Nerve Fibers and Resting Membrane Potential

• Nerve fibers transmit sensory impulses to the brain or spinal cord and motor impulses to muscles and organs.
• Electrical and chemical signals are used to transmit sensory and motor information from one body part to another.
• Nerve fibers are covered by myelin sheaths and/or a membrane, the neurilemma.

Nerve Type Classifications

• Structure (myelinated or unmyelinated)
• Origin (cranial or spinal)
• Function (afferent or efferent fibers)
• The neurotransmitter secreted
• Diameter

Resting Membrane Potential (RMP)

• The distribution of electrolytes across the cell membrane is unequal and creates a potential difference.
• At rest, the cell is polarized.
• Decreasing the RMP is termed depolarization.
• Returning to the RMP is repolarization.
• The RMP in a nerve is about -70mV>

Causes of RMP

• Selective permeability of the cell membrane, where hydrated Na+ size is bigger than hydrated K+.
• K+ permeability is 25-30 times greater than Na+ permeability.
• The Na+/K+ pump pumps 3 Na+ out for every 2 K+ in, using 1 ATP.
• Nondiffusible intracellular anions, such as proteins, contribute to the negative charge inside the cell.

Action Potential

• A transient change in the resting membrane potential, defined by rapid depolarization followed by repolarization.
• The depolarization phase results from cation influx, such as Na+ and Ca++.
• The repolarization phase results from cation efflux, such as K+, and some anion influx, such as Cl-.
• Action potential is the nerve impulse and the language of nerves.
• The response obeys an all-or-none principle
• The duration of action potential differs in nerve or muscle.

Components of Action Potential

• Threshold: the membrane potential value that, when reached, initiates an all-or-nothing action potential.
• Depolarization (upstroke): the initial or rising phase.
• Overshoot: the region between 0 mV and the peak amplitude.
• Repolarization: the return of the membrane potential to its resting state
• Hyperpolarization (undershoot): the phase where the membrane potential is more negative than the resting potential.

Depolarization Stage

• The membrane potential remains at the resting voltage until a stimulus occurs.
• Sodium (Na+) mechanical channels open, allowing Na+ to diffuse into the cell, making the membrane less negative.
• The action potential either occurs or does not, depending on whether the threshold is reached.
• If depolarization reaches -55 mV, the action potential continues to +40 mV.

Repolarization Stage

• Sodium channels close and potassium channels open after a brief period of membrane permeability to sodium ions.
• Potassium ions diffuse rapidly to the exterior, re-establishing the normal negative resting membrane potential, called repolarization.

Action Potential Steps

• The membrane is polarized at rest.
• Threshold stimulus is reached.
• Sodium channels open, and the membrane depolarizes.
• Potassium leaves the cytoplasm, and the membrane repolarizes.

Re-establishing Sodium and Potassium Ionic Gradient After Action Potential

• The Na+/K+ pump continuously pumps sodium ions out and potassium ions in.
• More positive charges are pumped outside versus inside (3 Na+ out for 2 K+ in), creating a net deficit of positive ions inside.
• The action above causes a negative potential inside the cell membrane at the resting membrane potential.

Propagation of Action Potential and the Neuron

• Basic neuron parts include cell body, dendrites, axon hillock, axon, and axon terminals.
• Action potentials start at the axon hillock and are conducted throughout the nerve fiber.

Types of Propagation

• Continuous Conduction occurs in unmyelinated fibers
• During continuous conduction the action potential spreads along every part of the membrane.
• Saltatory Conduction occurs is rapid In myelinated fibers
• During saltatory conduction the impulse jumps over sections of the fiber covered with myelin
• Saltatory conduction is about 50 times faster in myelinated fibers than in unmyelinated fibers.

Characteristics of Action Potential

• Action potentials occur in an all-or-none fashion depending on the stimulus strength.
• Subthreshold stimuli fail to elicit an action potential (AP).
• If an excitable cell is depolarized to threshold, the action potential's occurrence is inevitable.

