Muscle Tissue: Types, Properties, Functions

Muscle Tissue Types

• Skeletal muscle is voluntary and multinucleated
• Cardiac and smooth muscle are involuntary
• Smooth and cardiac muscle tissue have one or two nuclei
• Skeletal and cardiac muscle tissues are striated
• Cardiac muscle has intercalated discs
• Muscle tissue differences include nervous control, thin and thick filament binding

Four Special Muscle Properties

• Excitability (or irritability): Ability to receive and respond to stimuli
• Contractility: Ability to shorten forcibly when stimulated
• Extensibility: Ability to be stretched without damage
• Elasticity: Ability to recoil to its resting length after stretching

Major Functions of Muscle Tissue

• Movement
• Posture maintenance
• Movement of body structures
• Joint stabilization
• Heat generation
• Skeletal muscles facilitate locomotion and manipulation
• Skeletal muscles contract continuously to maintain posture via spinal reflexes
• Smooth muscle maintains blood pressure in blood vessel walls
• Smooth muscle contracts in digestive, urinary, and reproductive organs
• Muscles stabilize skeleton joints
• Skeletal muscles contract to generate heat, raising body temperature

Connective Tissue Layers

• Epimysium: surrounds entire skeletal muscle
• Perimysium: surrounds fascicle
• Endomysium: surrounds individual muscle fiber
• Muscle fascicle: bundle of muscle fibers running parallel in skeletal muscle

Sarcomere Structure

• Sarcomere definition: structural and functional unit of skeletal muscle
• Major regions of sarcomere: H zone, A band, Z disc (line), I band
• H zone: contains only actin fibers and disappears when muscle contracts
• A band: contains both myosin and actin filaments, appears darker, and runs the length of myosin
• Z disc (line): border of the sarcomere separating neighboring sarcomeres
• I band: contains only actin filaments and is lighter than A bands; has Z disc (line) in the center

Sarcolemma and Related Structures

• Sarcolemma: Plasma membrane of sarcomere with Na+ and K+ gates for action potentials
• Sarcoplasmic reticulum: Organelle storing Ca+ ions for cross-bridge coupling to be released into the sarcoplasm
• Myofibril: Thread-like contractile organelle of skeletal muscle fiber
• T (transverse) tubules: Sarcolemma extensions running inward to terminal cisternae of sarcoplasmic reticulum
• Sarcoplasm: Muscle cell cytoplasm, receiving Ca+ ions from sarcoplasmic reticulum
• Triad: T (transverse) tubule along with two terminal cisterns for sarcoplasmic reticulum
• Myoglobin: Oxygen-binding molecule in muscle
• Glycosomes: Organelles within skeletal muscle containing glycogen

Sliding Filament Model

• Thin filaments slide past thick filaments
• Thin filament consists of actin
• Thick filament consists of myosin
• Myosin filaments appear thicker and darker
• Actin filaments appear thinner and lighter

Neuromuscular Junction

• Neuromuscular junction definition: Axon terminal associated with skeletal muscle
• Somatic motor fibers: The nerves going to the skeletal muscle
• Motor end plate: Region of sarcolemma with acetylcholine (ACh) receptors

Steps for Skeletal Muscle Contraction

• ACh is released into synaptic cleft, binds to ACh receptors
• Na+ channels open, leading to Na+ influx and action potential initiation on sarcolemma

Excitation-Contraction Coupling

• Action potential excites sarcolemma, leading to depolarization and repolarization
• Depolarization: Na+ ions move from ECF to ICF through Na+ protein channel
• Repolarization: K+ ions move from ICF to ECF through K+ protein channel
• Action potential travels down sarcolemma into T (transverse) tubules
• Release of Ca2+ ions from terminal cisternae of sarcoplasmic reticulum, leading to contraction

Motor Unit

• Motor unit definition: A somatic motor neuron and all the skeletal muscle fibers it stimulates
• Motor unit variation:
  • Finely controlled motor units: one motor neuron, few muscle fibers (e.g., orbicularis oculi)
  • Coarsely controlled muscle: one motor neuron, many muscle fibers (e.g., gluteus maximus)

