Muscle Tissue: Types and Function

• Muscular tissue enables body movement through muscle cell contraction.
• Muscle cells, or fibers, convert chemical energy from nutrients into mechanical energy.
• There are three types of muscle tissue: skeletal, cardiac, and smooth.

Skeletal Muscle

• Skeletal muscle is also known as striated or voluntary muscle.
• It makes up 40% to 50% of body weight.
• Microscopically, it appears with crosswise stripes, called striations.
• Contractions are voluntarily controlled.

Cardiac Muscle

• Cardiac muscle forms the bulk of the heart.
• Cardiac muscle cells branch frequently.
• It contains intercalated disks.
• The interconnected nature of cells allows the heart to contract efficiently as a unit.

Smooth Muscle

• Smooth muscle is also called nonstriated, involuntary, or visceral muscle.
• It lacks cross stripes, appearing smooth under a microscope.
• It is found in the walls of hollow visceral structures.
• Contractions aren't under voluntary control.

Muscle Function

• All muscle cells specialize in contraction.

Structure of Skeletal Muscle

• Muscle organs consist of skeletal muscle cells and connective tissue.
• Connective tissue surrounds muscle fibers, fascicles, and the entire muscle.
• Fascia surrounds muscle organs and nearby structures.
• Most skeletal muscles extend from one bone across a joint to another bone.
• The origin is the attachment to a stationary bone.
• The insertion is the point of attachment to the moving bone.
• The body is the main part of the muscle.
• Muscles attach to bones through tendons.
• Tendon sheaths are synovial-lined tubes lubricating some tendons.
• Bursae are synovial-lined sacs between tendons and bones.

Muscle Fibers

• Contractile cells, or muscle fibers, are grouped into bundles.
• Myofilaments form cylinders: thick myofilaments (myosin) and thin myofilaments (actin).
• Sarcomeres are basic contractile units separated by Z lines.
• The sliding filament model explains muscle fiber contraction.
• Thick and thin myofilaments slide past each other during contraction.
• Contraction requires calcium and ATP.

Function of Skeletal Muscle

• Muscles contract, pulling on bones and producing movement.
• The insertion bone is pulled closer to the origin bone.
• Movement occurs at the joint between the origin and the insertion.
• Groups of muscles often contract to produce a single movement.
• The prime mover is mainly responsible for producing a given movement.
• A synergist assists the prime mover.
• An antagonist opposes the prime mover's action.

Posture

• Tonic contraction maintains body position.
• Only a few muscle fibers shorten at a time during tonic contraction.
• Tonic contractions produce no movement of body parts.
• Tonic contractions maintain muscle tone called posture.
• Good posture involves optimum body positioning.
• Skeletal muscle tone counteracts gravity to maintain posture.

Heat Production

• Body temperature maintenance is crucial for survival.
• Fever indicates an elevated body temperature.
• Hypothermia means the body temperature is below normal.
• Muscle fiber contraction produces heat to maintain body temperature.

Fatigue

• Fatigue is the reduced strength of muscle contraction.
• It's caused by repeated muscle stimulation without adequate rest.
• Repeated contraction depletes ATP stores and outstrips oxygen and nutrient supply.
• Inadequate oxygen leads to lactic acid production, causing muscle burn.
• Oxygen debt is the effort to burn excess lactic acid after exercise.
• Labored breathing after exercise "pays the debt" and restores energy and oxygen reserves.

Integration with Body Systems

• Muscle function relies on other body systems.
• Muscles cause movement by pulling on bones across movable joints.
• Respiratory, cardiovascular, nervous, muscular, and skeletal systems play roles in normal movements.
• Conditions like multiple sclerosis, brain hemorrhage, and spinal cord injury can affect movement.

Motor Unit

• A nerve impulse stimulates a muscle before contraction.
• A motor neuron transmits an impulse to a muscle.
• The neuromuscular junction (NMJ) is between a nerve ending and muscle fiber.
• Neurotransmitters cross the NMJ to trigger contraction.
• Acetylcholine (ACh) is the neurotransmitter at the NMJ.
• A motor unit is a motor neuron and the muscle cell(s) it innervates.

