Nerve Muscle Physiology MBBS 1st Year: RMP, AP, NMJ & Muscle Contraction

The Nerve Muscle Physiology is a crucial unit in the MBBS 1st year, as even a single unclear concept—such as the action potential—can impact performance across multiple questions. Analysis of previous university question papers indicates that Nerve Muscle Physiology consistently contributes to long answer questions, short notes, and viva assessments.

Here, you can learn the core excitability properties, from the generation and transmission of electrical signals in nerves to the intricate mechanisms of muscle contraction. Understanding these principles is crucial for grasping overall body function and for scoring well in MBBS first-year exams.

Also Read - NEET PG 2026 Preparation

Resting Membrane Potential (RMP): Origin and Development

The Resting Membrane Potential (RMP) is a negative potential across the membrane of an excitable cell (like a nerve or muscle) when it is in a resting state. This negativity is due to excess negative charges within the cell, specifically just inside the membrane. When discussing any membrane potential, we always refer to the inner aspect of the membrane.

• Specific RMP Values for Excitable Tissues:

• • Nerve RMP: -70 millivolts (mV)

  • Skeletal Muscle RMP: -90 millivolts (mV)

• Origin and Development of RMP:
  The RMP is primarily established by the differential permeability of the cell membrane to ions and their concentration gradients.

• • Equilibrium Potential for an Ion (Nernst Equation): This theoretical concept calculates the membrane potential if only one ion could move.

• • • Sodium (Na+): Higher outside the cell. If only Na+ moved, it would rush in, making the inside positive. Equilibrium potential for Na+ (ENa) is +61 mV.

    • Potassium (K+): Higher inside the cell. If only K+ moved, it would move out, making the inside negative. Equilibrium potential for K+ (EK) is -96 mV.

    • Key Contribution: Potassium diffusion contributes maximally to the development of RMP. This is because the membrane is 50 to 100 times more permeable to Potassium than to Sodium at rest. This high K+ permeability pulls the RMP close to K+'s equilibrium potential.

    • Chloride (Cl-): Equilibrium potential for Cl- (ECl) is -89 mV, very close to skeletal muscle RMP, so it hardly moves at rest. For nerves, ECl (-70 mV) is closest to nerve RMP.

• Goldman-Hodgkin-Katz (GHK) Equation: This equation calculates the membrane potential when multiple ions (Na+, K+, Cl-) move simultaneously, considering both concentration gradients and membrane permeability (P). For these three ions, their combined diffusion contributes approximately -86 mV to the RMP.

• Role of Sodium-Potassium Pump: The Sodium-Potassium Pump (Na+/K+ pump), an active transport mechanism, continuously pumps 3 Na+ ions out and 2 K+ ions in, contributing an additional -4 mV to the RMP.

• Total RMP: The sum of ion diffusion (-86 mV) and Na+/K+ pump activity (-4 mV) results in the -90 mV RMP for skeletal muscle.

Action Potential (AP)

An Action Potential is a transient reversal of membrane potential that travels or propagates through a nerve (or muscle). It signifies an excited state of the membrane. The inside of the membrane, normally negative at rest, temporarily becomes positive before returning to its resting negative state.

• Recording Action Potential: APs are recorded using a Cathode Ray Oscilloscope (CRO), showing electrical changes as a graph.

• Phases and Ionic Basis of Action Potential:

• • Basic Concepts:

• • • Polarized State: Negative inside (e.g., -70mV).

    • Depolarization: Inside becomes positive.

    • Repolarization: Inside returns to negative.

    • Hyperpolarization: Inside becomes excessively negative.

• • Key Ionic Movements:

• • • Depolarization is primarily due to Sodium (Na+) influx.

    • Repolarization is primarily due to Potassium (K+) efflux.

• • Detailed Graph and Phases:

1. Resting Membrane Potential (RMP): Starts at -70 mV.

2. Threshold: Around -55 mV. A stimulus causes a slight depolarization, opening a few voltage-gated sodium channels. This initiates a positive feedback cycle of sodium channels, where Na+ influx causes further depolarization and opens more channels. Once threshold is reached, an AP is inevitable. 

(Memory Tip: Like a firecracker's fuse burning slowly until it reaches the explosive part.)

3. Depolarization (Upstroke and Overshoot): At threshold, a massive, instantaneous influx of sodium occurs (Na+ permeability increases by up to 5000 times). This rapid influx makes the membrane potential peak at approximately +35 mV. This rapid rise and fall above 0mV is called the Spike Potential. Sodium channels rapidly close after this peak.

