9- Muscle Tissue

What is the order of muscle hierarchy?

whole muscle

• muscle fascicle

  • muscle fiber

    • myofibril

      • myofilament

Whole Muscle

complete organ surrounded by muscular fascia & epimysium

• BV & nerves (motor neurons!)

• made of several fascicles

Muscle Fascicle

bundle of muscle fibers w/in entire muscle

• surrounded by perimysium/CT

Muscle Fiber

individual muscle cell w/in fascicle

• surrounded by endomysium/CT

• made from several myofibrils 

Myofilament

protein filaments, myosin or actin, shorten muscle fiber during contraction

What are the functions of skeletal muscle?

locomotion, facial expressions, posture, respiratory movements, other movements 

• voluntary movements (communicate, breath, generate heat, hold up)

What are the functions of smooth muscle tissue?

involuntary movements, mix food in digestive tract, dilate/constrict pupils, regulate blood flow, moving things, propel urine

What controls smooth & cardiac muscle tissue?

involuntarily by endocrine & autonomic nervous systems

Where is smooth muscle tissue found?

walls of hollow organs (digestive), BV, eyes, glands, skin

What are the functions of cardiac muscle tissue?

involuntary contractions, pump blood throughout the body, regulate heart rhythm, maintain blood circulation

• autorhythmic

What is the cellular nature of skeletal muscle?

long, cylindrical, multinucleated cells

• nuclei peripheral/pushed out 

• no cell-cell attachments 

• striated 

Which muscle tissues are able of spontaneous contraction?

smooth & cardiac

T/F: Skeletal muscle is purely voluntary.

False, some involuntary movements w/ reflexes

What is the cellular nature of smooth muscle?

spindle shaped, single nucleus/central

• gap junctions join visceral smooth muscle cells 

• NOT striated

What is the cellular nature of cardiac muscle?

cylindrical, branched, single nucleus/central

• intercalated disks join cells 

• striated

What are the functions of the muscular system?

moving body, posture maintain, respiratory movements, make body heat, communication, organ/vessel constrict, heart contract

What are general properties of muscle tissue?

contractability, excitability, extensibility, elasticity

What is contractility?

ability of a muscle to shorten w/ force

What is excitability?

capacity of muscle to respond to stimulus (usually nerves, some reflexes)

What is extensibility?

muscle can be stretched beyond normal resting length, still able to contract (stretch & not destroy)

What is elasticity?

ability of muscle to recoil to original resting length after stretched

What 2 cycles do muscles undergo?

• contract & relax

• contract/stretch/relax

What are the CT coverings for skeletal muscles?

epimysium, perimysium, endomysium

What is the epimysium?

CT surrounds whole muscle, many fascicles

• merge w/ muscular fascia 

• wraps around several bundles

What is muscular fascia?

layer of CT b/w adjacent muscles & b/w muscles & skin

What is the perimysium?

loose CT surrounds group of muscle fibers, 1 fascicle

• passage for BV & nerves 

What are fascicles?

Bundles of muscle fibers grouped together w/in a muscle

What is the endomysium?

loose CT separating individual muscle fibers w/in each fascicle 

• wraps 1 fiber 

How does CT form tendons?

collagen from CT layers merge to form tendons/aponeuroses

• attach muscle to bone 

What are myoblasts?

Myoblasts are the precursor cells that fuse to form muscle fibers during development, contributing to muscle growth and repair.

How are skeletal muscle fibers developed?

develop from fusion of myoblasts

• make multinucleated muscle cells

• striated appearance due to contractile proteins organized in sarcomeres

Do muscle fibers change throughout life?

no, number of fibers stays constant after birth

How do muscles get larger?

hypertrophy of muscle fibers

What is hyperplasia?

more muscle fibers being made due to more cells, increasing muscle bigger 

How do hyperplasia & hypertrophy contribute to muscle growth?

Both processes increase muscle size

• hypertrophy enlarges existing fibers

• hyperplasia adds new fibers

What is the structure of muscle fibers?

sarcolemma, T tubules, sarcoplasmic reticulum, endomysium covering, myofibrils

What is the sarcolemma?

plasma membrane of muscles, surrounds sarcoplasm & other contents of cell

What are T tubules?

transverse tubules, inward folds of sarcolemma

• project into interior of muscle cell 

What is the sarcoplasmic reticulum?

specialized smooth ER, stores calcium

• have terminal cisternae

What are terminal cisternae?

enlarged portions are adjacent to T tubules

What is a triad?