Muscle Stimulus

• A muscle contracts only if the stimulus reaches a threshold level.
• Different muscle fibers are controlled by different motor units with varying thresholds.
• Varying numbers of motor units activate to execute contractions of graded force.

Types of Muscle Contraction

• Twitch contractions are quick, jerky responses, not significant in normal activity.
• Tetanic contractions are sustained and steady, caused by rapid series of stimuli.
• Isotonic contractions produce movement at a joint, changing muscle length.
• Concentric contractions shorten muscles
• Eccentric contractions lengthen muscles.
• Walking and running are caused by isotonic contractions.
• Isometric contractions do not produce movement, and the muscle doesn't shorten.
• Tension within the muscle increases during isometric contractions.

Effects of Exercise

• Regular exercise improves muscle tone and posture, enhances heart and lung function, and reduces fatigue.
• Muscles adapt based on the amount of work they do.
• Prolonged inactivity causes disuse atrophy.
• Regular exercise increases muscle size, called hypertrophy.
• Strength training involves contracting muscles against heavy resistance that increases the number of myofilaments.
• Endurance training increases a muscle’s ability to sustain moderate exercise.
• Endurance training allows efficient oxygen and nutrient delivery, and it does not usually result in muscle hypertrophy.

Movements Produced by Muscles

• Angular movements include flexion (decreasing an angle), extension (increasing an angle), abduction (away from the midline), and adduction (toward the midline).
• Circular movements include rotation (around an axis) and circumduction (moving a distal end in a circle).
• Special movements include dorsiflexion and plantar flexion (foot movements), and inversion and eversion (sideways foot movements).
• Muscles can be named or grouped according to function or movement.

Skeletal Muscle Groups

• Muscles of the head and neck include facial muscles (orbicularis oculi, orbicularis oris, zygomaticus), muscles of mastication (masseter, temporal), sternocleidomastoid (flexes head), and trapezius (elevates shoulders and extends head).
• Muscles of the upper extremities include pectoralis major (flexes arm), latissimus dorsi (extends arm), deltoid (abducts arm), biceps brachii (flexes forearm), brachialis (flexes forearm), triceps brachii (extends forearm), flexor muscles in forearm (flex wrist and hand), and extensor muscles in forearm (extend wrist and hand).
• Muscles of the trunk include abdominal muscles (rectus abdominis, external oblique, internal oblique, transversus abdominis) and respiratory muscles (intercostal muscles, diaphragm).
• Muscles of the lower extremities include iliopsoas (flexes thigh), gluteus maximus (extends thigh), adductor muscles (adduct thighs), hamstring muscles (semimembranosus, semitendinosus, biceps femoris - flex leg and extend thigh), quadriceps femoris group (rectus femoris, vastus muscles - extend leg), tibialis anterior (dorsiflexes foot), gastrocnemius (plantar flexes foot), and fibularis (peroneus) group (flexes foot).

Muscular Disorders

• Myopathies are muscle disorders ranging from mild to life-threatening.
• A strain is an injury from overexertion.
• A strain involves stretching or tearing of muscle fibers.
• Strains are often accompanied by myalgia (muscle pain).
• Strains may result in myositis (muscle inflammation) or fibromyositis (muscle and tendon inflammation).
• Sprains involve ligament damage near a joint.
• Cramps are painful muscle spasms.
• Crush injuries may cause kidney failure due to released cell contents.
• Stress-induced muscle tension can cause headaches and back pain.
• Muscle infections can be caused by bacteria, viruses, and parasites.
• Poliomyelitis is a viral infection of motor nerves.
• Tetanus infections are caused by C. tetani bacteria that release a toxin, causing tetanic muscle contractions.
• Muscular dystrophy (MD) is a group of genetic disorders with muscle atrophy.
• Duchenne muscular dystrophy (DMD) is the most common type, characterized by rapid weakness and atrophy.
• Myasthenia gravis is an autoimmune muscle disease causing weakness and fatigue.

Explore the three types of muscle tissue: skeletal, cardiac, and smooth. Learn about their unique characteristics, locations, and functions within the body. Understand how these tissues enable movement and maintain bodily functions.