4. Repolarization: With sodium channels closed, potassium exit gathers momentum. Voltage-gated K+ channels are slow to open but become fully active at +35mV, causing rapid efflux of K+. This outflow of positive charges restores the negative membrane potential.

5. After-depolarization: Repolarization slows around -40 mV due to accumulating positive charges outside, opposing further K+ efflux. (This term is a misnomer.)

6. Hyperpolarization (After-hyperpolarization): The membrane potential becomes excessively negative (e.g., to -90mV) due to excess potassium leaving the cell. This happens because potassium channels are slow to close, remaining open longer than needed to reach RMP.

Refractory Period

The refractory period is a time during which an excitable tissue cannot be excited again after an initial excitation.

• Absolute Refractory Period (ARP): From the start of depolarization until about -40mV during repolarization, no stimulus, no matter how strong, can elicit another Action Potential.

• Relative Refractory Period (RRP): Immediately following the ARP, a stronger-than-normal stimulus can elicit another Action Potential.

• Significance: A tissue with a long refractory period cannot be excited at high frequencies. The heart has a long refractory period, preventing rapid, unsustainable excitation.

Also Read - NEET PG 2026

Propagation of Action Potential

Action Potential propagation differs between unmyelinated and myelinated nerves.

• Unmyelinated Nerves: Local Circuit Flow
  A stimulus at one point causes Na+ influx. These positive charges flow to an adjacent resting point, depolarizing it to threshold. This opens Na+ channels at the new point, and the process repeats, spreading sequentially along the nerve.

 (Memory Tip: Like a chain of firecrackers.)

• Myelinated Nerves: Saltatory Conduction
  Myelinated nerves have gaps called Nodes of Ranvier, where sodium channels are concentrated.

1. Na+ ions enter at one Node of Ranvier.

2. These ions travel electrotonically (direct charge spread) to the next Node of Ranvier.

3. The next Node of Ranvier reaches threshold, opening its concentrated Na+ channels and generating a new AP.
   The term "jumping" in saltatory conduction is a perception, not a reality. Ions move internally via electrotonic conduction, giving the impression of skipping myelinated segments.

• Advantages of Saltatory Conduction:

• Faster Conduction: APs are generated only at nodes, and electrotonic conduction between nodes is rapid.

• Less Energy Consumption: APs are generated at fewer points, activating fewer Sodium-Potassium pumps, thus requiring less ATP for ion restoration.

Nerves

A neuron is the basic unit of the nervous system. A nerve fiber is a single axon. A nerve is a bundle of nerve fibers.

• Classification of Neurons:

• Multipolar Neuron: Most common type, with multiple dendrites and one axon (e.g., cerebral cortex).

• Bipolar Neuron: Two distinct poles (one input, one output). Pseudounipolar neurons (e.g., Dorsal Root Ganglion (DRG)) are functionally bipolar but appear unipolar.

• Anaxonal Neuron: Lacks a defined axon (e.g., Amacrine cells in the retina, postganglionic parasympathetic neurons).

• Classification of Nerve Fibers: This is the most important classification. Fibers are categorized by diameter and conduction velocity.

1. Erlanger and Gasser Classification: (A, B, C; A further subdivides into α, β, γ, δ)

2. Numerical Classification (for Sensory Fibers):

• Type 1A: Primary afferents from the muscle spindle.

• Type 1B: Afferents from Golgi Tendon Organ.

• Type II: Secondary afferents from the muscle spindle.

• Type III: Touch and pressure.

• Type IV: Pain and temperature.

Injury to Nerve Fibers and Degeneration

Susceptibility to Nerve Injury:

• Pressure-Related Injury: Type A fibers are most susceptible.

• Hypoxia: Type B fibers are most susceptible.

• Local Anesthetics: Type C fibers are most susceptible.

Degrees of Severity of Injury (Sunderland's Classification):

• Grade I: Temporary (neurapraxia).

• Grade II: Axonotmesis (axon damage, endoneurium intact).

• Grade III: Axonal transection, endoneurium disrupted.

• Grade IV: Disruption of the entire nerve bundle (fascicle).

• Grade V: Nerve trunk transection (complete loss of continuity).

Wallerian Degeneration: This process follows nerve transection, particularly Grade V nerve injury. The nerve is divided into a proximal stump (connected to the cell body) and a distal stump (separated). Wallerian degeneration occurs in the distal stump.

• Chronology of Changes:

• Within 24-48 hours: Chromatolysis in the nerve cell body (proximal stump).

• Up to 3 days: Distal stump remains functional.

• By 6th day: Axonal degeneration (axon fragments).