2 terminal cisternae & associated T tubule 

What is the general structure of muscle fibers?

myofibrils surrounded by SR & terminal cisternae, triads

What are the mechanical component structures allow muscles to contract?

myofibrils & myofilaments

What are myofibrils?

bundles of protein filaments

• contain protein filaments (myofilaments) causing contraction

What myofilaments?

actin/thin & myosin/thick myofilaments 

What are myofilaments arranged into?

sarcomeres

What is the order of organization for muscles from smallest to largest unit?

1. myofilament

2. myofibril

3. sarcomere

4. muscle fiber (endomysium)

5. fascicle (perimysium)

6. muscle (epimysium)

What is a sarcomere?

basic functional unit of muscle fiber, smallest part that can contract, Z disk

What is a Z disk?

filamentous network of protein, attachment for actin myofilaments

What are the regions of a sarcomere?

I band, A band, H zone, M line, 

What are I bands?

lighter staining regions, less things

• each have Z disk

• extend to ends of myosin myofilaments

• isotropic → angle always the same

• thin filaments/actin

What are A bands?

central dark staining region

• overlapping actin & myosin myofilaments

• myosin myofilaments surrounded by actin myofilaments

• no overlap at center!

What is the H zone?

region in A band, actin & myosin don’t overlap

What is the M line?

middle of H zone, delicate dilaments

• hold myosin in place

• w/in H, midline 

What produces a striated appearance?

A & I bands of parallel myofibrils are aligned

What are titin filaments?

elastic chains of amino acids, make muscles extensible & elastic 

What does F actin?

2 strands of fibrous actin → double helix

• double helix extends length of myofilament

• attached at either end of sarcomere

• G actin monomers

What is F actin made of?

consists of G actin monomers arranged in a double helix structure

• each has an active site

• actin site can bind myosin during muscle contraction

What is tropomyosin?

elongated protein winds along groove of F actin double helix

What is troponin?

3 subunit

• 1 → binds to actin

• 2 → binds to tropomyosin

• 3 → binds to Ca ions

spaced b/w ends of tropomyosin mols in groove b/w F actin strands

What does the troponin/tropomyosin complex regulate?

interaction b/w active sites on G actin & myosin

Myosin Myofilament

• elongated myosin mols shaped like golf clubs, thick & heavy

• myosin heavy chains wound together = rod portion (H zone)

  • parallel to myosin myofilament

  • 2 myosin heads extend laterally

What are myosin heads capable of?

• bind to active sites on actin mols to make cross bridges

• attached to rod portion of hinge region, bend & straighten during contraction

• ATPase enzymes → release E/ATP breakdown

  • E used to bend hinge region of the myosin mol during contraction/reset for next

What are motor neurons?

stimulate skeletal muscle contraction

• alpha motor neurons 

• 1 neuron → controls several muscle fibers

• form neuromuscular junctions to transmit signals

What blood vessels are seen in muscles?

• artery & 1-2 veins extend w/ nerve through CT layers 

• capillary beds surround muscle fibers

How are motor neurons involved in the neuromuscular junction?

carry electrical signals/action potentials

• stimulate muscle fiber action potentials

• followed by muscle contraction

• NMJ → synapse, contact b/w motor neuron & muscle fiber 

What is the NMJ made of?

• presynaptic terminal 

• synaptic cleft

• motor end plate/postsynaptic membrane

What is unique about the presynaptic terminal of the motor neuron?

synaptic vesicles with neurotransmitter

• nt binds to ligand gated ion channels W

Which neurotransmitter does the motor neuron release?

acetylcholine to stimulate muscle contraction.

What is the sliding filament model?

actin myofilaments slide over myosin to shorten sarcomeres

T/F: when contracting, actin & myosin change length.

False, actin & myosin don’t change length, crawl over e/o

What is structurally responsible for muscle contraction?

shortening sarcomeres

What is chemically responsibly for muscle contraction?

acetylcholine, neurotransmitter released by motor neuron

What causes muscle relaxation?

sarcomeres lengthen from external force

• ex → contraction of antagonistic muscles (do opposite effect of og)