Refractory Period

• Responsible for setting a limit on the frequency of action potentials.
• It's the period of repolarization, where is muscle fiber cannot respond to further stimulation

Stimulus

• It's a change in the external environment that elicits a response from excitable tissues.
• Types of stimulus include mechanical, thermal, electrical, and chemical.

Muscular System

• The biological system that allows movement in humans.
• The nervous system controls this system, but some muscles, like cardiac muscle, can be completely autonomous.
• Muscles move the body like strings move a puppet.

Muscle Tissue

• The muscle cell is known as a muscle fiber due to its elongated, fiber-like shape.
• It produces force and motion for locomotion or movement within or with internal or external organs.
• Elongated shape facilitates contractile function.

Muscle Classification

• Presence of cross-striation, being striated or non-striated.
• Nature of control, being voluntary or involuntary.
• Distribution: skeletal muscle is striated and voluntary, cardiac muscle is striated and involuntary and visceral muscle is non-striated and involuntary.

Smooth Muscle Characteristics

• Lacks striations
• Has spindle-shaped cells
• Usually has a single nucleus
• Involuntary
• Found mainly in walls of hollow organs.

Cardiac Muscle Characteristics

• The cardiac muscle has Striations
• The cardiac muscle usually has single nucleus
• Cardiac Muscle is joined to another muscle cell at an intercalated disc.
• It is Involuntary
• Cardiac Muscle is found only in heart.

Skeletal Muscle Characteristics

• Anchored by tendons to bones.
• Cells are multinucleated
• Skeletal muscle appears striated due to visible banding
• Voluntary, subject to conscious control
• Cells are surrounded and bundled by connective tissue

Connective Tissue Wrappings of Skeletal Muscle

• Endomysium: surrounds a single muscle fiber.
• Perimysium: surrounds a bundle of fibers.
• Epimysium: covers the entire skeletal muscle.
• Fascia: on the outside of the epimysium and is a deep fascia.

Myofibrils

• Myofibrils are the contractile units within muscle fibers and are bundles of protein filaments causing contraction.
• They appear alternately dark and light shaded (transverse striations).
• Myofibrils contain sarcomeres (functional contractile units)
• Sarcoplasm (cytoplasm of muscle fiber) consists of sarcosomes (mitochondria), a small Golgi apparatus, myoglobin (protein pigment), lipid, glycogen, and sarcoplasmic reticulum
• Fibers richer in sarcoplasm are darker in color and vice versa.

Sarcomere Organization

• Area between two adjacent Z lines.
• Thin filament consists of actin.
• Thick filament consists of myosin.
• I-band is the zone of thin filaments not superimposed by thick filaments (myosin).
• A-band contains the entire length of a single thick filament.
• H-band is the zone of the thick filaments that has no actin.

Muscle Contraction Process

• A cycle of repetitive events causing a thin filament to slide over a thick filament and generate tension.
• Spaces between thick and thin filaments are bridged by myosin projections (cross-bridges).
• Cross-bridges contact troponin molecules of thin filaments and exert force toward the center.
• ATP binding to myosin cross-bridges detaches myosin from actin, allowing myosin to bind to another actin molecule.

Sliding Filament Mechanism

• The process is aided by Calcium, and the concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum.
• Muscle contraction ceases when calcium ions are pumped back into the sarcoplasmic reticulum by Ca-ATPase, allowing the muscle cell to relax.
• During muscle contraction, the A-bands do not change length, whereas the I-bands and the H-zone shorten, causing Z lines to come closer.

Neuromuscular Junction

• Electrical and chemical transmission travels from a nerve to muscle.
• The Neuromuscular Junction is where the muscle is stimulated by a nerve.

Nerve Impulse Transmission to Muscle.