Contraction Molecules

• Calcium and ATP are required for skeletal muscle contraction
• Calcium interacts with troponin, making myosin binding site on actin available
• ATP interacts with myosin (myosin acts as an ATPase to breakdown ATP), forming cross-bridge with actin
• Troponin and tropomyosin are the two regulatory molecules that block the myosin binding site on actin
• Cross-bridge cycle: myosin forms cross bridges with actin
• Myosin heads (ATPases) energized by ATP move to bind to myosin binding sites on actin
• Two ATPs are required for each cycle

ATP and Muscle Contraction

• Myosin pulls actin towards sarcomere center, allowing myofilaments to slide
• Another ATP is required for myosin (ATPase) to detach from the actin binding site
• Muscle relaxation occurs when Ca+ returns to storage and ACh is deactivated by acetylcholinesterase

Rigor Mortis

• Rigor mortis definition: Process after death due to lack of ATP to break cross bridges in muscle tissue

Muscle Twitch Analysis

• Periods of a muscle twitch: dormant, contraction, and relaxation, measured on a myogram
• Latent period: Occurs after excitation-contraction coupling as Ca2+ begins exiting sarcoplasmic reticulum
• Contraction period: Cross bridges are active, myogram tracing rises to a peak
• Relaxation period: Ca2+ is pumped back into the sarcoplasmic reticulum

Energy and Muscle

• Three types of muscle metabolism: direct phosphorylation, anaerobic respiration, and aerobic respiration
• Muscle metabolism produces ATP
• Enzymes and heat increase metabolic rate

Direct Phosphorylation

• Direct phosphorylation defined: Transfer of phosphate group from creatine phosphate to ADP to make ATP
• Process provides energy for 15 seconds

Anaerobic Respiration

• Glycogen broken down into glucose, undergoes glycolysis without oxygen
• Pyruvic acid is converted to lactic acid
• Two ATP produced per glucose molecule
• Anaerobic respiration energy lasts approximately 60 seconds

Aerobic Respiration

• Glycogen converted to glucose and enters glycolysis in the presence of oxygen
• Pyruvic acid produced and leads to cellular metabolism for more ATP
• 36 ATP is produced per glucose molecule
• Complete glucose breakdown occurs, CO2, H2O, and 36 ATP are produced during Kreb's cycle (citric acid cycle)
• ATP production, 40% becomes mechanical energy (myosin, cross-bridges)
• Remaining 60% of ATP generates heat via shivering
• Blood moves heat to superficial layers through capillaries and sweating
• Aerobic respiration energy lasts for hours
• Anaerobic respiration provides the most strength
• Ionic imbalances, lactic acid production, and lack of muscle contraction occur when ATP production cannot keep up with demands of the body

Oxygen Debt

• Oxygen debt definition: Amount of oxygen needed to restore the muscle to its resting state

Muscle Twitch and Response

• Muscle twitch: Brief contraction of all muscle fibers in motor unit in response to action potential
• Threshold stimulus: Lowest stimulus strength required for muscle contraction
• Graded muscle responses: Number of action potentials affecting muscle

Contraction Variation

• Graded Muscle Responses vary based on motor unit recruitment level
• Treppe: Muscle contractions become more efficient, and wave summation begins
• Characteristics of treppe stage: Complete contractions, first contraction is weakest, subsequent contractions are stronger with an increase in heat

Tetanus

• Incomplete tetanus: Abnormal muscle contraction (unfused wave summation)
• Complete/fused tetanus: Loss of muscle relaxation, sustained contraction from twitches

Temporal Wave Summation

• Temporal wave summation definition: A second stimuli occurs before muscle has a chance to relax, leading to fused muscle summation

Types of Muscle Contractions

• Isometric vs isotonic (concentric and eccentric) contractions
• Isometric contractions: No change in length, no sarcomere shortening, cross-bridges form (no power stroke)
• Isotonic contractions: Myofilaments slide over one another because myosin was forming cross-bridges and pulling the actin filaments towards the H zone

Isotonic Contraction types

• Shortening in concentric contractions
• Lengthening in eccentric contractions occurs

