Physiology Exam 3

BIOL 2420 SLCC

Skeletal muscle

Voluntary, striated muscle controlled by the somatic nervous system.

Motor unit

A single motor neuron plus all of the skeletal muscle fibers it innervates.

Small motor units

Provide fine control and precision because one neuron controls fewer fibers.

Large motor units

Produce powerful movements because one neuron controls many fibers.

Motor end plate

Specialized region of the skeletal muscle fiber membrane where the motor neuron releases acetylcholine.

Muscle fiber

One skeletal muscle cell; long, multinucleated, and surrounded by sarcolemma.

Myofibril

Contractile cylinder inside the muscle fiber made of repeating sarcomeres.

Sarcomere

Functional contractile unit of skeletal and cardiac muscle.

Thin filament

Mostly actin plus troponin and tropomyosin.

Thick filament

Mostly myosin; myosin heads bind actin and generate the power stroke.

T-tubules

Membrane invaginations that carry the muscle action potential deep into the cell.

Sarcoplasmic reticulum (SR)

Specialized smooth ER that stores and releases $Ca^{2+}$ for contraction.

Troponin

Regulatory protein that changes shape when $Ca^{2+}$ binds.

Tropomyosin

Regulatory protein that normally blocks myosin-binding sites on actin.

Actin

Thin filament protein containing myosin-binding sites.

Myosin

Thick filament motor protein that uses ATP to pull actin.

Power stroke

Mechanical step in muscle contraction where a myosin head pivots and pulls an actin filament toward the center of the sarcomere. Driven by adenosine diphosphate (ADP) and inorganic phosphate $P_i$ dissociate from the myosin head

Rigor state

The attached state where myosin remains attached to actin after the power stroke. This occurs in the absence of ATP.

Excitation-contraction coupling

The process by which an action potential triggers $Ca^{2+}$ release and contraction.

Neurogenic muscle

Muscle that requires neural input to initiate contraction, such as skeletal muscle.

Acetylcholine (ACh)

Neurotransmitter released by somatic motor neurons at the neuromuscular junction.

Nicotinic cholinergic receptor

Ligand-gated ion channel on the motor end plate that allows $Na^+$ entry and $K^+$ exit.

End-plate potential (EPP)

Local depolarization at the motor end plate that triggers a skeletal muscle action potential.

DHP receptors

Receptors in the T-tubule membrane that detect voltage changes and mechanically open ryanodine receptors on the sarcoplasmic reticulum.

Ryanodine receptors

Receptors on the SR that open to allow $Ca^{2+}$ to exit from SR into the cytosol.

Myogenic muscle

Muscle that can initiate its own electrical activity, such as cardiac muscle.

Pacemaker cells

Specialized cardiac cells that spontaneously depolarize and set the rhythm of the heart.

Gap junctions

Connections that allow ions to move directly from cell to cell, spreading excitation through myocardium.

L-type $Ca^{2+}$ channels

Voltage-gated $Ca^{2+}$ channels that allow extracellular $Ca^{2+}$ to enter pacemaker and myocardial cells.

Calcium-induced calcium release (CICR)

The process where extracellular $Ca^{2+}$ entering the cardiac cell triggers additional $Ca^{2+}$ release from the SR.

Latent period

Time between the muscle action potential and the beginning of contraction; excitation-contraction coupling occurs.

Contraction phase

Cross bridge cycling is occuring; $Ca^{2+}$ release from the SR exceeds $Ca^{2+}$ reuptake.

Relaxation phase

$Ca^{2+}$ is actively pumped back into the SR , ryanodine receptors close, DHP returns to resting shape & No additional AP occurs.

SERCA pump

ATP-dependent pump that returns $Ca^{2+}$ to the SR.

Optimal resting length

The length at which actin and myosin overlap allows maximal force production.

If skeletal or cardiac muscle is not near optimal resting length, it cannot contract with maximum force at that time

Size Principle

The rule that small motor units are recruited first and large motor units are recruited last.

Recruitment

The process of adding more motor units to increase total muscle force.

Small motor neurons

Have smaller dendritic trees/shorter spread distance, so a smaller synaptic stimulus can reach threshold

Large motor neurons

have larger than Reddic trees/longer spread distance, so a stronger stimulus is needed to reach threshold

Fastest simple activity

Low load isolated movements usually take less time than complex full body or high resistance movements

Calmodulin

Protein that binds $Ca^{2+}$ in smooth muscle (instead of troponin)

Stroke volume (SV)

The amount of blood ejected from one ventricle during one cardiac cycle; blood ejected per beat; “blood filled minus blood left behind” calculated as $SV = EDV - ESV$.

End-diastolic volume (EDV)

Amount of blood in a ventricle at the end of diastole.

End-systolic volume (ESV)

Amount of blood left in a ventricle at the end of systole.

Preload

Initial stretch of ventricular muscle before contraction; increases with venous blood return.

Afterload

The force or pressure the heart must push against to open the valves and send blood out to the body.

Cardiac output (CO)

Liters of blood pumped by one ventricle per minute; blood pumped per minute;CO increases if heart rate or stroke volume increases; calculated as $CO = HR \times SV$ & $CO=\frac{MAP}{TPR}$

Epinephrine

Increases heart rate and stroke volume by increasing pacemaker activity and contractility

Starling’s Law of the Heart

Principle stating that anything that increases venous blood return intrinsically increases stroke volume.