• Neurotransmitters are chemicals released by nerves after a nerve impulse.
• Acetylcholine is the neurotransmitter for skeletal muscle, and attaches to receptors on the sarcolemma.
• The sarcolemma becomes permeable to Sodium (Na+).
• Sodium rushes into the cell and generates an action potential.
• A local potential in the muscle is called end plate potential (EPP).
• Once muscle contraction is started, it cannot be stopped.

Events Leading to Muscle Contraction

• Motor neuron depolarization
• The action potential generates
• Travels down the nerve fiber to the neuromuscular junction
• Depolarization of the axon terminal
• Opening of the voltage-gated Ca+2 channels
• Release of neurotransmitter (Acetylcholine; ACh)
• ACh diffuses across the synaptic cleft and binds to the post-synaptic receptor
• Opens the ligand-gated Na+ channels
• Depolarization of the sarcolemma travels down the t-tubules
• Release of Ca2+ from the sarcoplasmic reticulum
• Muscle contraction occurs

Properties of Skeletal Muscle

• Excitability: ability to be excited by adequate stimulus and contract simultaneously.
• Refractory period: brief period after stimulation when the muscle is not excitable to a second stimulus.
• Tonicity: state of partial contraction or light tension within the muscle.
• Conductivity: ability to propagate a wave along the muscle upon stimulation.
• Extensibility and Elasticity: the muscle recoils to its original length after tension is removed.

Smooth Muscle Contraction Characteristics

• Large numbers of actin filaments are attached to dense bodies.
• Dense bodies are attached to cell membranes or are spread inside cells
• Intercellular protein bridges connect cells and transmit force of contraction

Smooth Muscle Types

• Multi-unit smooth muscle is innervated by a single nerve ending where each fiber contracts independently like the ciliary muscle and iris of eye.
• Unitary (visceral) smooth muscle is a mass of hundreds to thousands of smooth muscle fibers that contract together as a single unit, and is controlled by non-nervous stimuli.
• The potentials or simple ion flow can travel from one fiber to the next thus causing contraction.

Physical Basis of Smooth Muscle Contraction

• Dense bodies of smooth muscle act like Z discs in skeletal muscle.
• Myosin filaments have "sidepolar" cross-bridges that hinge in opposite directions.
• Myosin pulls one actin filament in one direction and pulls another actin filament in the opposite direction on the other side.
• Smooth muscle cells can contract as much as 80% of their length, unlike skeletal muscle, which is limited to less than 30%.

Control of Smooth Muscle Contraction

• Nervous signals
• Hormonal stimulation
• Stretch of the muscle

Neuromuscular Junctions of Smooth Muscles

• Nerve fibers do not make direct contact with smooth muscle fiber cell membranes but instead form diffuse junctions that secrete transmitter substance into the matrix
• Vesicles of autonomic nerve fiber endings contain acetylcholine in some fibers and norepinephrine in others.
• Acetylcholine can be excitatory or inhibitory in different organs.
• When acetylcholine excites a muscle fiber, norepinephrine ordinarily inhibits it, and vice versa
• A substance can cause either excitation or inhibition in different locations, such as norepinephrine inhibiting contraction of smooth muscle in the intestine but stimulates their contraction in blood vessels.

Excitation by Stretch

• Visceral (unitary) stretches sufficiently generates action potentials.
• This response allows the excessively stretched gut wall to contract automatically and rhythmically.
• Local automatic contractions can set up peristaltic waves when the gut is overfilled by intestinal contents.

Non-Nervous Stimuli

• Half of all smooth muscle contraction is initiated without action potentials.
• Local tissue chemical factors are involved.
• Vasodilatation occurs when extra blood flow is needed because the vessels walls relax, allowing for increased flow.
• Factors for increased flow include lack of oxygen, an excess of carbon dioxide, increased hydrogen ion, lactic acid, and increased temperature.
• Hormones such as Epinephrine, Vasopressin, Acetylcholine and Histamine also provide non-nervous stimulation.