Neuroscience and Muscle Physiology Review
Myosin Structure
Myosin is a motor protein that plays a key role in muscle contraction. It possesses a long tail and a globular head.
The head region has ATPase activity, allowing it to hydrolyze ATP to generate movement.
Myosin interacts with actin, another protein that forms the thin filaments in muscle fibers, to enable contraction.
Regulatory Proteins
Two primary regulatory proteins involved in muscle contraction are:
Tropomyosin: This protein binds to actin filaments and blocks the myosin binding sites, preventing contraction when the muscle is at rest.
Troponin: This complex binds to calcium ions and tropomyosin, leading to a conformational change that moves tropomyosin away from myosin binding sites when calcium levels increase, allowing contraction to occur.
Anatomy and Characteristics of Neurons
Motor Neuron Example:
A motor neuron transmits signals from the central nervous system (CNS) to muscle fibers.
Nissl Bodies: These are large aggregations of rough endoplasmic reticulum and ribosomes found in the cell body of neurons, involved in synthesizing proteins, particularly neurotransmitters.
Neuron Parts:
Dendrites: Receive impulses (incoming signals).
Axon: Discharges impulses away from the cell body (outgoing signals).
Types of Gated Channels
Three Types of Gated Channels:
Ligand-Gated Channels: Open in response to the binding of a chemical messenger or neurotransmitter (e.g., acetylcholine receptor).
Voltage-Gated Channels: Open in response to changes in membrane potential (e.g., sodium and potassium channels).
Mechanically Gated Channels: Open in response to physical deformation of the receptor (e.g., touch receptors).
Resting Membrane Potential
Definition: Resting membrane potential is the difference in charge across a neuron's membrane when it is not transmitting an impulse. Typically, it is around -70 mV.
Ion Concentration:
Higher Concentration in ECF: Sodium ions (Na⁺).
Characteristics of Skeletal Muscles
General Characteristics:
Striated appearance, multinucleated, voluntary control, and capable of rapid contraction.
Tendon vs. Aponeurosis:
Tendon: A cord-like projection of connective tissue that attaches muscle to bone.
Aponeurosis: A flattened sheet of connective tissue that connects muscles to other muscles or structures.
Connective Tissue Coverings of Skeletal Muscle
Endomysium: Covers individual muscle fibers.
Perimysium: Surrounds bundles of muscle fibers (fascicles).
Epimysium: Envelops the entire muscle.
Major Parts of a Skeletal Muscle Fiber
Sarcolemma: The plasma membrane of a muscle fiber.
Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and myofibrils.
Myofibrils: Composed of myofilaments (actin and myosin) responsible for contraction.
Transverse Tubules (T-tubules): Invaginations of the sarcolemma that transmit action potentials into the cell.
They form a triad with the terminal cisternae of the sarcoplasmic reticulum.
Myofilaments
Myosin: The thick filament, composed of myosin molecules that can bind to actin during contraction.
Actin: The thin filament, polymerized globular proteins.
Z Lines: Define the boundaries of a sarcomere.
Myoglobin: An oxygen-binding protein found in muscle fibers.
Tropomyosin: Blocks myosin binding sites on actin when calcium is not bound to troponin.
Troponin: A complex of proteins that regulates the contraction process by its interactions with tropomyosin.
Sarcomere: The structural unit of muscle, the segment between two Z lines.
Structures of Actin
Actin Structure: Composed of globular actin (G-actin) subunits that polymerize to form filamentous actin (F-actin).
Protein Blocking Actin: Tropomyosin physically blocks myosin binding sites on actin when calcium is absent.
ICF and ECF Concentrations
Higher Concentration in ICF: Potassium ions (K⁺).
Leaky Channels: More potassium (K⁺) leaky channels are present in the membrane compared to sodium (Na⁺) channels.
Anions in Cell Membrane
Anions that Cannot Cross: Proteins and phosphates remain inside the cell due to their size and charge.
Synapse and Junctions
Neuron Synapses:
Neurons can form synapses with other neurons or with muscle cells.
Presynaptic Cell: The neuron that releases the neurotransmitter.
Postsynaptic Cell: The target neuron or muscle cell that receives the signal; can be excitatory/inhibitory.
Synaptic Junctions
Definition: Specialized junctions through which neurons communicate with target cells.
Neuromuscular Junction: A synapse between a motor neuron and a muscle fiber.
Contents of Synaptic Vesicles: Contain neurotransmitters such as acetylcholine.
Acetylcholine Removal: Acetylcholinesterase breaks down acetylcholine in the synaptic cleft.
