BIOL 1103 Week 11: Muscle Physiology

Motor unit

One motor neuron and all the muscle fibers it supplies

What is the primary ossification center?

Initial site of bone formation, typically in the diaphysis of a long bone during fetal developement

What do motor nerves divide into?

They divide into many synaptic end bulbs

Neuromuscular junction

Specialized synapse where a motor neuron communciates with a muscle fiber, triggering a muscle contraction

What is the name of the neurotransmitter in muscle contraction?

Acetylcholine

Acetycholine receptors

They are ligand-gated ion channels located on the motor end-plate. When ACh binds to them, they open, allowing sodium ions (Na+) to flow into the muscle cell, which triggers a new action potential in the muscle.

Events of the neuromuscular junction

Inward flow of Ca2+ → exocytosis & diffusion of ACh across neuromuscular junction → ACh binds to ligand-gated cation channels → channels open, Na+ rushes inside muscle cell → muscle action potential is propogated

What does the influx of calcium into the axon terminal in the neuromuscular junction trigger?

It triggers the exocytosis of vesicles filled with acetylcholine (ACh) into the synaptic cleft

What do acetylcholine (ACh) molecules bind to on the muscle fiber

ACh binds to ligand-gated cation channels on the muscle surface

How is the muscle action potential propogated into the muscle fiber’s interior?

Transverse tubules (T-tubules) propogate the action potential along the sarcolemma, moving it into the muscle fiber’s interior

Step 1 of muscle contraction (Ca2+ binding)

1. Ca2+ binds to troponin, pulling tropomyosin away from myoin binding sites on actin

2. Myosin heads hydrolyze ATP, releasing energy and orienting myosin in a “cocked” or activated position

What happens when Ca2+ binds to troponin?

It pulls tropomyosin away from the actin filament, exposing the myosin binding sites so contraction can begin

Why do myosin heads orient into a “cocked” or activated position during muscle contraction?

Because hydrolysis of ATP into ADP fuels myosin heads, forcing them to pivot into the “cocked” position

Step 2 of muscle contraction (release of phosphate)

1. Myosin heads attach to actin, forming crossbridges

2. Previously hydrolyzed phosphate is released (ADP remains attached)

Crossbridges (in muscle contraction)

Temporary attachments between myosin and actin filaments, necessary for power strokes

ADP

Adenosine diphosphate, formed when ATP is broken down to release energy

When ATP hydrolyzes into ADP, why is phosphate released?

Because ATP is triphosphate, and ADP is diphosphate. So, phosphate is released, turning the prefix from tri- into di-

Step 3 of muscle contraction (sliding filaments)

1. Myosin heads perform a power stroke, where they rotate towards the center of sarcomere

2. ADP is released during the power stroke

3. Thin filament slides past the thick filament towards the M-line, shortening the sarcomere

Power stroke

Movement of the myosin head that generates force during muscle contraction

What does movement of the myosin head do?

It pulls on the actin filament, shortening the sarcomere and causing muscle contraction

Step 4 of muscle contraction (release of actin)

1. ATP binds to myosin head, causing myosin to release actin

2. Cycle continues, if atp IS AVAILABLE AND CA2+ IS BOUND TO TROPONIN

What event triggers myosin to release actin? 

A new molecule of ATP binds to the myosin head causing myosin to release actin and relax the muscle

How does the movement at one sarcomere affect the whole muscle?

The simultaneous shortening of all sarcomeres arranged end-to-end within the myofibrils causes the entire muscle fiber to shorten, generating the force needed for whole muscle contraction.

Sliding filament mechanism

Process by which muscle contraction occurs, where filaments slide and the H zone and I band decrease in size

Acetylcholinesterase

Enzyme that breaks down acetylcholine to relax muscles/terminate nerve signals

Overall process of muscle decontraction

Acetylcholine breaks down, calcium release channels in the sarcoplasmic reticulum close, calcium pumps actively pump calcium back to the sarcoplasmic reticulum

Muscle tone

Sustained, partial contraction of skeletal muscle in response to activation of stretch receptors

Is the mechanism of contraction in cardiac muscles different from skeletal msucles?

They are mostly the same, but cardiac muscle must all contract synchrously to ensure normal heart function

Synchrous contraction of cardiac muscles

The synchronized contraction of cardiac muscles, where one cell’s contraction stimulates surrounding cells to contract

What causes cardiac muscles to contract synchrously?