Muscle Tone

• Muscle tone definition: Slightly contracted state from motor unit stimulation alteration, due to spinal reflexes
• Force influence: Degree of muscle stretch, frequency of stimulation, and number of muscle fibers

Skeletal Muscle Fibers

• Slow oxidative fibers: High oxygen environment, fatigue-resistant, slow ATPases (myosin), and aerobic metabolism (36 ATP/glucose)
• Fast glycolytic fibers: Low oxygen environment, high glycogen, quick to fatigue, fast ATPases (myosin), and anaerobic metabolism (2 ATP/glucose)
• Fast glycolytic fibers have more strength but less speed due to limited ATP
• Muscles cannot contract due to fatigue after prolonged tetanus until stores are replenished
• Fast oxidative fibers are both aerobic and anaerobic

Muscle Growth

• Hypertrophy: Excessive muscle development
• Atrophy: Muscle degeneration from lack of use for extended period
• Hyperplasia: increase in the amount of muscle cells

Smooth Muscle Functions

• Maintaining blood pressure in cardiovascular system
• Food propulsion in gastrointestinal (GI) tract
• Shunting blood between organs in cardiovascular system

Smooth Muscle Layers

• Circular layer is directly surrounding the lumen
• Longitudinal layer

Key Differences - Skeletal Muscle vs Smooth Muscle

• Calcium binds to calmodulin (smooth muscle) instead of troponin (skeletal muscle)
• Varicosities for neurotransmitters to bind to in smooth muscle
• Smooth muscle adapts to stretch, while skeletal muscle does not
• Caveolae (smooth muscle) instead of T-tubules (skeletal muscle)
• Smooth muscle contracts more slowly than skeletal muscle

Smooth Muscle Arrangement

• Smooth muscle cells arranged in sheets around hollow organs (blood vessels or GI tract) Contraction in smooth muscle cells In unison
• Peristalsis: Alternating contraction and relaxation of smooth muscle in the GI tract that moves food along
• Acetylcholine causes smooth muscle contraction
• Norepinephrine relaxes bronchioles and contracts blood vessels

Organization of Skeletal Muscles

• Skeletal muscles organized into functional groups
• These groups are separated by connective tissue
• Functional groups include prime mover (agonist), antagonist, synergist, and fixator.
• Prime mover (agonist): responsible for producing specific movement
• Antagonist: opposes prime mover (agonist) or reverses movement
• Synergist: Reduces undesirable or unnecessary movement
• Fixator: Stabilizes agonist origin

Naming Muscles

• Muscle names based on location, shape, size, direction of fibers/fascicles, number of origins, attachment location, and action
• Triangular fascicle arrangement converges at single tendon
• Examples of direction of muscle fibers/fascicles: rectus, transversus, and oblique.
• Oblique direction in diagonal arrangement
• Circular fascicle arrangement surrounds opening (sphincter), e.g., orbicularis oris
• Multipennate fascicle arrangement inserts into multiple tendons tapering to common tendon, e.g., deltoid
• Convergent fascicle arrangement: widespread expansion that come to a point, e.g., pectoralis major
• fusiform arrangement: spindle shaped w/ wide centre, biceps brachii
• Parallel fascicle arrangement: with fibres that run parallel, sartorius
• Bipennate fascicle arrangement: converge on both sides, rectus femoris
• Unipennate fascicle arrangement: fibers converge to 1 side of tendon, extensor digitorum longus
• Maximus, medius, and minimus used to describe relative muscle size

Origins

• Quadriceps femoris origins: Four
• Biceps brachii origins: Two
• Triceps brachii origins: Three
• Example of a attachment origin: Sternocleidomastoid
• Example of a action origin: Flexor carpi radialis

Muscle attachment

• Muscles attach to bone via tendons (cordlike epimysium extension)
• Aponeurosis: Sheetlike attachment from muscle to bone
• Two attachments of a muscle to bones: origin and insertion

Muscle parts

• Origin: Attachment to stationary or less moveable bone
• Insertion: Attachment to moveable bone

Lever Systems

• Three lever systems: 1st, 2nd, and 3rd degree
• Effort, load, and fulcrum.
• Effort: Force applied by muscle contraction (to move load)
• Load: bone and surrounding tissues (provides resistance)
• Motion occurs when: effort exceeds load

