Autonomic Nervous System, Cardiac, Skeletal, and Smooth Muscle Physiology Vocabulary

Vocabulary flashcards focusing on the autonomic nervous system, cardiac electrical conduction, and skeletal and smooth muscle contraction mechanisms.

Sympathetic Nervous System

A division of the autonomic nervous system whose outflow originates in the thoracolumbar regions of the spinal cord (segments $\text{T1}$ to $\text{L2-L3}$). It utilizes short myelinated preganglionic neurons that release acetylcholine (ACh) and long unmyelinated postganglionic neurons that release noradrenaline (NA), except at sweat glands.

Parasympathetic Nervous System

A division of the autonomic nervous system with craniosacral outflow originating from cranial nerves $3$, $7$, $9$, and $10$ (ocular motor, facial, glossopharyngeal, and vagus) and sacral segments $\text{S2}$ to $\text{S4}$. It utilizes long myelinated preganglionic neurons and short unmyelinated postganglionic neurons, both releasing acetylcholine (ACh).

Ganglion

A collection of neural cell bodies located outside the Central Nervous System (CNS) where preganglionic autonomic neurons synapse onto postganglionic neurons.

Preganglionic Neuron

A myelinated neuron whose cell body originates in the ventral horn of the spinal cord, exits along the ventral root, and extends to synapse within an autonomic ganglion.

Postganglionic Neuron

An unmyelinated autonomic neuron with its cell body housed inside a ganglion that extends its axon to innervate target effector organs.

Sympathetic Trunk Ganglion

A ganglion located in the sympathetic chain close to the vertebral column where short sympathetic preganglionic axons synapse with postganglionic neurons.

Adrenal Medulla

An endocrine tissue directly innervated by preganglionic sympathetic neurons that secretes adrenaline (epinephrine) and noradrenaline (NA) directly into the bloodstream.

Sweat Gland Innervation

An exceptional sympathetic pathway where postganglionic sympathetic neurons are cholinergic and release acetylcholine (ACh) to target sweat glands.

Pacemaker Cells

Modified cardiac myocytes within the cardiac conduction system that lack contractile machinery and generate self-initiated action potentials to drive heartbeats.

Sinoatrial (SA) Node

The primary pacemaker of the heart located in the right atrium that spontaneously fires about $80$ action potentials per minute to control heart rate.

Atrioventricular (AV) Node

A conduction structure where electrical impulses coming from the SA node briefly slow down to allow atrial contraction prior to ventricular activation.

Funny Current

A slow influx of $\text{Na}^+$ through voltage-gated Funny channels that open below $-\text{40}\,\text{mV}$, producing the spontaneous pacemaker potential from $-\text{60}\,\text{mV}$ to the threshold of $-\text{40}\,\text{mV}$.

Gap Junctions

Intercellular protein channels located within intercalated discs connecting neighboring myocytes that allow rapid ionic diffusion for electrical coupling and synchronous contraction.

Plateau Phase

A prolonged phase in contractile myocyte action potentials lasting about $200\,\text{msec}$ created by a balance between L-type $\text{Ca}^{2+}$ channel influx and $\text{K}^+$ channel efflux.

Absolute Refractory Period

A period following stimulation during which muscle tissue cannot respond to further stimuli, lasting $250\,\text{msec}$ in cardiac muscle versus $1\,\text{msec}$ in skeletal muscle to prevent tetanus.

Calcium-Induced Calcium Release (CICR)

A mechanism in cardiac myocytes where entry of extracellular $\text{Ca}^{2+}$ through L-type channels triggers a substantially larger release of $\text{Ca}^{2+}$ from the sarcoplasmic reticulum via ryanodine receptors.

SERCA

Sarco-endoplasmic reticulum calcium-ATPase, an ATP-dependent pump that actively sequesters cytosolic $\text{Ca}^{2+}$ back into the sarcoplasmic reticulum during muscle relaxation.

Fascicles

Organized bundles of skeletal muscle fibers bound together by a surrounding sheath of connective tissue called the perimysium.

Endomysium

A thin layer of collagen and reticular fibers (predominantly type III collagen) encasing individual muscle fibers that contains extracellular fluid and nutrients.

Sarcolemma

The specialized plasma membrane enclosing a muscle fiber cell.

Sarcomere

The key functional contractile unit of skeletal and cardiac muscle fibers bounded by Z discs, containing overlapping actin (thin) and myosin (thick) filaments.

Z Discs

Anchoring structures inside striated muscle composed primarily of $\alpha\text{-actinin}$ that anchor thin actin filaments at the borders of each sarcomere.

Tropomyosin

A thin regulatory protein draped around actin filaments that covers myosin-binding sites in resting muscle to block cross-bridge formation.

Troponin

A regulatory protein complex bound to tropomyosin that binds $\text{Ca}^{2+}$, undergoing a conformational shift that pulls tropomyosin away to expose myosin-binding sites on actin.

Dense Bodies

Dark, specialized anchoring structures in smooth muscle cells composed mainly of $\alpha\text{-actinin}$ that function like Z-discs by securing actin and intermediate filaments.

Caveolae

Small invaginations in the cell membrane of smooth muscle cells containing calcium channels that facilitate the entry of extracellular $\text{Ca}^{2+}$ located near the sarcoplasmic reticulum.

Calmodulin (CaM)

An intracellular calcium-binding protein in smooth muscle that binds $\text{Ca}^{2+}$ to form a complex that activates myosin light chain kinase (MLCK).

Myosin Light Chain Kinase (MLCK)

An enzyme in smooth muscle activated by the $\text{Ca}^{2+}\text{-CaM}$ complex that phosphorylates myosin heads, giving them the ATPase activity required to bind actin and drive contraction.

Myosin Light Chain Phosphatase

An enzyme that dephosphorylates smooth muscle myosin heads when cytosolic $\text{Ca}^{2+}$ declines, preventing actin binding and causing muscle relaxation.

Peristalsis

A coordinated, multidirectional contracting and squeezing motion of smooth muscle in hollow organ walls that pushes materials along the digestive tract.