anatomy exam 3

Movement:

The contraction of the skeletal muscle is responsible for overall movement.

Maintain Posture:

skeletal muscle constantly maintains posture

Production of body heat:

 Skeletal muscle contractions help maintain body temperature

Communication:

 Skeletal muscles are responsible for speaking, writing, typing, gesturing, and facial expressions.

Glycemic control:

skeletal muscle absorbs, stores, and uses a large amount of glucose, thus regulating/stabilizing glucose concentration.

Contractibility:

ability to contract/shorten.

Excitability:

the capacity of muscle cells to respond to stimuli

Conductivity:

electrical changes trigger a wave of excitation

The extensibility

 muscle can be stretched beyond its normal resting length and still be able to contract.

Elasticity

 the ability of a muscle to return to its original length

Skeletal muscle:

attached to the skeletal system.

• Contain long, threat-like cells that are multinucleated (located peripherally/edges)

• Striations present (light and dark bands)

• Under voluntary control (manual contraction)

Function of skeletal muscle

body movement, posture, body heat

Smooth muscle:

 long, thin cells that taper at the end

• Single, centrally located nucleus

• Under involuntary control

• No striations

Function of smooth muscle

 makeup and function of visceral internal organs, such as digestion, emptying of bladder//both involuntary and voluntary control, and regulation of blood vessels. 

Cardiac muscle:

cells are called cardiocytes, short, branched cells

• Single, centrally located nucleus

• Striations present

• Involuntary control Autonomic (cardiac): self-excitable and able to generate an action potential without external stimulation by nerve cells

Intercalated discs (cardiac):

gap junctions and mechanical junctions (desmosomes) that join cardiocytes

Belly:

 enlarged fleshy body of muscle located between the slender points of attachment

Fascia:

 layers of tough connective tissue

Epimysium:

outer layer of fascia that can extend to the bone and form tendons

Perimysium:

connective tissue layer that surrounds fascicles (bundles of muscle fibers)

Endomysium:

connective tissue layer around individual muscle fibers (located in fascicles)

Sarcomere:

 contractile units that make up myofibrils; the smallest portion of muscle that can contract, contains myofilaments

Actin (thin):

 composed of 3 separate proteins

Tropomysin:

• a long fibrous protein that lies in the groove along the actin strand. COvers the “attachment/active site” in relaxed muscles. (muscles can't contract until tropomyosin moves away from the active site) NERDS ROPE

Troponin:

a protein with 3 parts

• Anchor troponin to actin

• Prevents tropomyosin from uncovering the actin attachment site in a relaxed muscle

• Binds calcium ions

Myosin (thick):

 composed of elongated myosin molecules shaped like golf clubs

Rod portion:

lying parallel to the myosin molecule

Myosin heads:

2 structures that extend laterally (stick out to the side). 

Myosin heads have 3 important functions: 

• Heads bind to the active site on actin to form a cross-bridge

• Heads are attached to the rod with a hinge region that bends and straightens during contraction

• Heads break down ATP, releasing energy

Sarcomere structure

• Sarcomeres join end to end, forming myofibrils

• Arrangement of actin and myosin gives muscles their striated appearance

Z discs:

•  stationary anchors for actin myofilaments. Creates the boundary for the sarcomere. 

I bands:

• two light-staining regions. contains only actin. Includes Z disks (anchors) and extends to myosin

A band:

• a darker staining band in the center of each sarcomere. Contains both actin and myosin

H zone:

• center of A band (middle), contains only myosin

M-line:

• middle of H zone. Consists of protein filaments that hold myosin in place

Neuromuscular junction (synapse):

• point of contact between a motor neuron axon and muscle fiber. Consists 5 things

 The presynaptic terminal/synaptic knob:

• enlarged axon terminals

Synaptic vesicles:

• small spherical sacs that contains acetylocholine (neurotransmitter)

Synaptic cleft:

•  space between presynaptic terminal and muscle

Postsynaptic membrane (motor end plate/junctional folds):

• a portion of the sarcolemma of muscle fiber

• It Contains high concentration of ACh receptors (transmembrane proteins)

Acetylcholinesterase (AChE):

• enzyme that degrades ACh and stop stimulation of muscle fibers.

the first step of muscle contraction

Excitation

action potential in nerve fiber leads to action potential in muscle fiber

the second step of muscle contraction

Excitation-Contraction Coupling:

• events that connect the action potential of the sarcolemma to activation of myofilaments

the third step of muscle contraction

contraction

muscle fiber shortens by the sliding filament theory (explains muscle contraction as the result of actin and myosin filaments sliding past each other, shortening the sarcomere, the basic unit of muscle contraction)

the fourth and final step of muscle contraction

Relaxation:

•  nerve fibers stop stimulating a muscle fiber, and the muscle fiber returns to its resting length

Electrical potential/voltage:

difference in electrical charges from one point to another. (inside of cell vs outside)

Polarized:

 separation of charges between the inside of the cell and the outside

Resting membrane potential for sarcolemma

 higher negative charges inside the cell as compared to outside the cell.