SA node

Fastest pacemaker and normal starting point for cardiac excitation.

AV node

Part of the conduction system that delays conduction so the ventricles fill before ventricular contraction.

Bundle branches

Structures that carry excitation down the interventricular septum toward the apex.

Purkinje fibers

Fibers that distribute excitation upward through ventricular myocardium for coordinated ventricular contraction.

Funny $Na^+$ channels

Important in cardiac pacemaker cells; help pacemaker cells spontaneously depolarize.

$Ca^{2+}$ T-type channels

Present in cardiac pacemaker cells; contribute to pacemaker depolarization.

$Ca^{2+}$ L-type channels

Important and pacemaker action potentials and myocardial action potentials

P wave

ECG wave representing atrial depolarization (electrical event), leading to atrial contraction(mechanical Result)

QRS complex

ECG wave representing ventricular depolarization;atrial repolarization is hidden (electrical event), ventricular contraction (mechanical Result)

T wave

ECG wave representing ventricular repolarization(electrical event), leading to ventricular relaxation (mechanical Result)

Ventricular filling

Cardiac cycle phase where :

AV valves are open and semilunar valves are closed.

Ventricular volume increases during diastole.

Isovolumetric contraction

Cardiac cycle phase where :

All valves are closed

ventricular pressure/force increases but volume does NOT change.

Ventricular ejection

Cardiac cycle phase where :

All Valves are closed; semilunar valves open

Blood leaves the ventricles. The last phase of systole.

Isovolumetric relaxation

Cardiac cycle phase where :

All valves are closed

Ventricular pressure/force decreases but volume does NOT change.

Flow

Movement of blood from high pressure to low pressure; equation predicts how pressure gradient and resistance affect blood flow; equation $Flow = \frac{\text{Delta Pressure}}{Resistance}$.

Resistance

Opposition to blood flow, strongly influenced by vessel radius and blood viscosity.

Mean arterial pressure (MAP)

Average driving pressure in systemic arteries; relates to cardiac output as $CO = \frac{MAP}{TPR}$ or $MAP=CO\cdot TPR$

Total peripheral resistance (TPR)

Total resistance across systemic blood vessels.

Arterioles

Major/primary resistance vessels & the largest drop in blood pressure occurs across arterioles.

Valves

Prevent backward flow of blood in veins and heart

Baroreceptors

Stretch receptors that monitor blood pressure in major arteries.

Purpose is to maintain MAP(Mean Arterial Pressure) within a homeostatic range by adjusting autonomic output.

High MAP: Increases Baroreceptor firing

Low MAP: Decreases Baroreceptor firing

Capillary hydrostatic pressure

Fluid pressure inside capillaries that promotes filtration; dominant at the arteriole end.

Arteriole end of capillary

Capillary hydrostatic pressure dominates; filtration occurs

Venule end of capillary

Capillary osmotic pressure dominates; absorption occurs

Capillary osmotic pressure

Osmotic pull created mainly by plasma proteins that promotes absorption; dominant at the venous end.

Continuous / Tight Capillaries

Tight junctions form the blood-brain barrier

Fenestrated capillary beds

Found in organs specialized for filtration or absorption, including bone marrow, kidneys, liver, and intestines.

Filtration

Bulk flow of fluid out of the capillary into interstitial fluid.

Absorption

Bulk flow of fluid from interstitial fluid back into the capillary.

Lymphatic system

Returns excess filtered fluid and proteins to the blood.

Arteriosclerosis

Hardening and loss of elasticity of arteries, often associated with calcium deposition in the vessel wall

Atherosclerosis

Narrowing of arteries caused by fatty plaque deposits within the vessel wall.

Blood viscosity

Thickness of blood

If increased, it increases resistance and may increase blood pressure.(can result from dehydration or increased hematocrit)

Plasma

Liquid portion of blood containing water, proteins, ions, nutrients, gases, and wastes.

Formed elements

Cells and cell fragments in blood, including erythrocytes, leukocytes, and platelets.

Erythrocytes(RBCs)

Transport oxygen using hemoglobin.

Leukocytes (WBCs)

Defend against infections and foreign bodies.

Lymphocytes

Leukocytes involved in adaptive immune responses.

Platelets

Cell fragments involved in hemostasis.

Albumins

Major plasma proteins; help maintain plasma osmotic pressure and transport some substances.

Globulins

Major plasma proteins; include antibodies and transport proteins.

Fibrinogen

Major plasma protein; Clotting protein converted into fibrin during coagulation.

Complement Proteins

Major plasma proteins; immune defense proteins that help fight pathogens

Erythropoietin (EPO)

Hormone released by the kidneys that stimulates erythropoiesis in red bone marrow.

Bilirubin

Product of heme breakdown that travels to the liver for processing.

Transferrin

Protein that transports iron in the blood back to red bone marrow.

Ferritin

Protein that binds and stores extra iron in the liver.

Hemostasis

Process that stops bleeding after vessel damage

Process includes: vascular spasm→platelet plug formation→coagulation

Vascular spasm

Localized vasoconstriction that reduces blood loss after vessel damage.

Platelet Plug Formation

Platelets adhere to exposed collagen and become activated

ADP & Thromboxane-A2 recruit platelets

Coagulation

Clotting cascade that produces fibrin to stabilize the platelet plug.

Prothrombin→thrombin→fibrinogen→ fibrin