Unit 3

Skeletal muscle structure

1. Filaments-actin and myosin; tropomyosin and troponin 2. Myofibrils-fill muscle fiber;sarcomeres;SR and T tubules 3. Muscle fiber-sarcoplasm and sarcolemma 4.Endomysium-covers each muscle fiber 5. Fascicle-bundle of cells 6. Perimysium-cover each fascicle 7.All fascicles 8. Epimysium-cover all fascicles;beneath fascia 9. Fascia-layer of dense connective tissue covering surface of muscle;extends beyond muscle:tendons and aponeuroses

Discuss three ways that activity at the neuromuscular junction may be inhibited

1. botulinus toxin inhibits ACh release from a motor neuron, causing the fiber to never get a stimulus to contract 2. Curare prevents ACh from binding to muscle, causing a fiber to never get a stimulus to contract 3. Nerve gases, pesticides inhibit action of acetylcholinesterase, which causes a muscle to not be able to relax (stuck in contraction)

Explain potential consequences of neurotransmitter imbalances and the influence of drugs on neurotransmitters, including chemical addiction

ACh: nicotine (activates receptors, muscle paralysis) and Curare (blocks receptor binding, increases alertness) Norepinephrine/Epinephrine: reserpine (decreases packaging of neurotransmitter into vesicles, decreases blood pressure), Tricyclic antidepressants (blocks reuptake, antidepressant), monoamine oxidase inhibitors, block enzymatic degradation of neurotransmitter in presynaptic cell, antidepressant), dual reuptake inhibitors (blocks reuptake, mood elevation) Serotonin: tryptophan (stimulates neurotransmitter synthesis, sleepiness), tricyclic antidepressants (blocks reuptake, antidepressant), selective serotonin reuptake (blocks reuptake, antidepressant, anti-anxiety agent) Dopamine: nicotine (elevates levels, sense of pleasure), cocaine (blocks reuptake, euphoria)

Explain excitation-contraction coupling, and describe the roles of calcium and acetylcholine in this process

Acetylcholine (ACh) is the neurotransmitter that motor neurons use to control skeletal muscle contraction. ACh diffuses across synaptic cleft and binds to protein receptors in fiber membrane, increasing permeability to sodium and potassium ions. Sodium enters the muscle cell faster than potassium, which opens nearby sodium channels on the sarcolemma. Result of second set of channels opening is an impulse which spreads throughout the muscle cell. The electrical impulse is what triggers the release of calcium ions from the sarcoplasmic reticulum, leading to muscle contraction. At rest, troponin and tropomyosin block binding site on actin molecules from myosin. As calcium concentration rises due to electrical impulse, calcium ions bind to troponin and pull tropomyosin out of the way, which allows myosin heads to bind to actin, forming cross bridges.

List several types of neurotransmitters and explain their actions and fates

Acetylcholine (ACh): control skeletal muscle actions/stimulates skeletal muscle contraction at neuromuscular junctions; may excite or inhabit at autonomy nervous systems. Decomposed by acetylcholinesterase Norepinephrine & Epinephrine: in the CNS & PNS, creates a sense of well-being; low levels may lead to depression; may excite or inhibit autonomic nervous system actions, depending on receptors; decomposition by monoamine oxidase Dopamine: in the CNS, creates a sense of well-being; deficiency in some brain areas associated with Parkinson disease; not made from scratch, reuptake Serotonin: in the CNS, primarily inhibitory; leads to sleepiness; action is blocked by LSD, enhanced by selective serotonin reuptake inhibitor antidepressant drugs; not made from scratch, reuptake

Distinguish between excitatory and inhibitory postsynaptic potentials

Excitatory postsynaptic potential (EPSP): a neurotransmitter binds to a postsynaptic receptor and opens sodium ion channels, the ions diffuse inward, depolarizing the membrane, possibly triggering and action potential (lasts about 15 milliseconds) Inhibitory postsynaptic potential (IPSP): a different neurotransmitter binds other receptors and increases membrane permeability to potassium ions, these ions diffuse outward, hyperpolarizing the membrane. action potential is now less likely to occur Temporal summation: keep sending signal (one nerve) over short period of time and adding them together to reach threshold Spatial summation: adding together signals at same time from different locations to reach thresholds

Describe the general anatomy of a skeletal muscle, including all connective tissue coverings

Fascia: layers of dense connective tissue that cover the surface of and extend beyond muscle forming tendon; separate individual skeletal muscle from adjacent muscles; hold skeletal muscles in position Tendon: extension of fascia; attach muscle to bone; ropelike structures Aponeuroses: extension of fascia; broad, flat, fibrous sheets; ex. abdominal region and top of head Epimysium: layer of connective tissue that closely surrounds a skeletal muscle; beneath fascia Perimysium: surrounds each fascicle (bundle of cells); extends inward from epimysium and separates the muscle tissue into small sections (fascicles) Endomysium: surrounds each muscle cell (fiber)

Describe the process of muscle relation

First, an enzyme called acetylcholinesterase destroys acetylcholine remaining in the synapse, which closes voltage gated calcium channels and gets rid of the impulse. This enzyme prevents a single action potential from continuously stimulating a muscle fiber. The calcium pump moves calcium ions back into the sarcoplasmic reticulum and cross-bridge linkages break (which requires ATP)

Discuss ways that neural impulses may be processed by facilitation, convergence, and divergence

