Muscle Tissue and Physiology

Types of Muscle Tissue

Muscle tissue is classified into three main types, each with distinct locations, structures, and control mechanisms.

• Skeletal Muscle: - Location: Attached to bones - Structure: Long, cylindrical, multinucleated, striated fibers - Control: Voluntary

• Cardiac Muscle: - Location: Heart - Structure: Branched, usually single nucleus, striated, connected by intercalated discs - Control: Involuntary

• Smooth Muscle: - Location: Walls of hollow organs (e.g., intestines, blood vessels) - Structure: Spindle-shaped, single nucleus, non-striated - Control: Involuntary

Intercalated discs are specialized connections between cardiac muscle cells that contain gap junctions and desmosomes, allowing for synchronized contraction and communication.

Properties of Muscle Tissue

• Contractility: Ability to shorten forcefully.

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to conduct electrical signals.

• Distensibility: Ability to stretch without being damaged.

• Elasticity: Ability to return to original shape after stretching.

Muscle Cell Components

• Sarcoplasm: Cytoplasm of a muscle cell, containing organelles and myofibrils.

• Sarcolemma: Plasma membrane of a muscle cell.

• Myofibrils: Bundles of contractile proteins within muscle fibers.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions.

• Transverse Tubules (T-tubules): Invaginations of the sarcolemma that transmit action potentials into the cell.

• Triad: Structure formed by a T-tubule and two adjacent terminal cisternae of the SR.

Myofilaments and Associated Proteins

• Thick Filament: Composed mainly of myosin proteins.

• Thin Filament: Composed of actin, tropomyosin, and troponin.

• Elastic Filament: Composed of titin, providing elasticity and stability.

Sarcomere Structure

• I band: Contains only thin filaments (actin).

• A band: Contains thick filaments (myosin) and overlapping thin filaments.

• H zone: Central region of A band with only thick filaments.

• M line: Center of the sarcomere, holds thick filaments together.

• Z disc: Boundary of sarcomere, anchors thin filaments.

Sliding Filament Mechanism

During contraction, thin filaments slide past thick filaments, shortening the sarcomere without changing the length of the filaments themselves.

• H zone: Narrows or disappears.

• Z discs: Move closer together.

• A band: Remains the same length.

Electrical Properties of Muscle Cells

• Electrical Gradient: Difference in electrical charge across the cell membrane.

• Resting Membrane Potential: The voltage difference across the membrane at rest, typically around -70 mV.

• Leak Channels: Always open, allow passive ion movement.

• Gated Channels: Open or close in response to stimuli.

• Ligand-Gated Channels: Open in response to chemical signals.

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ into the cell, maintaining gradients.

Action Potentials

• Action Potential: Rapid, temporary change in membrane potential.

• Depolarization: Na+ enters, membrane potential becomes less negative.

• Repolarization: K+ exits, membrane potential returns to resting value.

Neuromuscular Junction (NMJ)

• Synaptic Vesicles: Contain acetylcholine (ACh).

• Acetylcholine: Neurotransmitter released into synaptic cleft.

• Synaptic Cleft: Space between neuron and muscle fiber.

• After release, ACh binds to receptors on the sarcolemma and is broken down by acetylcholinesterase.

Phases of Muscle Contraction

• Excitation: ACh binds to receptors, Na+ and K+ enter, depolarizing the muscle fiber.

• Excitation-Contraction Coupling: Action potential triggers Ca2+ release from SR; Ca2+ binds to troponin, exposing actin sites.

• Contraction: Ca2+ binding initiates crossbridge cycling.

Crossbridge Cycle

1. ATP hydrolysis cocks myosin head.

2. Myosin binds actin (crossbridge formation).

3. Power stroke: Myosin pulls actin, ADP released.

4. New ATP binds, myosin detaches from actin.

Crossbridge: Connection between myosin head and actin filament.

Muscle Relaxation

• Actin active sites covered by tropomyosin.

• Acetylcholine broken down in synaptic cleft.

• Calcium pumped back into SR.

Tetanus and Muscle Twitch

• Unfused (Incomplete) Tetanus: Partial relaxation between stimuli.

• Fused (Complete) Tetanus: No relaxation, sustained contraction.

• Muscle Twitch Phases: Latent, contraction, relaxation.

ATP Synthesis in Muscle

Length-Tension Relationship

• Force generated depends on sarcomere length at contraction onset.

• Optimal Length: Sarcomere length with maximal overlap of actin and myosin.

Muscle Fiber Types

Motor Units and Muscle Tone

• Motor Unit: One motor neuron and all muscle fibers it innervates.

• Muscle Tone: Baseline tension in resting muscle.

Types of Muscle Contractions

• Isotonic Concentric: Muscle shortens while generating force.

• Isotonic Eccentric: Muscle lengthens while generating force.

• Isometric: Muscle length does not change, but tension increases.

Muscle Adaptations

• Endurance Training: Increases mitochondria, capillaries, and fatigue resistance.

• Resistance Training: Increases muscle fiber size (hypertrophy) and strength.

