Block 2 Module 4: Muscle - PNB 2774

characteristics of muscle tissue

excitability, contractility, elasticity, extensibility

excitability

a characteristic of muscle tissue; outside stimuli can initiate electrical changes in the muscle fiber, leading to contraction of that muscle fiber

contractility

a characteristic of muscle tissue; stimulation of muscle fiber can lead to contraction or shortening of the muscle fiber

elasticity

a characteristic of muscle tissue; a muscle fiber’s ability to return to its original length when the tension of contraction is released

extensibility

a characteristic of muscle tissue; the ability of a muscle fiber to be stretched beyond its relaxed length

muscle tissue types

skeletal muscle (to move skeleton and joints), cardiac muscle (to pump heart), and smooth muscle (involuntary, in digestive system and blood vessles)

components of muscle tissue

connective tissue (connects latter components), muscle fascicles (bundles composed of individual muscle fibers), blood vessels, and nerves

muscle fiber

synonymous with muscle cell

motor unit

consists of a single motor neuron and the muscle fibers it controls (cannot have more than one neuron controlling it), controls only a few muscle fibers in an entire muscle (so an entire muscle may have many of these); larger muscles have more of these than do smaller muscles

structural components of motor unit

efferent motor neuron, which connects to a few muscle fibers, at neuromuscular junctions

all or nothing principle

similar to neural firing; the concept that muscle fibers either completely contract or do not contract at all - when its motor neuron is stimulated, all the muscle fibers under its control contract

Henneman’s size principle

as force increases in a muscle, more and larger motor units are recruited to generate larger force; there is a progressive increase in neural input to different types of motor neurons

properties of muscle tissue

not all muscle cells in a muscle contract at the same time, only the ones activated by the same neuron; the number and type(s) of motor neurons activated determines the strength of the contraction (small units or small number of units at low frequency is a weak contraction, large units or large number of units at greater frequency is a strong contraction)

muscle tone

the continued steady, low level of contraction that stabilizes joints and maintains general muscle health

S type motor neuron

small motor neurons with high excitability, “slow”, recruited first

FR type motor neuron

big motor neurons with average excitability, “fatigue resistant”, recruited second

FF type motor neuron

very big motor neurons with low excitability, “fatigable”, recruited third

type I muscle fibers

slow twitch muscle fibers; slow oxidative (primarily aerobic processes), high mitochondria concentration and high resistance to fatigue; contains low glycogen

type IIa muscle fibers

fast twitch muscle fibers; fast oxidative (combination aerobic and anaerobic processes), high or moderate mitochondria concentration and high or moderate resistance to fatigue; contains moderate glycogen

type IIx muscle fibers

fast twitch muscle fibers; fast glycolytic (primarily anaerobic processes), low mitochondria concentration and low resistance to fatigue; contains high glycogen

synaptic knob

the expanded end of the neuron

synaptic vesicles

membrane-bound sacs filled with acetylcholine (ACh)

synaptic cleft

the narrow space separating the synaptic knob from the motor end plate

motor end plate

region of sarcolemma (muscle cell plasma membrane) across from the synaptic knob that has folds and indentations to increase the surface area in that region

ACh receptors

ionotropic receptors (calcium) in the motor end plate that bind to ACh

acetylcholinesterase (AChE)

an enzyme in the synaptic cleft that rapidly breaks down ACh (prevents reuptake, etc.)

sarcolemma

the plasma membrane in muscle cells

sarcoplasm

the cytoplasm in muscle cells

sarcoplasmic reticulum

a membrane-bound organelle found within muscle cells that is similar to the smooth endoplasmic reticulum in other cells; main function is to store calcium ions

transverse tubules (t-tubules)

deep invaginations of sarcolemma; extend into sarcoplasm

terminal cisternae

Calcium-filled sacs at end of sarcoplasmic reticulum

steps of excitation-contraction coupling

motor neuron releases ACh at the junction; net entry of sodium ions through cation nonselective ACh receptor-channel initiates a muscle action potential; action potential (voltage gate) across t-tubule membrane alters conformation of DHP; DHP opens RyR calcium release channels, and calcium enters the cytoplasm; calcium binds to troponin, allowing actin-myosin binding; myosin heads execute a power stroke - actin filament slides toward the center of sarcomere

DHP receptor

protein complexes in t-tubule membranes (in muscle fibers); when membrane depolarizes in excitation-contraction coupling, they open (voltage gated), mechanically pulling RyR channel with it