Cardiac muscles are branched and connected to each other. This special connection allows for synchrous contraction

What is found at the ends of cardiac muscle branches?

Gap junctions and desmosomes

Intercalated disc

Structure formed by gap junctions and desmosomes, with a folding of cell membranes

Why are intercalated discs important in the heart?

They utilize desmosomes for mechanical strength and gap junctions for electrical continuity, enabling the rapid spread of impulses

Gap junctions

Channels formed by proteins in membranes of adjoining cells, allowing ions and impulses to pass from one cell to the next uninterrupted

Desmosomes

Strong anchoring junctions between adjacent cells, preventing them from separating. Contains “velcro-like spots” that connect cells

What are the “velcro-like” spots in desmosomes?

Interactions of specialized proteins that provide structural integrtiy

Why is cardiac muscle “self-excitable?”

Because the heart contains a sinoatrial node, which produces regular depolarizations that spread throughout the cardiac muscle, causing contractions

Process of contraction in cardiac muscle

Pacemaker cell AP spreads via gap junctions to cardiomyocytes, causing an influx and massive release of calcium. This Ca2+ binds Troponin, allowing Myosin to pull Actin toward the M-line, causing the muscle to contract.

What specialized cells initiate the electrical signal across the myocardium?

Pacemaker cells (in the SA node).

Source of Ca2+ in smooth muscle

Interstitial space

Why must the troponin-tropomyosin complex be removed from the myosin binding site for contraction?

Its removal unblocks the binding sites, allowing myosin to bind to actin and initiate contraction.

Sinoatrial (SA) node

Specialized group of cells in the upper right chamber of the heart that acts as the heart’s natural pacemaker

Organization of smooth muscles

Most smooth muscle organize into at least two layers of muscle oriented at different angles in walls of various hollow organs

How does the organization of smooth muscle allow peristalsis?

Alternating contraction of the two smooth muscle layers mixes substances in the lumen of intestine, moving them along

Structure of a smooth muscle

Unstriated, actin and myosin not arranged as sarcomeres, dense bodies and intermediate filaments form a lattice-like network of noncontractile proteims, NO troponin

Caveolae

Gap junctions in smooth muscle cells

Dense bodies in smooth muscle

Protein structures in smooth muscle cells that act as anchors for thin filaments

Why do contracted smooth muscles have a “puffy” shape?

The contraction of criscrossing myofilaments cause the unsupported membrane areas between the anchors to bulge out

Varicosities

Swollen regions on axons of neurons that form neuromuscular junctions with smooth muscle cells

Diffuse junction

Other name for a neuromuscular junction in smooth muscles

Process of smooth muscle contraction

Ca2+ enters cell → Ca 2+ binds to calmodulin which activates myosin kinase → kinase phosphorylates myosin head → myosin head binds to actin and starts sliding filament contraction cycle

Calmodulin

Regulatory protein in cytoplasm, whose activated form activates an enzyme called myosin kinase

How does calmodulin activate, and what does it do?

When Ca2+ binds to calmodulin, it activates. When activated, it can activate myosin kinase

Where does Ca2+ influx occur in smooth muscles?

It occurs via channels in the caveoli in the smooth cell’s membrane

Why does Ca2+ influx in smooth muscle cells occur in the caveoli?

Because smooth muscle cells lack T-tubules and have very little sarcoplasmic reticulum

Phosphorylate meaning

To add a phosphate group (PO4) to a molecule, by using an enzyme called kinase

Neurontransmitters regulating smooth muscle contraction

Acetylcholine, norepinephrine (norepinephrine can cause either relaxation or contraction)

Chemicals regulating smooth muscle contraction

Histamine, high CO2 levels, low pH, low O2 levels

How can chemicals stimulate contractions without generating an action potential?

They can increase intracellular calcium levels in other ways than generating an action potential

Hormones regulating smooth muscle contraction

Cholecystokinin (digestion and motility in small intestine), gastrin (digestion and motility of stomack), oxytocin (uterine contraction

Lumen

Inside cavity of a tubular structure like an intestine, where contents (like food) within it trigger the peristalsis reflex

Source of Ca2+ in cardiac muscle

Extracellular space, a little from the sarcoplasmic reticulum

Source of Ca2+ in skeletal muscle

Sarcoplasmic reticum