ex. one of these covers each surface of the wrist like a bracelet; the tendons of several forearm muscles pass under them on their way to the hand

Intrinsic muscle

Any muscle contained entirely within a region of interest

ex. the muscles of the hand that are contained entirely within the hand

Extrinsic muscles

Muscle that acts upon a designated organ or region but arises from another region

ex. acts upon the hand but originates in the forearm due to size

Additional intrinsic-extrinsic muscle examples

Tongue, larynx, back, foot, other regions

Action

Effect produced by a muscle, whether it is to produce or prevent a movement

• muscles usually act in groups

• four categories: prime mover, synergist, antagonist, fixator

Prime mover/agonist (category of muscle action)

Muscle that produces most of the force during a particular joint action

ex. brachialis flexes elbow

Synergist (category of muscle action)

Muscle that aids in the prime mover

• two or more acting on a joint can produce more power than a single larger muscle

• might not be identical and redundant with prime mover; can cause rotation or other undesirable movements of a bone; may stabilize a joint and restrict movement/modify the direction of movement to be more coordinated and specific

ex. biceps brachii assists brachialis in flexing elbow

Antagonist (category of muscle action)

Muscle that opposes the prime mover

• may relax to give prime mover almost complete control over an action

• mostly maintains some tension on a joint, limits speed/range of prime mover and prevents excessive movement, joint injury, inappropriate actions

ex. triceps brachii extends elbow while brachialis slows extension to stop at appropriate point (represents antagonistic pair)

Antagonistic pair

Muscles that act on opposite sides of a joint

• necessary as muscles can only pull, not push; ex. a single muscle can’t flex and extend elbow

Fixator (category of muscle action)

Muscle that prevents a bone from moving

• holds bone steady, allowing another muscle attached to it to pull on something else

ex. rhomboid muscles fix scapula when biceps contract

Synergistic and antagonistic muscle pairs

Innervation

Nerve that supplies a given muscle

• if nerve is severed, muscle is paralyzed

• knowing innervation enables diagnosis of nerve, spinal cord, and brainstem injuries from muscle tests

• muscles are innervated by nerve branches from spinal and cranial nerves

Spinal nerves

Arise from the spinal cord, emerge through the intervertebral foramina, supply branches to muscles below the neck

• identified by letters and numbers that refer to the adjacent vertebrae, ex. T6 for sixth thoracic nerve, S2 for second sacral nerve

• branches into posterior and anterior ramus

Cranial nerves

Arise from base of the brain, emerge through the skull foramina, innervate muscles of the head and neck

• identified by roman numbers (CN I to CN XII); not all 12 innervate skeletal muscles

Blood supply to muscular system

Receives ~1.24 L of blood per minute at rest (one-quarter of blood pumped by heart)

• may rise as high as three-quarters of cardiac output (11.6 L/min)

• blood capillaries branch extensively through endomysium to reach every muscle fiber; fibers may have surface indentations to accommodate them

• capillaries undulate/coil when muscle is contracted, allowing enough slack to stretch out straight, without breaking, when muscle lengthens

Muscular system, anterior view

Muscular system, posterior view