RyR channel

a protein in the sarcoplasmic reticulum, holds calcium in the sarcoplasmic reticulum back from the cytoplasm until an excitation-contraction coupling event occurs

how calcium triggers contraction

binds to troponin, allowing actin-myosin binding; myosin heads execute a power stroke - actin filament slides toward the center of sarcomere

myofibrils

muscle fiber sarcoplasm contains 100s-1000s of cylindrical protein-based fibrous structures that extend the entire length of the cell; these structures have the ability to shorten, resulting in the contraction of the muscle; contain multiple repeating sarcomeres

sarcomere

the functional contractile unit in a skeletal muscle fiber; defined by the area between two adjacent Z discs (made of alpha-actinin), middle is the M-line (made of myomesin, C-protein, and creatine kinase)

Z discs

also called Z line, the end/staring points marking different sarcomeres; made of alpha-actinin

M line

the middle of a sarcomere; made of myomesin, C-protein, and creatine kinase

myofilaments

also sarcomeres, repetitive groupings within myofibrils; two types of filaments are thin and thick

thin myofilaments

connected to Z line; made of actin twisted around each other, and associated proteins; individual actin molecules - G-actin - globular actin monomers; strands of actin molecules - F-actin - filamentous actin polymers; ALSO contains two primary regulatory proteins (tropomyosin and troponin C)

thick myofilaments

connected to M line; made of myosin, which contains thick globular heads and long filamentous tails; myosin heads form crossbridges with actin thin filaments during contraction

skeletal muscle contraction process

calcium levels increase in cytosol, calcium binds to troponin in thin filaments, troponin-calcium complex pulls tropomyosin away from actin’s binding site (exposes it), myosin binds strongly to actin and completes power stroke, actin filament moves

myosin binding process

myosin heads bind to actin, they release a Pi and perform a power stroke, ADP is released, ATP binds and cross-bridge detaches, ATP is hydrolyzed so myosin heads return to relaxed state with one ADP and one Pi bound

smooth muscle histology

found in the walls of the gut and in blood vessels; composed of short fusiform cells (wide in the middle, tapered at the end) - much smaller than skeletal muscle cells; contain one long, centrally located nucleus; no striations (smooth), and have less sarcoplasmic reticulum; held together by “attachment plaques” (similar to adhering junctions), and may be linked by gap junctions in single-unit situations

smooth muscle physiology

not arranged in sarcomeres, have longer actin filaments from different dense bodies; pulled anti-parallel by myosin, which has slow heavy chain ATPase activity; myosin light chain (actin) plays regulatory role

IP3 receptor channel

the primary calcium channel used in smooth muscle (as opposed to RyR in skeletal muscle - smooth muscle does have RyR, and recieves calcium from ECF through voltage-gated calcium channels, but uses this most); on the sarcoplasmic reticulum; either opened (activated) by calcium or activated by a G-protein coupled receptor production of IP3

smooth muscle contraction process

intracellular calcium increases from ECF and SR, calcium binds to calmodulin, Ca-CaM activates myosin light chain kinase, MLCK phosphorylates MLC increasing myosin heavy-chain ATPase activity, active myosin crossbridges pull on actin, creating muscle tension

smooth muscle differences

use CaM as a calcium sensor (skeletal and cardiac uses Troponin C); filaments are pulled anti-parallel (skeletal and cardiac pulls actin thin filaments together); requires MLC phosphatase to dephosphorylate (S & C myosin heads do that themself)

phosphorylation

effects protein shape, activity, binding, stability, etc.; often attached to proteins by kinases and removed (protein inactivated) by phosphatases

smooth muscle relaxation process

calcium in cytosol decreases when calcium is pumped out of cell or back into SR (through plasma membrane calcium primary active pumps, and secondary active sodium-calcium exchangers, also sarco-/endoplasmic reticulum calcium pump - SERCA - takes calcium back into SR), calcium unbinds from CaM, myosin phosphatase removes phosphate from myosin decreasing myosin ATPase activity - less of this activity results in decreased muscle tension

smooth muscle contraction control mechanisms

smooth muscle operates on slow wave potentials, which occurs over several seconds and usually involves multiple slow waves before reaching action potentials

pharmacomechanical coupling

occurs when chemical signals change muscle tension (usually in smooth muscle) through STPs with little or no change in membrane potential

intercalated discs

a structural component of cardiac myocytes, contains desmosomes (to resist stress on membranes during contraction) and gap junctions (electrical communication)

cellular properties of cardiac muscle fibers

almost exclusively found in heart walls, striated, mono or multinucleated, feature y shaped branching for better intercellular electrical communication, join other cells at intercalated discs

cardiac muscle control process

autorhythmic involuntary pacemaker cells are spontaneously and set the pace (drive beating) for heart rate beginning at the SA node (where P cells are); intercalated discs (gap junx.) help spread signals; the pace can be altered by hormones: norepinephrine (faster) and acetylcholine (slower), and autonomic neurons