• Atrophy: Decrease in muscle size due to inactivity or disease.

Muscle Fatigue and Recovery

• Muscle Fatigue: Inability to maintain contraction strength due to ATP depletion, ion imbalances, or lactic acid buildup.

• Postexercise Oxygen Consumption (EPOC): Increased oxygen intake after exercise to restore homeostasis (replenish ATP, remove lactic acid, restore ion gradients).

Smooth Muscle

• Functions: Moves substances through organs, regulates vessel diameter, controls pupil size, etc.

• Dense Bodies: Anchor actin filaments in smooth muscle cells.

• Pacemaker Cells: Generate spontaneous action potentials.

• Latch State: Sustained contraction with minimal ATP use (e.g., in blood vessels).

Introduction to the Nervous System and Nervous Tissue

Organization of the Nervous System

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

• Peripheral Nervous System (PNS): Cranial and spinal nerves.

• Nerve: Bundle of axons in the PNS.

• Cranial Nerves: Originate from the brain.

• Spinal Nerves: Originate from the spinal cord.

Functions and Divisions

• Sensory (Afferent) Division: Transmits sensory information to CNS.

• Somatic Sensory: Detects stimuli consciously perceived (e.g., touch, pain).

• Visceral Sensory: Detects internal stimuli not consciously perceived (e.g., blood pressure).

• Motor (Efferent) Division: Transmits commands from CNS to effectors.

• Somatic Motor: Controls voluntary movements (skeletal muscle).

• Visceral Motor (Autonomic): Controls involuntary movements (smooth muscle, glands).

Cells of Nervous Tissue

• Neurons: Excitable, amitotic, long-lived cells that transmit electrical signals.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

Neuron Structure and Function

• Cell Body (Soma): Contains nucleus and organelles.

• Dendrites: Receive signals.

• Axon: Conducts impulses away from cell body.

• Axon Hillock: Initiates action potentials.

• Axon Terminals: Release neurotransmitters.

• Axolemma: Axon membrane.

• Axoplasm: Axon cytoplasm.

• Anterograde Transport: Movement from cell body to axon terminal.

• Retrograde Transport: Movement from axon terminal to cell body.

• Receptive Region: Dendrites and cell body (receives input).

• Conducting Region: Axon (transmits action potentials).

• Secretory Region: Axon terminals (release neurotransmitters).

Neuron Classifications

• Structural: Multipolar, bipolar, unipolar (based on number of processes).

• Functional: Sensory (afferent), motor (efferent), interneurons (association).

• Nuclei: Clusters of neuron cell bodies in CNS.

• Tracts: Bundles of axons in CNS.

• Ganglia: Clusters of neuron cell bodies in PNS.

• Nerves: Bundles of axons in PNS.

Neuroglia

• Myelin Formation: Oligodendrocytes (CNS) myelinate multiple axons; Schwann cells (PNS) myelinate one axon segment each.

• PNS Regeneration: Schwann cells form regeneration tube, guide axon regrowth.

Membrane Potentials and Ion Movement

• Depolarization: Na+ influx, membrane potential becomes less negative.

• Repolarization: K+ efflux, membrane potential returns to rest.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Local (Graded) Potential: Small, localized change in membrane potential.

• Sodium Channel States: Closed, open, inactivated; ions move freely only when open.

Action Potentials

• Depolarization: Threshold reached, Na+ channels open.

• Repolarization: Na+ channels inactivate, K+ channels open.

• Hyperpolarization: K+ channels remain open briefly.

• Unidirectional Propagation: Inactivation of Na+ channels prevents backward movement.

• Absolute Refractory Period: No new action potential possible.

• Relative Refractory Period: Stronger stimulus needed for new action potential.

• All-or-None Principle: Action potentials occur fully or not at all.

• Conduction Speed: Increased by myelination and larger axon diameter.

• Continuous Conduction: Unmyelinated axons; slower.

• Saltatory Conduction: Myelinated axons; action potential jumps between nodes of Ranvier; faster.

Synapses

• Axodendritic: Axon to dendrite.

• Axosomatic: Axon to cell body.

• Axoaxonic: Axon to axon.

• Presynaptic Neuron: Sends signal.

• Postsynaptic Neuron: Receives signal.

• Chemical Synapse: Uses neurotransmitters.

• Electrical Synapse: Direct ion flow via gap junctions.

• Events at Chemical Synapse: Action potential arrives, Ca2+ influx, neurotransmitter release, binding to postsynaptic receptors, response initiated.

• EPSP (Excitatory Postsynaptic Potential): Depolarizes postsynaptic membrane.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes postsynaptic membrane.

• Temporal Summation: Multiple signals from one neuron over time.

• Spatial Summation: Signals from multiple neurons at once.

• Termination of Synaptic Transmission: Neurotransmitter diffusion, reuptake, or enzymatic degradation.

Neurotransmitters and Neuromodulators

• Neuromodulators: Chemicals that modulate synaptic transmission, affecting strength or duration of signals.