Physiology Exam #1

3 types of muscles

skeletal, cardiac, and smooth muscle

cardiac muscle

striated

involuntary

skeletal muscle

striated

voluntary

smooth muscle

non striated

involuntary

cardiac muscle

skeletal muscle

smooth muscle

Factors that contribute to skeletal muscle diversity

shape

size

number of fibers per area

length of fiber

angle of pennation

why is skeletal muscle diverse

diversity in skeletal muscle shape and structure allows for diverse function

Pennation angle

the angle between muscle fibers and tendons

A greater pennation angle can

allows more fibers per area

allows muscle to produce more force within a limited anatomical space

result in a smaller length change during contraction

Biomechanical Advantage

pennation —> more fibers in an area —> greater ability to produce force

Characteristics of skeletal muscle

Excitable & Contractile

Excitable

controlled voluntarily by the nervous system

Contractile

able to contract and produce force

Skeletal Muscle functions

movement

• the only tissue capable of this function

support

protection

energy storage

heat generation

Skeletal muscle: organ —> cell

whole skeletal muscle —> fascicle —> muscle fiber —> myofibril —> sarcomere

Fascicle (2)

a group of muscle fibers packed tightly together

Muscle Fiber (3)

groups of myofibrils packed together

Myofibrils (4)

cylindrical organells that contain sarcomeres inside the muscle fiber

Sarcomere (5)

the contractile unit of a myofibril

3 layers of Connective Tissue

1) epimysium

2) perimysium

3) endomysium

Epimysium

surrounds the entire muscle organ

Perimysium

surrounds the fascicles

• it contains lots of blood vessels and nerves

Endomysium

thin layer surrounds the individual muscle fibers

• contains tiny capillaries individual neurons

Sarcolemma

cell/plasma membrane (lipid bilayer) surrounding each muscle fiber

• NOT A CT

Connective Tissue

resists excessive passive stretching of the muscle and distributes forces to minimize damage to the muscle fibers

Function of Connective Tissues

provide a scaffolding for muscle fibers

• holds muscle fibers together

• help determine the gross structure of the muscle belly

provides a pathway/conduit for blood vessels and nerves = perimysium

resists excessive passive stretching

helps distribute forces to minimize damage to muscle fibers

Tendon

tough, fibrous, cord like tissue

extension of the epimysium, perimysium, and endomysium

• the CT layers don’t just stop at the muscle, but continue into the tendon

Functions of the tendon

physically connects muscle to bone (or another structure)

wraps around and within the muscle to transmit force

Myotendinous Junction

muscle contracts —> force —> MTJ —> tendon —> bone

Myotendinous Junction (MTJ)

the MTJ is the site where the muscle and tendon connect

function = force transmission

the contractile proteins of the muscle fivers transmit force to the

extracellular connective tissue proteins of the tendon

Blood Vessels and Nerve Fibers

provide nutrient delivery

help regulate muscle contraction

Skeletal Muscle within the Cell

1) sarcolemma

2) sarcoplasm

3) sarcoplasmic reticulum

4) sarcomere

5) myofibril

6) myofiber

sarcolemma (1)

the lipid bilayer surrounding the muscle fiber

• also called: plasma membrane, cell membrane

functions of sarcolemma

Regulates ion concentrations

Receives and transmits action potentials

• signals at the neuromuscular junction

• carries AP into muscle fiber through T-tubules

provides scaffolding

• fiber regeneration

• neighboring muscle cell attachment

Transverse (T) Tubules (2)

extensions of the sarcolemma

run deep into muscle fiver

carry action potential deep

interacts w/ end of the terminal cisternae of the sarcoplasmic reticulum

Sarcoplasmic Reticulum (SR) (3)

Function = stores calcium

runs parallel to the muscle fiber

is a membrane bound structure

functions similarly to the ER in other cells

Sarcoplasm (4)

the cytosol/cytoplasm of the muscle cell

stores: glycogen, myoglobin, proteins, minerals, fats, organelles

Myofibrils (5)

is a structure inside the muscle fiber that contains sarcomeres

sarcomere (5)

is the contractile unit of a myofibril

• thick filament = Myosin

• thin filament = Actin

the arrangement of the filaments creates the striated appearance

Myosin

thick filament

Head

• binds to actin

• contains ATPase activity

Actin

thin filament

contain myosin binding sites

Tropomyosin

part of thin filament

wraps around actin

blocks the myosin binding sites

Troponin

part of thin filament

bound to actin

holds tropomyosin in place

Nuclei/Myonuclei (6)

skeletal muscle fibers = multinucleated

myonuclei located near the edge of muscle fibers

function of nuclei/myonuclei

genetic control

control gene expression

replication of DNA during the cell cycle

Mitochondria (7)

are responsible for producing ATP

use oxygen and water to convert energy into a form the cell can use

muscle cells need ATP to generate force

Mitochondria produce ATP w/ oxidative metabolism including:

krebs cycle

electron transport chain

oxidative phosphorylation

What is membrane potential (MP)

the electrical potential difference/voltage between the intracellular and extracellular spaces

What is Resting Membrane Potential (RMP)

for a neuron, -70mV (at rest)

