Muscular Tissue and Function

Functional Overview

• Muscles control all body movements
• Muscles help maintain posture in several positions
• Sphincter control, blood pumping, and the digestive system are enabled via muscles
• Thermogenesis, core temperature regulation, and shivering occurs with generated heat

Types of Muscular Tissue

• Skeletal muscles move the bones of the skeleton to produce movements
• Smooth muscles comprise the walls of hollow organs, blood vessels, and the digestive tract
• Cardiac muscle contraction occurs in the heart chambers

Properties of Muscular Tissue

• Muscles exhibit electrical excitability via producing and propagating action potentials
• Muscles show contractility via contracting and relaxing to produce force/movement
• Muscles are extensible via their capability to stretch within limits
• Muscles exhibit elasticity by returning to their original length after stretching

Skeletal Muscles

• Skeletal muscle tissue comprises muscle fibres (myocytes) containing connective tissue, blood vessels, and nerves
• Separating muscles from the skin is the subcutaneous layer (hypodermis)
• The hypodermis is made of adipose tissue, which stores energy reduces heat loss and cushions trauma
• A pathway is provided for nerves, blood vessels, and lymph vessels in and out of muscle tissue

Skeletal Muscles - Connective Tissue

• Fascia is a band of irregular connective tissue lining body walls / limbs supporting / surrounding muscles and organs
• Muscle groups with similar functions are held together
• Three connective tissue layers extend from the fascia which protect and strengthen the skeletal muscle
• Epimysium is the outer layer
• Perimysium is the middle layer, surrounding groups of muscle fibres and bundling them into fascicles
• Endomysium is the inner layer

Skeletal Muscles - Tendons, Nerves and Blood

• Epimysium, perimysium, and endomysium band together and form tendons (rope-like band of tissue)
• Tendons connect skeletal muscle to bone
• Every nerve penetrating skeletal muscle is accompanied by one artery and two veins
• High metabolic demand creates a rich capillary bed within muscle tissue
• Somatic motor neurons stimulate skeletal muscles

Microscopic Anatomy of Skeletal Myocytes

• Sarcolemma is the plasma membrane of a muscle fibre
• Transverse tubules (T tubules) are tunnels within the sarcolemma which are filled with interstitial fluid
• Myocyte action potentials travel along the sarcolemma and through the T-tubules
• Action potentials ensure simultaneous excitation (and contraction) of the entire muscle fibre
• Myocyte cytoplasm is called sarcoplasm which contains glycogen (to convert to glucose), and myoglobin, which stores and releases oxygen
• Myofibrils are the contractile organelles within the sarcoplasm
• Myofibrils extend the muscle fibre length giving the cell its striped (striated) appearance
• Sarcoplasmic Reticulum (SR) encircles each myofibril
• Sarcoplasmic Reticulum is like endoplasmic reticulum in non-muscular cells
• Sarcoplasmic Reticulum stores and releases calcium ions (Ca2+) to trigger muscle contraction

Microscopic Anatomy Cont.

• Filaments (or myofilaments), are small protein structures within myofibrils responsible for the contractile process
• Thick filaments are myosin proteins
• Thin filaments are actin proteins
• Sarcomeres are compartments of two thin filaments and one thick filament
• A Z-disc separates the sarcomere from the next

Microscopic Anatomy - Muscle Proteins

• Skeletal myocytes contain many different proteins that are contractile, regulatory, and structural
• Actin and Myosin are the primary contractile proteins
• Troponin regulates muscle contraction.
• When muscle is damaged, troponin leaks from the cell into circulation showing high levels when blood tested

Contraction and Relaxation of Skeletal Myocytes

• During a muscular contraction, the size and length of the filaments does not change
• The thin and thick filaments slide past each other pulling on the z-disc which shortens the Sarcomere
• Shortening muscle fibres = muscle contraction
• Sarcoplasm Ca2+ levels trigger muscle contraction. When these levels drop, the muscle relaxes
• Muscle action potentials travelling along the sarcolemma (and through the T-tubules) stimulate the release of calcium ions from the Sarcoplasmic Reticulum (SR) using voltage-gated Ca2+ channels
• Calcium binds to troponin causing a cascading reaction and contraction
• Calcium is continually pumped back into the SR via Ca2+-ATPase pumps during stimulation

Contraction and Relaxation - Rigor Mortis

• Calcium-release channels remain open and Ca2+ flows into the sarcoplasm faster than Ca2+-ATPase pumps as long as action potentials propagate.
• Voltage-gated calcium-release channels close and remaining calcium ions are pumped back into the sarcoplasmic reticulum when action potentials cease.
• Cell membranes become leaky after death and Ca2+ leaks out of the SR (rigor mortis a.k.a. rigidity of death)
• Calcium is not pumped back into the SR without metabolic process (ATP).

