Functions of Muscles

• Muscles make up nearly half of the body’s weight.
• There are approximately 600 human skeletal muscles.
• The three types of muscle tissue are: skeletal, cardiac, and smooth.
• Muscles are specialized to convert the chemical energy in ATP into mechanical motion.

Functions of Muscle

• Movement is achieved through muscle contractions.
  • Skeletal muscles move the skeleton.
  • Cardiac muscle moves blood.
  • Smooth muscle moves "stuff" through hollow organs.
• Skeletal muscles maintain posture and provide joint stabilization.
• Thermogenesis, or heat generation, occurs when contracting skeletal muscles produce approximately 85% of body heat through working out or shivering.
• Venous Return: Muscle pump action during dynamic exercise pumps blood back to the left ventricle, increasing venous blood return via rhythmic contraction/relaxation.
• Protein becomes an alternative energy source during malnutrition or starvation.
• Glycemic Control: Muscles absorb and store glucose, regulating blood sugar within a normal range.
• Sphincters, which are internal smooth muscle rings, control the movement of food, blood, and other materials within the body, controlling openings and passageways.

Functional Characteristics of Muscle

• Contractility is the ability to shorten and thicken, generating force to do work, and muscles only pull.
• Excitability is the ability to respond to stimuli by producing electrical signals.
• Extensibility is the ability to stretch without being damaged.
• Elasticity is the ability to return to its original length/shape following distension.

Classification by Function

• Action: effect produced by a muscle to produce or prevent movement.
• Four Functional Groups of muscles: Agonists, Antagonists, Synergists, and Fixators.
• Agonists (Prime Movers): provide the major force for producing a specific movement.
• Antagonists: oppose or reverse a particular movement.
• Synergists: muscles that aid the prime mover.
• Fixators: muscles that prevent movement of bone.
• Muscles may perform any of these roles depending on the movement.

Naming Skeletal Muscles

• Latin names explain the Muscle's Location, Shape, Relative Size, Fiber Direction, Attachment Location, Number of Origins, and Action.
  • Location: Intercostal (between ribs)
  • Shape: Deltoid (triangle)
  • Relative Size: Maximus, Minimus, Longus, Brevis
  • Direction of Fibers: Rectus, Transverse, Oblique
  • Location of Attachments: Sternocleidomastoid
  • Number of Origins: Biceps, Triceps
  • Actions: Flexor, Extensor, Abductor, Adductor

Muscle Mechanics: Arrangement of Fascicles

• Circular: Fascicles are arranged in concentric rings (e.g., orbicularis oris).
• Convergent: Fascicles converge toward a single tendon insertion (e.g., pectoralis major).
• Parallel: Fascicles are parallel to the long axis of a straplike muscle (e.g., sartorius).
• Fusiform: Spindle-shaped muscles with parallel fibers (e.g., biceps brachii).
• Pennate: Short fascicles attach obliquely to a central tendon running the muscle's length (e.g., rectus femoris).

Muscle Attachments

• Indirect attachment to bone: tendons connect muscle to bone.
  • Collagen fibers of the endo-, peri-, and epimysium continue into the tendon and from there into periosteum and the matrix of bone.
• Aponeurosis: tendon is a broad, flat sheet.
• Retinaculum: connective tissue band that tendons from separate muscles pass under.

Sites of Attachment

• Origin: the point of muscle attachment on the immovable or less movable bone; in limbs, the origin is usually proximal to the insertion.
• Insertion: the point of muscle attachment that moves toward the origin.

Muscle Innervation

• Innervation: refers to the identity of the nerve that stimulates it.
  • Knowing innervation enables diagnosis of nerve, spinal cord, and brainstem injuries from muscle tests.
• Spinal nerves arise from the spinal cord.
  • They emerge through intervertebral foramina.
  • Spinal Nerves immediately branch into posterior and anterior rami.
  • Spinal Nerves innervate muscles below the neck.
  • Plexus: web-like network of spinal nerves adjacent to the vertebral column.
• Cranial nerves arise from the base of the brain.
  • Cranial Nerves emerge through skull foramina.
  • Cranial Nerves innervate the muscles of the head and neck.
  • Cranial Nerves are numbered CN I to CN XII.