Muscle Tissue

Three Types of Muscle Tissue

Muscle tissue is classified into three main types, each with distinct structural and functional characteristics.

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels); controls movement of substances within these organs.

Example: Skeletal muscle enables walking, cardiac muscle contracts the heart, and smooth muscle regulates blood vessel diameter.

Unique Features of Muscles

Muscle cells possess specialized features that enable contraction and force generation.

• Cardiac muscle fibers are syncytia—formed by joining individual cells (myocytes) end-to-end at intercalated discs.

• Cardiac muscle cells do not regenerate after injury.

• The functional unit of a skeletal muscle is the sarcomere.

• The sarcoplasmic reticulum (SR) in skeletal muscle cells stores calcium ions, essential for contraction.

• Skeletal muscle fibers are innervated by somatic motor neurons.

Additional info: The sarcomere is the repeating contractile unit within myofibrils, defined by Z discs.

Connective Tissues in Skeletal Muscle

Connective tissues provide structural support and organization to skeletal muscles.

• Outside muscles: Hypodermis and fascia surround and separate muscles from other tissues.

• Part of muscles:

  • Epimysium surrounds the whole muscle.

  • Perimysium surrounds fascicles (bundles of muscle fibers).

  • Endomysium surrounds each individual muscle fiber.

• Connective tissues within muscles extend to form tendons at the ends of muscles, connecting muscle to bone.

Example: The Achilles tendon is formed by connective tissue extensions from calf muscles.

Internal Structure of a Muscle Fiber

Muscle fibers contain specialized organelles and molecules for contraction.

• Sarcolemma: Plasma membrane of the muscle fiber.

• Transverse (T) tubules: Tunnel-like extensions of the sarcolemma that conduct electrical signals.

• Sarcoplasm: Cytoplasm of the muscle fiber, containing myoglobin and glycogen.

• Myoglobin: Oxygen-binding protein in muscle cells.

• Glycogen: Storage form of glucose.

• Sarcoplasmic reticulum (SR): Network of membranous sacs storing calcium ions.

Microscopic Anatomy of Muscle Cells

Muscle fibers are unique in their structure and development.

• Formed by fusion of many myoblasts, resulting in multinucleated, long cells (10–30 cm).

• Do not divide after birth.

• Filled with myofibrils, surrounded by sarcoplasmic reticulum.

Proteins of Thick and Thin Filaments

Muscle contraction is driven by interactions between thick and thin filaments.

• Thick filaments: Composed of myosin, which uses ATP to generate movement. Myosin heads project toward thin filaments.

• Thin filaments: Composed of three proteins:

  • Actin: Contains myosin-binding sites.

  • Tropomyosin: Regulatory protein that covers myosin-binding sites on actin.

  • Troponin: Regulatory protein that binds calcium ions.

Additional info: The sliding filament mechanism describes how these proteins interact to produce contraction.

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.

• Acetylcholine (ACh) is released from the neuron into the synaptic cleft.

• ACh binds to receptors on the muscle fiber, generating a muscle action potential.

• ACh is broken down by acetylcholinesterase.

Sliding Filament Mechanism

Muscle contraction occurs as thick and thin filaments slide past each other, shortening the sarcomere.

• Calcium ions stimulate contraction by binding to troponin, moving tropomyosin, and exposing myosin-binding sites on actin.

• Myosin heads bind to actin, forming cross-bridges and pulling the filaments.

Additional info: The process is powered by ATP hydrolysis.

Excitation-Contraction Coupling

This process links the muscle action potential to contraction.

• Action potential triggers Ca2+ release from SR.

• Ca2+ binds to troponin, moving tropomyosin and exposing binding sites.

• Myosin binds to actin, initiating contraction.

Contraction Cycle

The contraction cycle consists of repeating steps that cause filaments to slide and muscle to shorten.

• ATP hydrolysis powers myosin head movement.

• Contraction continues as long as Ca2+ and ATP are available.

Relaxation

Muscle relaxation requires cessation of neural stimulation and removal of Ca2+ from the cytoplasm.

• Motor neuron stops stimulating the muscle.

• ACh is removed from the synapse.

• SR Ca2+ channels close; Ca2+ is pumped back into SR.

• Tropomyosin covers binding sites; muscle returns to resting length.

Muscle Metabolism: Energy for Contraction

Muscle cells require ATP for contraction, which can be generated by three mechanisms:

• Aerobic cellular respiration: Uses oxygen to produce ATP from glucose or fatty acids; efficient and supports prolonged activity.