Muscle Functions

• Buccinator function: Mastication, powerful sucking, whistling, cheek compression
• Frontalis function: Produces horizontal forehead wrinkles, raises eyebrows
• Masseter function: Mastication, elevates mandible
• Occipitalis function: Pulls skin posteriorly
• Orbicularis oculi function: Closes eye, blinking/squinting
• Orbicularis oris function: Kissing muscle, closes/purses/protrudes lips
• Platysma function: Depresses mandible
• Temporalis function: Closes jaw, elevates/retracts mandible
• Zygomaticus function: Participates in mastication, smiling, raises corners of the mouth upward

Neck Muscles

• Sternocleidomastoid: Flexes and laterally rotates head
• Trapezius: Stabilizes, raises, retracts, and rotates scapula

Shoulder Muscles

• Deltoid: Prime mover of arm abduction
• Latissimus dorsi: Prime mover of arm extension, powerful adductor, medially rotates arm at shoulder
• Levator scapula: Elevates scapula

Shoulder Rotator Cuff Group

• Supraspinatus, infraspinatus, teres minor, and subscapularis.
• Supraspinatus: Initiates arm abduction, stabilizes shoulder joint
• Infraspinatus: Stabilizes shoulder joint, rotates humerus laterally
• Teres minor: Stabilizes shoulder joint, rotates humerus laterally
• Subscapularis: Chief medial rotator of arm

Stabilizes Scapula

• Rhomboid major: Stabilizes scapula
• Teres major: Extends, medially rotates, adducts humerus

Arm Muscles

• Biceps brachii: Prime mover of flexing the elbow.
• Brachialis: Forearm flexor
• Triceps brachii: Extends the elbow

Forearm Muscles

• Brachioradialis: Flexing forearm
• Flexor carpi radialis: Flex and abducts the hand at wrist joint
• Flexor carpi ulnaris: Flex and adducts the hand at wrist joint
• Extensor carpi radialis: Extend and abducts the hand at wrist joint
• Extensor carpi ulnaris: Extends and adducts the hand at wrist joint
• Extensor digitorum: Prime mover of finger extension
• Pronator teres: Pronates forearm
• Palmaris longus: Wrist flexor

Thorax Muscles

• Diaphragm: Normal breathing in/out
• External intercostals: Breathing, pulling ribs upward/outward for inspiration
• Internal intercostals: Breathing, pulling ribs downward/inward for expiration
• Pectoralis major: Prime mover of arm flexion, rotates arm medially, adducts and flexes the arm
• Pectoralis minor: Draws scapula forward and downward with ribs fixed
• Serratus anterior: Rotates scapula so its inferior angle moves laterally and upward

Abdomen Muscles

• External obliques (superficial): Flexes vertebral column, compresses abdominal wall
• Internal oblique: Flexes vertebral column, compresses abdominal wall
• Rectus abdominus: Flex/rotate lumbar region of vertebral column
• Transversus abdominis: Compresses abdominal contents

Hip Muscles

• Gluteus maximus: Extends thigh
• Gluteus medius: Abducts and medially rotates thigh

Thigh Muscles

• Adductor longus: Adducts, flexes, medially rotates thigh
• Adductor magnus: Adducts and medially rotates thigh
• Gracilis: Adduction of thigh, flexes and medially rotates leg

Hamstring Muscles

• Biceps femoris: Extends thigh and flexes knee
• Semitendinosus: Extends thigh and flexes knee
• Semimembranosus: Extends thigh and flexes knee

Quadriceps Muscles

• Rectus femoris: Extends knee, flexes thigh at hip
• Vastus lateralis: Extends and stabilizes knee
• Vastus medialis: Extends knee
• Vastus intermedius: Extend knee

Sartorius

• Function: Flexes, abducts, laterally rotates thigh, also flexes knee

Tensor Fascia Lata

• Function: Steadies knee and trunk on thigh by making iliotibial band taut.