Neuronal pools are groups of neurons that synapse with each other and perform a common function even though their cell bodies may be in different parts of the CNS. Facilitation: repeated impulses on excitatory presynaptic neurons may result in the increased release of neurotransmitter in response to an impulse, making it more likely to bring the postsynaptic cell to threshold Convergence: axons originating from different neurons leading to the same postsynaptic neuron; allows the nervous system to collect, process, and respond to information; makes it possible for a neuron to sum impulses from different sources (summation from two sources) Divergence: impulses conducted by a neuron of a neuronal pool may exhibit divergence by reaching several other neurons. one neuron stimulates two others, each of which stimulated several others, and so forth. Pattern of diverging axons allows an impulse to reach increasing numbers of neurons within the pool. As a result of divergence, an impulse originating from a single neuron in the CNS may stimulate several motor units i a skeletal muscle to contract. Similarly, an impulse originating from a sensory receptor may diverge and reach several regions of the CNS, where the information can be processes and evoke a response.

Describe the structure of a skeletal muscle fiber, including the sarcolemma, sarcoplasm, actin, myosin, sarcomere, I band, A band, Z lines, tropomyosin, troponin, sarcoplasmic reticulum, and transverse tubules

Sarcolemma: muscle cell membrane (plastic part of straw) Sarcoplasm: cytoplasm of skeletal muscle cell Myosin: thick filament; filament in sarcoplasm of cell; regular arrangement that creates striations; binds to actin for contraction Actin: thin filament; filament in sarcoplasm of cell; regular arrangement that creates striations Sarcomere: region between two Z lines; contracts I band: light bands; composed of thin actin filaments only A band: dark bands; composed of either only myosin or overlap of myosin and actin; length of myosin Z line: found at the center of light actin bands (I bands); where actin is anchored to Tropomyosin: Tod shaped and occupy the longitudinal grooves of the actin helix Troponin: holds tropomyosin in place; have three protein subunits attached to actin Sarcoplasmic Reticulum (SR): network of membranous channels that surround each myofibril; made of phospholipids; stores calcium Transverse Tubules (T Tubules): connect SR to cell membrane

Describe the neuromuscular junction and define motor unit

Synapse is the functional connection between neuron and cell (not physical connection). Neurons communicate through neurotransmitters (chemicals released by neuron for communication). Neurotransmitters diffuse to cell being controlled. Motor neuron control effectors, including skeletal muscle fibers. Neuromuscular junction is where a motor neuron axon and a skeletal muscle fiber meet. Motor end plate is the specialized part of a muscle fiber membrane at the neuromuscular junction. Small gap called the synaptic cleft separates the membrane of the neuron and the membrane of the muscle fiber Motor unit: motor neuron and all the muscle fibers it innervates

Explain the sliding filament model of skeletal muscle contraction, including cross-bridge formation and cycling, and the roles of tropomyosin, toponym, dystrophin, and calcium

When sarcomeres shorten, the thick and thin filaments do not change length but rather slide past one another with thin filaments moving toward the center of the sarcomere from both ends. The force that shortens the sarcomeres comes from cross-bridges pulling on thin filaments. A myosin head attaches to an actin binding site forming a cross-bridge and bend slightly pulling on the actin filament. Myosin heads contain the enzyme ATPase which catalyzes the breakdown of ATP to ADP and phosphate. This reaction transfers energy that provides the force for muscle contraction. Breakdown of ATP put the myosin head in a "cocked" position. When a muscle is stimulated to contract, a cocked myosin head attaches to actin forming a cross-bridge that pulls the actin filament toward the center of the sarcomere. When another ATP binds, the myosin head first detaches from the actin binding site, then breaks down the ATP to return to the cocked position. This cross-bridge cycle may repeat as long as ATP is present and action potentials release ACh at the neuromuscular junction. Troponin and tropomyosin block myosin from binding to actin Dystrophin sits right under cell membrane and reinforces it from wear and tear; no dystrophin causes the cell membrane to fail and the cell to die; adds strength to the sarcolemma so that the cell can repeatedly undergo contraction (shortening) Calcium ions trigger regulatory proteins (T&T) to shift position when it binds to troponin

Explain rigor mortis

state of muscle contraction/muscle stiffness after death where muscles are 'stuck'. it results from an increase in membrane permeability to calcium ions, which promotes cross-bridge formation, and a decrease in availability of ATP in the muscle fibers, which prevents myosin release from actin. The filaments of the muscle fibers remain linked until the proteins begin to decompose

Discuss how a neural signal is transmitted from the presynaptic potentials

synaptic transmission is carried out by neurotransmitters. the impulse runs down the the axon of the presynaptic neuron, where voltage-sensitive calcium channels open and calcium diffuses inward from the extracellular fluid. the increased calcium concentration inside the cell initiates a series of events that fuses the synaptic vesicles with the cell membrane, where they release their neurotransmitter by exocytosis. once the neurotransmitter binds to receptors on a postsynaptic cell, the action is either excitatory or inhibitory; 1. Action potential passes along an axon and over the surface of its synaptic knob 2. Synaptic knob membrane becomes more permeable to calcium ions, and they diffuse inward 3. In the presence of calcium ions, synaptic vesicles fuse to synaptic knob membrane 4. Synaptic vesicles release their neurotransmitter by exocytosis into the synaptic cleft 5. Synaptic vesicles become part of the membrane 6. The added membrane provides material for endocytosis vesicles