Neuromuscular Junction

• The origin of muscle cell action potential is at the neuromuscular junction
• A neuromuscular junction (NMJ) is the synapse between a somatic motor neuron, and a skeletal muscle fibre
• Thousands of acetylcholine (ACh) molecules - cholinergic neurotransmitter, are contained within the axon terminal of the motor neuron
• Sarcolemma opposite the motor neuron includes acetylcholine receptors
• Nerve impulse stimulates ACh release across the synapse (Release of ACh)
• Na+ influx is caused by Activation of ACh receptors - and other ions flow across the cell membrane
• Inflow of Na+ makes the inside of a muscle fibre more positively charged (Production of muscle action potential)
• Acetylcholinesterase (AChE) - breaks down excess ACh remaining in the synaptic cleft (Termination of ACh activity)

Muscle Metabolism

• Skeletal muscle fibres are energy dependent on activity
• Myocytes need a large amount of ATP for metabolic processes
• ATP stored within muscle fibres lasts for a few seconds of contraction, then ATP is produced via one of 3 methods:
  • Creatine Phosphate
  • Anaerobic Glycolysis
  • Aerobic Respiration

Muscle Metabolism - Creatine Phosphate

• While relaxed, muscle fibres make excess ATP, which is converted and stored as creatine phosphate
• The enzyme creatine kinase (CK) catalyses the creatine to ATP process when muscle contraction begins
• Energy stores last for about 15 seconds of maximal muscle contraction

Muscle Metabolism - Glycolysis and Aerobic Respiration

• Anaerobic - without the use of oxygen.
• Glycolysis - catabolism of glucose into pyruvic acid
• Pyruvic acid converts to ATP in presence of oxygen
• During anaerobic conditions, pyruvic acid converts into lactic acid and ATP via anaerobic glycolysis (lactic acid fermentation)
• Anaerobic glycolysis = ~ 2 minutes of muscle activity
• Pyruvic acid turns into ATP in mitochondria during aerobic respiration
• The Krebs Cycle is relevant with Aerobic Respiration
• Requires oxygen, glucose, fatty acids, and proteins for light / moderate exercise

Muscle Metabolism - Fatigue and Oxygen Debt

• Muscle Fatigue - inability to maintain force after prolonged activity
• Inadequate Ca2+ from SR, depleted creatine phosphate, insufficient oxygen, depleted glycogen & buildup of lactic acid are the causes of faigue
• Oxygen Debt (recovery oxygen uptake) is additional oxygen required beyond the resting oxygen consumption after exercise
• During exercise, high RR and HR = more oxygen delivery to cells
• Extra oxygen is used to aid in the restoration of baseline metabolic conditions
• Involves converting lactic acid back to glycogen, resynthesizing creatine phosphate and replacing oxygen released from myoglobin

Muscle Tension

• Motor unit, one somatic motor neuron synapses with ~ 150 skeletal muscles
• Muscles exhibit muscle tone, even at rest
• Muscle tone is important in smooth muscle (digestion, BP)
• Damage to a motor neuron means the muscle becomes flaccid and loses tone

Muscle Tension -Contractions

• Isotonic Contraction - the force of contraction (tension) developed in the muscle, remains almost constant and the muscle changes its length (body movements, moving objects)
• Isometric Contractions - tension does not exceed resistance of the object (holding arm out, body posture, stabilizing joints)

Cardiac and Smooth Muscle Tissure

• Cardiac muscle fibres have several differences from skeletal muscle fibres
• Intercalated discs (irregular transverse thickening of sarcolemma) connect adjacent cardiac myocytes
• Longer contraction-time frame
• Action potential contains a plateau period
• Autorhythmic muscles stimulate their own action (not by impulse)
• Sustained by aerobic respiration Smooth muscle tissues also differ from skeletal muscles
• There are no sarcomeres, so there is no "striated" appearance
• Sarcoplasmic reticulum / transverse tubules are absent
• Contractions last longer

Muscle Movement

• Skeletal muscles produce movement by exerting force via tendons
• Movement is produced when bones act as levers and joints act as fulcrums
  • Rigid structures that move are levers
  • Fixed points are fulcrums
• Levers are activated by two forces: the effort which CAUSES the movement, and the load (resistance) which opposes movement

Muscle Names

• Muscle names are characterized based on direction, size, action, shape, number of origins, and location
• Direction- orientation of muscle fascicles
• Size- relative size of the muscle
• Action- principal action of the muscle
• Shape- relative shape of the muscle
• Number of origins
• Location- structure near which a muscle is found

Major Muscle Groups

• Sternocleidomastoid
• Scalene
• Trapezius
• Deltoid
• Biceps Brachii
• Rectus Abdominis
• Quadriceps group
• Latissimus Dorsi
• Triceps Brachii

Major Muscle- Neck and Shoulder

• The Deltoid is in the shoulder (anterior, lateral, posterior) which is a main prehospital IM injection site
• Neck Muscles are accessory respiratory muscles
  • Sternocleidomastoid
  • Scalene
  • Trapezius

Major Muscle - Quadriceps

• The quadriceps consists of the following muscles:
  • Rectus femoris
  • Vastus lateralis
  • Vastus medialis
  • Vastus intermedius

Building and Losing Muscle

• Mature skeletal muscle fibres cannot undergo cell division
• The growth of skeletal muscle after birth is due to hypertrophy - the enlargement of existing cells
• Exercise promotes skeletal muscle hypertrophy
• Between the ages of 30-50, humans undergo a atrophy/progressive loss of skeletal muscle mass, replaced by fibrous connective tissue and adipose tissue
• Loss causes decreased strength, slowed muscle reflexes and loss of flexibility