• By 10th day: Myelin degeneration (myelin breaks down).

• By 15th day: Repair starts with sprouting from the proximal end and Schwann cell proliferation (forming guiding tubes) from the distal end.

• By 80th day: Repair is generally completed.

Muscle

Skeletal muscle physiology includes the Neuromuscular Junction (NMJ) and Excitation-Contraction (EC) Coupling. Myasthenia Gravis and Malignant Hyperthermia are clinically important aspects.

Neuromuscular Junction (NMJ)

The Neuromuscular Junction is a specialised synapse where a nerve axon terminal meets a muscle fiber. It is a biological transducer, converting an electrical signal (nerve AP) into a chemical signal (neurotransmitter) that causes electrical excitation of the muscle. Despite the absence of structural continuity, nerve impulses effectively excite the muscle.

• Structure:

• Presynaptic Terminal (Nerve Terminal): Contains synaptic vesicles filled with Acetylcholine (ACh), mitochondria, and voltage-gated calcium channels (VGCCs).

• Postsynaptic Membrane (Motor End Plate): Specialized region of the muscle fiber membrane. Contains Nicotinic Acetylcholine Receptors (nAChRs). These are ligand-gated ion channels that are pentameric proteins. Acetylcholine binds to these receptors.

Nicotinic Acetylcholine Receptor (nAChR)

The nAChR is a non-specific cation channel. When ACh binds, it opens, allowing both sodium influx and potassium efflux. However, sodium influx predominates due to the resting membrane potential's distance from sodium's equilibrium potential, leading to muscle depolarization.

Transmission of Impulse (Neuromuscular Transmission)

1. Action Potential (AP) arrives at the nerve terminal.

2. Voltage-gated calcium channels open.

3. Calcium influx occurs into the nerve terminal.

4. Vesicles containing acetylcholine migrate towards and fuse with the presynaptic membrane (involving SNARE proteins).

5. Acetylcholine is released by exocytosis into the synaptic cleft.

6. Acetylcholine traverses the synaptic gap.

7. Acetylcholine reaches the motor end plate and binds to nicotinic acetylcholine receptors.

8. Opening of the cation channel occurs.

9. Sodium influx predominates, causing depolarization and generating an End Plate Potential (EPP).

10. The EPP converts into an Action Potential (AP) if the threshold is reached.

11. The AP travels into the muscle fiber, resulting in muscle contraction.

End Plate Potential (EPP)

The End Plate Potential (EPP) is an important electrical event in neuromuscular transmission that occurs at the motor end plate of a muscle fiber. It plays a key role in converting a chemical signal (release of acetylcholine) into an electrical response, ultimately leading to muscle contraction.

• End Plate Potential (EPP) is a localized, depolarizing potential generated at the motor end plate following acetylcholine (ACh) binding.

• It has a large amplitude (approximately 40 mV).

• EPP acts as a precursor to the muscle action potential.

• Typically, one EPP is sufficient to trigger one action potential due to its high magnitude.

• It is a graded potential, meaning its amplitude can vary.

• EPP does not follow the all-or-none law, unlike action potentials.

Applied Aspects: Myasthenia Gravis

Myasthenia Gravis is an autoimmune disease where antibodies develop against nicotinic acetylcholine receptors at the NMJ, destroying them.

• Signs and Symptoms: Weakness, easy fatigability, ptosis (drooping eyelids), and in severe cases, respiratory depression.

• Reasoning Question: Why do Myasthenia Gravis patients feel worse at the end of the day? Repeated muscle contractions exhaust available ACh and further compromise transmission at the few remaining receptors, exacerbating weakness.

• Treatment: Anticholinesterases (e.g., Neostigmine) inhibit ACh breakdown; steroids (immunosuppressants) reduce antibody production; thymectomy (thymus removal) may help in some cases.

Neuromuscular Blocking Drugs

These drugs interfere with neuromuscular transmission.

Excitation-Contraction (EC) Coupling

EC coupling is the process linking electrical excitation (muscle AP) to muscle contraction. The coupling agent is calcium. This occurs within the sarcotubular system (T-tubules and L-tubules/sarcoplasmic reticulum).

• Components:

• T-tubules: Invaginations of the muscle membrane carrying the AP deep into the fiber.

• L-tubules (Sarcoplasmic Reticulum): Stores calcium.

• DHP Receptor (Dihydropyridine Receptor): In T-tubule membrane, acts as a voltage sensor.

• RyR (Ryanodine Receptor): In sarcoplasmic reticulum, functions as a calcium release channel.

