Excitable Tissues: Membrane & Ion Channels

Vocabulary-style flashcards covering the electrical excitability of cells, ion channels, resting membrane potentials, action potentials, skeletal muscle structure, and pharmacological effects on neurotransmission.

Excitable tissues

Nerve and muscle cells that have the ability to generate and propagate electrical signals.

Lipid bilayer

The structure of the cell membrane through which electrically charged ions cannot pass directly.

Ion channels

Transmembrane proteins that create selective pores allowing specific ions to pass through the cell membrane.

Leak channels

Ion channels that are open all the time; they are mainly permeable to $K^+$.

Gated ion channels

Ion channels that only open in response to specific stimulation of the cell.

Voltage-gated channels

Channels that open in response to a change in membrane voltage.

Ligand-gated channels

Channels that open in response to a chemical, such as the ACh receptor.

Intracellular Potassium ($K^+$)

The main cation inside the cell, with a high concentration of approximately $140\,mM$.

Extracellular Sodium ($Na^+$)

The main cation outside the cell, found in high concentrations compared to the intracellular environment.

Nernst Equation

The formula used to calculate the equilibrium potential for any ion: $V = (RT/zF) \times \ln(K_o/K_i)$.

$E_K$ (Equilibrium potential for $K^+$)

The potential at which chemical and electrical forces balance for potassium, calculated as $-94\,mV$ when $K_o = 4\,mM$ and $K_i = 140\,mM$.

$E_{Na}$ (Equilibrium potential for $Na^+$)

The sodium equilibrium potential, which is $+61\,mV$.

$V_m$ (Actual Resting Membrane Potential)

The voltage across the membrane at rest, typically between $-70\,mV$ and $-90\,mV$, sitting closer to $E_K$ due to higher permeability to $K^+$.

Action Potential

A regenerating depolarisation of membrane potential that propagates along an excitable membrane, generated when voltage-gated $Na^+$ channels open.

Depolarisation phase

The phase of the action potential where $Na^+$ rushes into the cell through open voltage-gated $Na^+$ channels, causing $V_m$ to peak at approximately $+30\,mV$.

Repolarisation phase

The phase where $Na^+$ channels close and $K^+$ channels open, allowing $K^+$ to rush out and return $V_m$ toward $-70\,mV$.

Hyperpolarisation

A brief period where $V_m$ dips below the resting potential because $K^+$ continues to move out of the cell.

$Na^+/K^+$ ATPase

An active transport pump that requires ATP to restore ion gradients by pumping $3\,Na^+$ out and $2\,K^+$ into the cell per cycle.

All-or-none property

The principle that an action potential either fires fully or not at all; the threshold is usually $15\,mV$ positive to resting potential.

Absolute refractory period

The period after an action potential during which no new action potential can be generated because $Na^+$ channels are inactivated.

Nodes of Ranvier

Gaps in the myelin sheath where voltage-gated $Na^+$ channels are concentrated.

Saltatory conduction

The process in myelinated fibres where the action potential appears to 'jump' from node to node, resulting in faster conduction.

Epimysium

Dense irregular connective tissue surrounding the entire muscle that reduces friction during contraction.

Perimysium

Fibrous connective tissue surrounding fascicles, which are groups of muscle fibres.

Endomysium

Fine areolar or reticular connective tissue surrounding each individual muscle fibre.

Sarcolemma

The plasma membrane of a muscle cell, containing invaginations called T-tubules.

Sarcoplasmic Reticulum (SR)

The endoplasmic reticulum of muscle cells that serves as a storage site for $Ca^{2+}$.

Sarcomere

The basic contractile unit of muscle that runs from Z disc to Z disc.

Z disc

The boundary of the sarcomere that anchors thin filaments and is composed of connectins.

A band

The dark band of the sarcomere that represents the full length of the thick filaments (myosin).

I band

The light band of the sarcomere containing only thin filaments (actin).

H zone

The center of the A band where thin filaments do not overlap; it contains myosin only.

M line

The center of the H zone, composed of desmin, which holds thick filaments together.

Titin

Elastic filaments that run from the Z disc to the thick filaments, providing a spring-back function.

DHP receptors

Voltage sensors located in the T-tubule membrane that activate in response to an action potential.

Ryanodine receptor channels

$Ca^{2+}\!$ release channels in the SR terminal cisternae that are opened by activated DHP receptors.

Triad

The junction where excitation-contraction coupling occurs, consisting of one T-tubule flanked by two terminal cisternae of the SR.

Calsequestrin

The protein inside the sarcoplasmic reticulum to which $Ca^{2+}\!$ is bound during storage.

Acetylcholinesterase (AChE)

The enzyme in the synaptic cleft that terminates the signal by cleaving acetylcholine into acetate and choline.

Lignocaine

A local anaesthetic that binds to an intracellular epitope within the voltage-gated $Na^+$ channel pore to block neural conduction.

Botulinum Toxin (Botox)

A substance that cleaves SNARE proteins (synaptobrevin, SNAP-25, syntaxin), preventing vesicle fusion and the release of ACh.

Organophosphates

AChE inhibitors (such as Novichok) that cause ACh to accumulate in the synaptic cleft, leading to persistent depolarisation and skeletal muscle paralysis.

Curariform Drugs

Competitive antagonists (such as D-tubocurarine) at nicotinic ACh receptors that block ACh binding, preventing action potentials and causing muscle paralysis.