Leg Muscles

• Gastrocnemius: Plantar flexion
• Fibularis (peroneus) longus: Plantar flexion
• Soleus: Plantar flexion
• Tibialis anterior: Dorsiflexion
• Extensor digitorum longus: Extends the toes

Nervous and Endocrine Comparison

• Nervous and endocrine systems: Controlling systems concerned with maintaining homeostasis
• Nervous system communicates with other neurons via electrical impulses and neurotransmitters
• Endocrine systems communicate with other organs by releasing hormones into the blood or ECF.
• Nervous system communication is milliseconds
• Endocrine system communication is seconds, minutes, hours, weeks, or months

Three Main Functions of Nervous System

• Sensory input: Gathers information from monitoring changes inside/outside the body (afferent)
• Integration: Nervous system processes sensory input, decides on action
• Motor output: Motor (efferent) output activates effector organs (muscles and glands), causes a response

Communication Methods

• Nervous system communicates activity using graded or action potentials along nerves
• Central and peripheral nervous systems: The two main parts of the nervous system

CNS vs PNS

• Central nervous system: brain and spinal cord
• Peripheral nervous system: cranial nerves, spinal nerves, and ganglia
• The two functional divisions of the peripheral nervous system: sensory (afferent) and motor (efferent) divisions
• Somatic nervous system (PNS): division is voluntary, control over skeletal muscles
• Autonomic nervous system (PNS): division is involuntary, controls cardiac muscle, smooth muscle, and glands

Fiber Types

• Two types of sensory (afferent) division fibers: somatic sensory fibers and visceral sensory fibers.
• Somatic sensory fibers convey impulses from the skin, skeletal muscles and joints and voluntary information towards the CNS
• Visceral sensory fibers convey impulses from from visceral organs

Glial cells

• Supportive cells that do not conduct electrical nerve impulses.
• Includes: astrocytes, microglial cells, ependymal cells, oligodendrocytes, Schwann cells, and satellite cells
• CNS: Astrocytes, microglial cells, ependymal cells, and oligodendrocytes
• PNS: Schwann cells and satellite cells

Astrocytes Functions

• Support and brace neurons and anchor them to capillaries
• Control the chemical environment around neurons
• Remove leaked potassium ions or leaked neurotransmitters.

Microglial Cells Function

• Type of macrophage that phagocytizes microorganisms.

Ependymal Cells Function

• Line ventricles and central cavities of brain and spinal cord
• Circulate CSF around the CNS

Oligodendrocytes Function

• Produce myelin sheaths that wrap their processes tightly around the fibers.

Myelin

• It is made up of Lipids.

Schwann Cells Function

• Surround all axons (nerve fibers) in the PNS
• Produce myelin sheath around thicker nerve fibers

Satellite Cells

• Surround neuron cell bodies and help to regulate ion concentration

Structures and Conductors

• Neurons (nerve cells): structural units of the nervous system
• Neurons conduct action potentials, function for a lifetime, and have a high metabolic rate

Main Parts of a Neuron

• Dendrite
• Cell body (soma)
• Axon hillock (trigger zone)
• Axon

Neuron structure

• Dendrite: Shorter, numerous regions of the neuron that receives information
• Cell body (soma): Integrative region of neuron containing nucleus
• Nissl bodies: ribosome name in cell body
• Axon hillock (trigger zone): Initial region of axon where action potential generated and has highest amount of Na+ gates
• Axon: Conducting region from cellular body

Neuron Type

• Multipolar
• Bipolar
• Unipolar

Neuron Function

• Multipolar:Most dendrites, single nucleus and are most common
• Bipolar- one of each dendrite and axon
• Unipolar - One process that comprises axon

Membrane Potential

• Resting membrane potential of plasma membrane is -70 mV, meaning ICF is more negative than ECF
• ECF has ions Na+ and Cl-
• ICF has K+ and amino acids
• Depolarization: Na+ channels open to allow Na+ from ECF to ICF to reach -55mV of threshold
• Repolarization: A +30 mV, there is a reversal of action potential
• During repolarization: K+ channels open to K+ exits cell from ICF to ECF
• Hyperpolarization: More K+ exits the cell to make ICF even more negative
• During absolute refractory period: stimulation ends
• During Relative Refractory period: Some K exits while some Na re enter to balance ECF and ICF