• Mechanism:

1. An action potential travels via T-tubules.

2. The DHP receptor senses the membrane potential change.

3. DHP receptor mechanically interacts with RyR.

4. This interaction opens the RyR calcium release channel.

5. Calcium is released into the sarcoplasm.

6. Sarcoplasmic calcium combines with Troponin C.

7. This initiates actin-myosin interaction, leading to muscle contraction.

8. SERCA pumps transport calcium back into the sarcoplasmic reticulum, causing muscle relaxation.

Applied Aspects: Malignant Hyperthermia

Malignant Hyperthermia is a grave, inherited condition triggered by certain anesthetics. It involves a genetic defect in the gene that codes for the Ryanodine Receptor (RyR).

• Pathophysiology: When exposed to triggers, the defective RyR causes a massive, uncontrolled release of calcium from the sarcoplasmic reticulum. This leads to greatly increased muscle metabolism, severe hyperthermia, muscle rigidity, and other symptoms.

• Treatment: Dantrolene sodium is the specific treatment. It is an uncoupling agent that inhibits calcium release from the sarcoplasmic reticulum, reducing contraction and heat generation.

Sarcomere Structure

The sarcomere is the functional unit of a muscle fiber.

Structural Components:

• Z-lines: Define sarcomere boundaries.

• Thin Filaments: Primarily actin, anchored to Z-lines.

• Thick Filaments: Primarily myosin, centrally located.

• Sarcomere Length: The distance between two Z-lines. Optimal length (L_O) is ~2.2 microns, where maximum contraction occurs (Starling's Law).

• Cross-sectional View: Each thick filament is surrounded by six thin filaments, and each thin filament by three thick filaments (a 2:1 ratio of thin to thick filaments in skeletal muscle).

Muscle Proteins

• Titin: The largest known human protein. It aligns thick filaments and acts as a molecular spring. Defects lead to Limb-Girdle Muscular Dystrophy.

• Alpha-actinin and CapZ Protein: Anchor thin filaments to the Z-line.

• Dystrophin: Transfers tension from contractile elements to the sarcolemma. Defects cause Duchenne's Muscular Dystrophy (DMD), characterized by proximal muscle weakness and Gower's sign.

Bands and Zones in the Sarcomere

The arrangement of filaments creates distinct bands:

• I-Band: Contains only thin filaments; appears light.

• A-Band: Spans the entire length of thick filaments; contains both thin and thick filaments (in overlap regions); appears dark.

• H-Zone: Central region within the A-band; contains only thick filaments; appears lighter.

• M-Line: Dark line at the center of the H-zone, holding thick filaments in place.

• Sarcomere Composition: One sarcomere consists of one full A-band + two half I-bands.

Changes in the Sarcomere with Contraction (Sliding Filament Theory)

The Sliding Filament Theory explains muscle contraction: thin filaments slide past thick filaments, pulling Z-lines closer.

• During Contraction:

• I-bands shorten.

• H-zone disappears.

• A-band remains constant in length.

• M-line becomes more prominent.

Molecular Mechanism of Contraction (Cross-Bridge Cycling)

Muscle contraction involves interaction between myosin (thick filament) and actin (thin filament).

A. Thick Filament (Myosin): Each myosin molecule has a head (contains ATPase and actin-binding site), neck, and tail.

• Rigor Mortis: Post-mortem muscle stiffening. Occurs because ATP is required for myosin heads to detach from actin. Without ATP after death, heads remain rigidly attached.

B. Thin Filament (Actin, Troponin, Tropomyosin):

• Actin: Provides active sites for myosin binding.

• Tropomyosin: Covers actin's active sites in a relaxed muscle.

• Troponin: Complex of three subunits:

• Troponin C (TnC): High affinity for calcium.

• Troponin T (TnT): Affinity for tropomyosin.

• Troponin I (TnI): Affinity for actin (inhibitory).

C. Cross-Bridge Cycling (Ratchet Theory):

1. Activation: Calcium released from sarcoplasmic reticulum binds to Troponin C.

2. Conformational Change: TnC binding pulls Tropomyosin away, exposing active sites on actin.

3. Cross-Bridge Formation: Myosin head (already energized by ATP hydrolysis to ADP+Pi) binds to the exposed active site on actin.

4. Power Stroke: Release of ADP+Pi causes the myosin head to pivot, pulling the thin filament towards the sarcomere center.

5. Detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin.

6. Re-cocking: ATP is hydrolyzed to ADP+Pi, re-energizing the myosin head to its cocked position, ready for the next cycle.
   This cycle repeats as long as calcium and ATP are available, causing muscle shortening.