• the inside of the cell is more negative

• outside is more positive

What determines RMP

ion channels

gated channels

ion pumps

ion channels

always open

ions diffuse through them down their gradient

gated channels

closed until a stimulus acts on the channel

once open, ions diffuse through the channel down their gradient

ion pumps

actively transport ions across the membrane

requires ATP

Potassium (K+) channels

allow K+ to leak out of the cell

Voltage gated Sodium (Na+) channels

open/close depending on membrane voltage

important during the action potential

Voltage gated Potassium (K+) channels

open/close depending on membrane voltage

important during action potential

Sodium/Potassium Pump (Na+/K+ pump)

pumps 3 Na+ OUT

pumps 2 K+ IN

requires ATP

what maintains RMP

primarily through the Na+/K+ pump and K+ channels

Action Potential (AP)

change in membrane potential that occurs when the cell reaches threshold

1) stimulus

2) depolarization

3) repolarization

4) hyperpolarization

5) resting membrane potential (RMP)

Threshold

MUST be reached to initiate an action potential

once threshold is reached:

• both Na+ and K+ voltage gated channels open

  • but they open at DIFFERENT speeds

At Rest of AP

cell is -70mV

no movement

voltage gated Na+ channel = closed

voltage gated K+ channel = closed

Threshold of AP

stimulus caused MP to move toward threshold

• trigger point for AP

a little Na+ move IN

voltage gated Na+ channel = stimulated to open

voltage gated K+ channel = stimulated to open

Depolarization of AP

rapid upward portion

Na+ moves into the cell A LOT

voltage gated Na+ channels = open

voltage gated K+ channels = closed

the inside of cell become more positive = graph goes up

Peak of AP

top of waveform

a little Na+ moves IN

a little K+ moves OUT

VG Na+ channels = closing

VG K+ channels = opening

transition point

Repolarization of AP

downward portion of AP

a lot of K+ moves OUT

VG Na+ channels = closed

VG K+ channels = open

inside becomes more negative

Hyperpolarization of AP

membrane potential becomes more negative than normal RMP

a little K+ move OUT

VG Na+ channels = closed

VG K+ channels = closing

then the membrane return to normal RMP

EPSP & IPSPs

summation of all incoming signals determines whether threshold is reached and an AP is generated

Excitatory Postsynaptic Potential (EPSP)

depolarizes the membrane

makes it more positive

brings the cell closer to threshold

Ex: opening of a ligand gated Na+ channel

Inhibitory Postsynaptic Potential (IPSP)

makes membrane more negative

moves cell further away from threshold

Ex: opening of ligand gated K+ channel

How does action potential travel

Na+ moves more negative areas inside the cell

causes next section to reach threshold

a new AP begins in next section

• doesn’t travel backwards b/c of VG Na+ channels

Orthodromic (Electrical Nerve Stimulation)

AP travels in the same direction as physiological signaling

Antidromic

AP travels in the opposite direction from physiological signaling

What determines Action Potential Conduction Velocity?

1) Myelination

• more myelin = faster conduction

2) Axon Diameter

• larger axon = faster conduction

Motor Unit

consists of the motor neuron and all the scattered muscle fibers which it innervates

Motor Neuron

is the nerve cell that communicates with the skeletal muscle

Motor Unit = motor neuron + all muscle fibers it controls

Parts of the motor neuron

Dendrites

Cell body

Axon

Axon Terminals

Dendrites

receive input/signals from the dendrites

Cell body

receives the signals from the dendrites

decides what to do with the summed inputs

Axon

transmits the signal along the length of the neuron

Axon Terminals

transmit signal to the adjacent neurons/cells

Synapse

connection between axon terminal from neuron 1 and dendrites of neuron 2

Neuromuscular Junction (NMJ)

the synapse that allows communication between the motor neuron and ALL the muscle fibers it innervates

Neurotransmitter

Acetylcholine (ACh)

Receptor

Nicotinic Acetylcholine receptor

Motor End Plate

specialized area of the sarcolemma at the NMJ

Motor Endplates Contain

Post Junctional Folds

• within the sarcolemma

• increase the surface area of the motor endplate by 8-10x

Nicotinic Acetylcholine Receptors

• motor endplate is covered w/ Nicotinic ACh receptors

• binding of ACh opens ligand gated Na+ channels

  • Na+ enter cell and MP changes

EXCITATION

getting the signal from the motor neuron —> along the muscle fiber

Excitation Step 1

Action potential reaches the motor neuron axon terminal

• signal travels down motor neuron toward axon terminal

• axon terminal = motor neuron communicates w/ muscle

Excitation Step 2

ACh is released from synaptic vesicles

• then diffuses across the synaptic cleft

Excitation Step 3

ACh binds to Nicotinic Acetylcholine Receptor on motor endplate

• opens Ligand Gated Na+ channel

Excitation Step 4

Na + enters the muscle fiber

• changes the membrane potential

• if threshold reached, AP is generated

Excitation Step 5

action potential propagates along the sarcolemma

neuron RMP = -70 mV

skeletal muscle RMP = -85 mV

skeletal muscle threshold = -55 mV

Characteristic of Action Potential

resting membrane potential = - 85 mV (neuron -70 mV)

threshold potential of skeletal muscle = - 55mV

all or nothing

propagated wave of sarcolemma depolarization at a constant velocity

Triad

is the interface between: T tubule + sarcoplasmic reticulum (SR) on both sides

Excitation-Contraction Coupling

rapid communication between electrical events at the sarcolemma of skeletal muscle fibers Ca2+ release from the SR

Step 1 of EC Coupling

initiation and propagation of an AP along the plasma membrane

Step 2 of EC Coupling

the electrical signal spreads radially/deep into the muscle fiber through the T=tubule

• Why?

  • b/c the muscle fiber is large and the signal needs to reach the deeper portions of the cell