~ due to the high concentration of K+ in ICF and high Na+ in ECF (maintained by the SODIUM POTASSIUM PUMP)

Depolarization:

change in the membrane potential; for muscles, this occurs at the site of the receptors as the interior of the cell becomes slightly positive (less neg) than before. SODIUM RUSHES IN

~ influx of Na+ into cells 

Repolarization:

the interior membrane returns to a negative charge. POTASSIUM RUSHES OUT

~ influx of K+ out of cells

Action potential:

quick up and down shift in voltage that spreads along the sarcolemma

Excitation STEPS

step 1

A nerve signal arrives at the synaptic knob and it opens voltage gated calcium channels and ca+ ions eneter from extracellular fluid.

excitation steps

2

Calcium stimulates vesicles to release acetylcholine into the synaptic cleft via exocytosis.

excitation steps

3

ACh (acetylcholine) diffuses across the cleft and binds to ACh receptors on the carcolemma

excitation step

4

Receptors open, allowing Na+ to quickly flow into the cell and K+ to flow out of the cell, causing the sarcolemma to reverse polarity and return to its original resting state. Forms and end plate potential (EPP) rapid change in polarity at the motor end plate. THE MUSCLE IS POTENTIALLY CHARGED

excitation step 5

Action potential created from Na+ moving into and K+ moving out of areas near the EPP.. A muscle fiber is now excited!

Excitation, contracting, coupling steps!

step 1

A wave of action potentials spreads down into the cell interior via T-tubules

Excitation, contracting, coupling steps!

2

An action potential opens voltage-gated channels in the terminal cisternae, which causes SR to diffuse Ca+ into the cytosol

Excitation, contracting, coupling steps!

3

Calcium binds to troponin

Excitation, contracting, coupling steps!

4

Troponin-tropomyosin complex changes its shape and exposes the active site on actin

Sliding filament theory steps

1

Myosin must have ATP bound to it to initiate the connection. Myosin ATPase hydrolyzes this ATP to ADP and phosphate. Energy released activated the head to “cock” into high energy position (extended). ADP and phosphate still bound to the head

Sliding filament theory steps

2

Extend myosin binds to the active site on actin, forming cross-bridge

Sliding filament theory steps

3

Myosin releases ADP and phosphate, flexes into a bent (low energy) position, tugging the thin filament along with it, called the power stroke

Sliding filament theory steps

4

ATP binds to myosin and destabilizes the myosin/actin bond, breaking the cross-bridge. Myosin repeats the whole process and performs a recovery stroke (myosin attaches to a new actin site further down the thin filament)

Relaxation steps!

1

Nerve signals stop arriving at the neuromuscular junction, and ACh is no longer released.

Relaxation steps!

2

ACh (acetylcholine) dissociates from receptors, AChE breaks it down into fragments. Synaptic knob reabsorbs the fragments for recycling. 

 ~ This is also done while the muscle is being stimulated, but it increases when the nerve signal stops

Relaxation steps!

3

Ca+ release stops in the SR ( circoplasmic reticulum), and Ca+ reabsorption increases. In SR, calcium binds to the protein calsequestrin, allowing SR to store large amounts of calcium without forming salts

Relaxation steps!

4

Levels of free Ca+ decrease in the cytosol because of S.R. reabsorption. Ca+ dissociates from troponin

Relaxation steps!

5

Tropomyosin moves back to its resting location, blocking the active site. Myosin can no longer attach to actin.

cord of collegen fibers that attaches muscle to bone

epimysium

series of contractile units that make up each myofibrils

sarcomere

connective tissue that surrounds muscle fibers

endomysium

extension of the cell membrane that penetrates into the interior of the muscle

transverse (t) tubules

connective tissue unsheathing the fascicles

perimysium

actin or myosin containing structure

myofibrils

calcium stores in this structure in the relaxed muscle

sarcoplasmic reticulum

connective tissue that surrounds the whole skeletal muscle

fascia

muscle cell is

a muscle fiber

the gap between a motor neuron axon and muscle cell membrane

synaptic cleft

swelling at the end of each nerve fiber in a synapse

synaptic knob

pre synapse that contains this neurotransmitter

acetylcholine (ach)

the point where a nerve fiber meets a muscle fiber

neuromuscular junction

increases surface area of ach-sensitive membranes

neuromuscular junction