Role of Calcium in Axon Terminal
Calcium ions (Ca²⁺) play a critical role in neurotransmitter release by causing synaptic vesicles to dock and fuse with the axonal membrane.
Neurotransmitter Release Process
Ion Entry: The entry of calcium ions through voltage-gated channels stimulates the fusion of neurotransmitter-containing vesicles with the presynaptic membrane, leading to the release of neurotransmitters into the synaptic cleft.
Removal of Neurotransmitter
Three Methods of Removal:
Reuptake: Transporter proteins bring neurotransmitters back into the presynaptic neuron.
Enzymatic Degradation: Enzymes break down neurotransmitters (e.g., acetylcholinesterase).
Diffusion: Neurotransmitters diffuse away from the synaptic cleft.
Postsynaptic Potential Generation
Generation of Graded Potential: Excitatory postsynaptic potentials (EPSP) or inhibitory postsynaptic potentials (IPSP) occur in the postsynaptic cell.
Definition of Graded Potential: A change in the membrane potential of the postsynaptic neuron that varies in magnitude based on the strength of the stimulus.
Ion Influx: Usually, sodium ions (Na⁺) enter the neuron, causing depolarization and generating EPSP or K⁺ influx causing hyperpolarization and generating IPSP.
Definition of EPSP and IPSP
EPSP (Excitatory Postsynaptic Potential): Depolarization that makes the neuron more likely to fire an action potential (graded potential).
Generated by depolarization.
IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization that makes the neuron less likely to fire an action potential.
Generated by hyperpolarization.
Threshold Stimulus
Definition: The minimum level of stimulus required to cause a neuron to generate an action potential.
All-or-None Response
Explanation: Once the threshold is reached, the action potential is always initiated at full magnitude; there are no partial action potentials.
Action Potential Mechanics
Stages of Action Potential:
Depolarization: Opening of voltage-gated sodium channels leading to sodium influx.
Repolarization: Opening of voltage-gated potassium channels leading to potassium efflux.
Hyperpolarization: Further outflow of potassium leads to a temporary negative state.
Opening of Ion Gates: At various stages, sodium (Na⁺) and potassium (K⁺) channels open.
Neuron Specifics
Graded Potentials vs. Action Potentials: Graded potentials occur at dendrites and cell bodies, while action potentials are generated at the axon hillock.
Junctional Folds
Definition: Infoldings of the postsynaptic membrane at the neuromuscular junction that increase surface area for receptor proteins.
Receptors for ACH: Located on the muscle cell membrane at the motor end plate.
Motor End Plate Events
Motor End Plate: When an action potential reaches here, it stimulates voltage-gated channels, allowing Na⁺ influx into the muscle.
Calcium and Contraction Processes
Calcium Binding: Released calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin, allowing contraction.
Detachment of Myosin heads: Myosin heads detach from actin through ATP hydrolysis after a power stroke.
Rigor Mortis
Causes: Occurs after death due to lack of ATP, which prevents myosin from detaching from actin, leading to sustained contraction.
Neuromuscular Junction Agents
Curare: Causes paralysis by blocking acetylcholine receptors.
Acetylcholinesterase inhibitors (e.g., nerve gas): Prevent breakdown of acetylcholine, leading to prolonged muscle contraction (spasms).
Botulism toxin: Blocks acetylcholine release, resulting in paralysis.
Tetanus toxin: Causes sustained muscle contraction, resulting in spasms.
Myogram and Muscle Twitch Stages
Myogram: A graphical representation of muscle contraction.
Three Stages of Muscle Twitch:
Latent Phase: Time period between stimulus and onset of contraction.
Contraction Phase: Muscle fibers shorten and generate force.
Relaxation Phase: Muscle returns to its resting state.
Muscle Contraction Types
Isotonic Contraction: Muscle shortens while maintaining force (
lifting an object).Isometric Contraction: Muscle tension increases but length remains the same (pushing against a wall).
Concentric: Muscle shortens against resistance (lifting).
Eccentric: Muscle lengthens while contracting (setting an object down).
Muscle Contraction Types Distinctions
Tetanic Contraction: A sustained contraction with no relaxation.
Muscle Tone: A continuous low-level contraction that keeps muscles firm but not actively contracting.
Staircase Effect
Definition: A series of twitches of increasing strength due to repeated stimuli, demonstrating increased contractility with successive twitching.
Recruitment of Muscle Fibers
Recruitment: The process of activating additional motor units in response to increased stimulus intensity to produce stronger contractions.