• Anaerobic cellular respiration: Produces ATP via glycolysis; pyruvate is converted to lactic acid; supports short bursts of activity (30–40 seconds).

• Creatine phosphate: Stores energy for rapid ATP regeneration; supports very short bursts (about 15 seconds).

Equation:

• Aerobic respiration:

• Anaerobic glycolysis:

• Creatine phosphate:

Muscle Tone

Muscle tone is a basal level of contraction that maintains firmness without producing movement.

• Generated by low-frequency action potentials from the brain and spinal cord.

• Muscle without tone is described as flaccid.

Clinical Implications

Certain conditions affect neuromuscular function:

• Curare: Plant extract that blocks ACh receptors, causing paralysis.

• Myasthenia gravis: Autoimmune disorder causing loss of ACh receptors, leading to muscle weakness.

Nervous System

Organization of the Nervous System

The nervous system is divided into central and peripheral components.

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): All nervous tissue outside the CNS.

• Somatic Nervous System (SNS): Controls voluntary movements.

• Enteric Nervous System (ENS): Regulates gastrointestinal function.

Structural Diversity and Classification of Neurons

Neurons are classified by structure and function.

• Structural:

  • Multipolar: Most common in CNS.

  • Bipolar: Found in retina and olfactory epithelium.

  • Unipolar: Sensory neurons.

• Functional:

  • Sensory (afferent): Transmit impulses to CNS.

  • Motor (efferent): Transmit impulses from CNS to effectors.

  • Interneurons: Connect neurons within CNS.

Neuroglia

Neuroglia are supporting cells in the nervous system.

• CNS: Oligodendrocytes form myelin sheaths around axons; astrocytes maintain the blood-brain barrier.

• PNS: Schwann cells form myelin sheaths; satellite cells support neuron cell bodies in ganglia.

Myelinated and Unmyelinated Axons

Myelination increases the speed of nerve impulse conduction.

• Myelinated axons: Wrapped in myelin sheath; conduct impulses rapidly via saltatory conduction.

• Unmyelinated axons: Lack myelin; conduct impulses slowly via continuous conduction.

Distribution of Gray and White Matter

Gray and white matter are distinguished by their composition.

• White matter: Primarily myelinated axons.

• Gray matter: Neuronal cell bodies, dendrites, unmyelinated axons, terminals, neuroglia.

Action Potential

Action potentials are rapid changes in membrane potential that propagate along neurons.

• Depolarizing phase: Na+ channels open, Na+ enters cell, membrane potential rises.

• Repolarizing phase: K+ channels open, K+ leaves cell, membrane potential returns to resting.

• Refractory period: Time during which a neuron cannot fire another action potential.

Equation:

Propagation of Action Potentials

Action potentials travel from the trigger zone to axon terminals.

• Continuous conduction: Occurs in unmyelinated fibers; depolarization spreads stepwise.

• Saltatory conduction: Occurs in myelinated fibers; depolarization jumps between nodes of Ranvier.

Blood-Brain Barrier (BBB)

The BBB protects the brain from harmful substances.

• Formed by tight junctions, basement membrane, and astrocyte end-feet.

• Prevents entry of polar chemicals; allows small, nonpolar molecules.

Cerebrospinal Fluid (CSF)

CSF is a nutrient-rich fluid that circulates in and around the CNS.

• Provides mechanical protection, chemical protection, and circulation.

• Produced by ependymal cells in brain ventricles.

Main Parts of the CNS

The CNS consists of several major regions:

• Brain:

  • Cerebrum (Telencephalon): Higher cognitive functions.

  • Diencephalon: Thalamus (relay center), hypothalamus (homeostasis), epithalamus.

  • Brainstem: Midbrain, pons, medulla (basic life functions).

  • Cerebellum (Metencephalon): Coordination and balance.

• Spinal cord: Conducts signals between brain and body.

Functional Anatomy of the Brainstem

The brainstem controls vital functions and alertness.

• Reticular formation: Regulates alertness.

• Medulla oblongata: Controls heart rate, blood pressure, and breathing.

Major CNS Tracts

Tracts are bundles of axons carrying specific information.

• Corticospinal tracts: Carry motor signals for voluntary movement.

• Spinothalamic tracts: Convey sensory information (temperature, pain) to the brain.

• Cerebellar tracts: Carry information about muscle coordination to the cerebellum.

Table: Connective Tissue Layers in Skeletal Muscle

Table: Types of Muscle Tissue