Lactic Acid Accumulation
Cause: Occurs during anaerobic respiration in muscles under strenuous exercise when oxygen is limited.
Terminology Definitions
Muscle Tone: Continuous partial contraction of muscle.
Hypertrophy: Increase in muscle fiber size due to strength training.
Hyperplasia: Increase in the number of muscle fibers.
Atrophy: Decrease in muscle size and strength due to disuse.
Fast vs. Slow Contracting Muscles
Fast-Contracting (Red): Muscle fibers adapted for quick responses, fatigue quickly.
Slow-Contracting (White): Muscle fibers designed for endurance, resist fatigue.
Duchenne Muscular Dystrophy
Characteristics: A genetic disorder affecting muscle fibers, leading to weakness and degeneration.
Gender Prevalence: Predominantly affects males due to its X-linked inheritance pattern.
Characteristics of Smooth Muscle Fibers
Generally have a single nucleus.
Elongated with tapering ends.
Thinner actin and myosin filaments compared to skeletal muscle.
No T-tubules or sarcomeres.
Only surrounded by endomysium.
Contains caveolae, which serve as sites for calcium storage.
Calmodulin replaces troponin for calcium binding and regulation.
Varicosities of nerve fibers release neurotransmitters to smooth muscle fibers.
Types of Smooth Muscle
Multiunit Smooth Muscle: Composed of individual fibers that can function independently; found in the iris and blood vessels.
Visceral Smooth Muscle: Composed of sheets of fibers that contract together; found in hollow organs (e.g., stomach).
Peristalsis Definition
Peristalsis: A wave-like contraction of intestinal muscles that propels contents along the digestive tract by alternating their contractions.
Nervous System Organization and Function
Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for processing information.
Peripheral Nervous System (PNS): Comprises nerves extending from the CNS to the rest of the body; responsible for communication between the CNS and limbs/organs.
Functions of Neuroglia (Supporting Cells)
CNS: Astrocytes (support neurons, regulate blood flow), microglia (immune defense), oligodendrocytes (myelin sheath production), ependymal cells (cerebrospinal fluid circulation).
PNS: Schwann cells (myelin sheath production) and satellite cells (supporting role around neuron cell bodies).
Myelin Sheath Function
Myelin Sheath: Insulates axons to increase the speed of electrical impulses; produced by Schwann cells in the PNS and oligodendrocytes in the CNS.
Neuron Types
Unipolar Neurons: Have one axon with the cell body off to the side; typically sensory neurons.
Bipolar Neurons: Have one axon and one dendrite; found in specific sensory pathways such as vision and smell.
Multipolar Neurons: Most common type; multiple dendrites and one axon; typically motor neurons or interneurons.
Neuron Functions
Motor Neuron: Transmits impulses away from the CNS to effector organs (muscles or glands).
Sensory Neuron: Transmits sensory information to the CNS (afferent neurons).
Interneuron: Connects sensory and motor neurons; processes information within the CNS.
Node of Ranvier
Definition: Gaps in the myelin sheath where ions can flow in and out of the axon, aiding in the propagation of action potentials through saltatory conduction.
Saltatory Conduction
Explanation: The insulated sections of the myelin sheath speed up action potentials, allowing them to jump from one node of Ranvier to another.
Action Potential Speed Factors
Factors Affecting Speed: 1. Presence of myelin sheath. 2. Diameter of the axon (larger diameters conduct faster).
A, B, and C Nerve Fibers
Classification:
A fibers: Large, myelinated, fast-conducting (e.g., motor neurons).
B fibers: Medium diameter, myelinated, moderately fast.
C fibers: Small, unmyelinated, slow-conducting (e.g., pain receptors).
Electrical Synapses
Location: Found in certain areas of the brain, retina, and cardiac muscle; allow rapid signal transference.
Mechanism: Ions flow through gap junctions, rapidly depolarizing connected cells.
Deficient Neurotransmitter Conditions
Dopamine Deficiency: Linked to Parkinson's disease.
Excitatory Neurotransmitter: Acetylcholine at the neuromuscular junction.
Serotonin: Linked to sleep and mood disorders; its reuptake is inhibited by the antidepressant Prozac.
Main Inhibitory Neurotransmitter: Gamma-aminobutyric acid (GABA).
Glutamate: Called “the stroke neurotransmitter” due to its role in neural injury and neurodegeneration.
Pain Mediator: Substance P is known to transmit pain signals.
Endorphins: Neurotransmitters that inhibit pain and produce feelings of pleasure; released during stress, exercise, or pain.