Skeletal Muscle - Anatomy & Excitation Flashcards
What is muscle?
Muscle is one of the four basic types of tissues
What is the function of muscle tissue?
Muscle tissue is able to contract and produce mechanical force
- contracts to produce movement
- contributes the most to body weight
Found in skeletal muscle, cardiac muscle, and smooth muscle
What are the three distinct subtypes of muscle tissues?
- Striated muscle
a) Skeletal muscle tissue
b) Cardiac muscle tissue - Smooth muscle
c) Smooth muscle tissue
What is skeletal muscle tissue (appearance/functions)?
Appearance: long, cylindrical, and banded (stripped/striated), contain multi nuclei (multinucleate)
- moves the skeleton/body by pulling on bones of the skeleton
- every skeletal muscle is attached to the skeleton except for external urinary sphincter
- voluntary movements
What is cardiac muscle tissue (appearance/functions)?
Appearance: stripped/striated, branched, short, single nuclei, cells are interconnected at specialized intercellular junctions called intercallated discs
- produces contractions that move blood within the heart and through blood vessels
- pumps blood through the heart
- involuntary movement
What is smooth muscle tissue (appearance/functions)?
Appearance: no stripes, spindle-shaped, non-striated, single central nucleus
- found in blood vessels, digestive tracts, etc.
- contractions move fluids and solids along the digestive tract and regulate the diameters of the small arteries
- involuntary movement
Each skeletal muscle is connected to the skeletal system via ______________________________
Dense regular connective tissue:
Tendons - narrow and/or rounded
- attach the muscle to a specific point on a bone
Aponeuroses - broad/flat
- provides attachment over a broad area that may involve more than one bone
- anchor bones to muscles (skeletal muscles to the skeletomuscular system)
What are 6 functions of skeletal muscle tissue?
- produce body movement
- maintain posture and body position
- support soft tissues
- guard body entrances and exits
- maintain body temperature
- store nutrients
Where are skeletal muscles often organized?
Skeletal muscles are often organized within layers of fasciae / connective tissue not associated with epithelia
- fascial compartments can have more than one muscle
Briefly describe the structure/tissues involved in a muscle
Tendon - connects muscle to bone
Fascicles - bundles of skeletal muscles
- one fascicle will contain several muscle fibers (myofibers)
- perimysium (inside)
- epimysium (outside)
- endomysium (inside)
- nerve fibers
- epithelia
What are the three connective tissue layers that surround, support, and attach a muscle
- epimysium
- perimysium
- endomysium
What is epimysium
- surrounds the whole muscle
- connects to tendon / surrounds the entire muscle
- made of dense irregular connective tissue (dense layer of collagen fibers)
What is perimysium
- fibrous layer that divides the skeletal muscle into compartments
- surrounds a fascicle
- dense elastic connective tissue
What is endomysium
- surrounds each muscle fiber
- loose/areolar connective tissue
- thin layer of areolar connective tissue that surrounds each muscle fiber
What is a muscle fascicle?
a bundle of muscle fibers - fascicles are separated from each other via the fibrous perimysium layer on the outside
Differentiate epimysium, perimysium, and endomysium
Epimysium - surrounds the entire muscle
Perimysium - surrounds the fascicles in muscle tissues
Endomysium - surrounds the muscle fibers in fascicles
Skeletal muscles
bundles of fascicles
bundles of several muscle fibers (myofibers)
myofibrils
memorize :,(
What is a myofiber?
a muscle fiber or muscle cell
- a single cell containing many nuclei that stretch from the muscle origin to its insertion
- stretches from TENDON TO TENDON
- can contain A LOT of nuclei within the cell membrane -> nuclei are all superficial (close to the surface of the muscle fiber)
What are myofibrils?
bundles of protein filaments that make up myofibres
- special protein filaments that contract in myofiber cells
Differentiate muscle, fascicle, myofibre, and myofibril
Muscles are made of bundles of fascicles
Fascicles are made of bundles of myofibres
Myofibres are made of bundles of myofibril rods
What does a muscle fiber contain?
- blood vessels
- axons of neurons
- myosatellite cells
- myofibrils
What are myosatellite cells?
Stem cells for muscle tissues within endomysium
- stem cells that function in the repair of damaged muscle tissue
What are the functions of axons in myofibers?
Axons of the neurons control the muscle fibers
How are myofibers produced? (embryonic development)
Myofibers are produced by the FUSION of many myoblasts during development
During embryonic development:
1. Myoblasts fuse to form multinucleate cells
2. Develop into skeletal muscle fibers
3. Myoblasts which do not fuse with the developing muscle fibers remain in the endomysium of adult skeletal muscle tissue as myosatelite cells
How does skeletal muscle repair itself? (as an adult)
Myosatellite cells fuse with damaged muscle fibers to repair the tissue
- repair through myosatellite proliferation with existing myofibers
Each mature myofiber has ONE _________________________ and is part of a ___________________
- ONE NEUROMUSCULAR JUNCTION
- MOTOR UNIT
What is a motor unit
The combination of a motor neuron and muscle cell(s)
How many NMJs can a myofibre have?
Each myofibre, no matter how large, has only ONE neuromuscular junction (NMJ)
HOWEVER, a single motor neuron can make NMJs with anywhere between 1-1000 myofibres
Where is epithelia found in a muscle?
Epithelia is found in the endothelium of blood vessels in the muscle
What do nerve fibers do in muscles?
Nerve fibers activate and monitor muscle contractions
- involve in excitation(?)
What is an NMJ
a neuromuscular junction
- site in which the axon of a neuron connects to the muscle fibre
What controls the contraction of skeletal muscle fibers?
The nervous system
- at a specialized site known as the NEUROMUSCULAR JUNCTION (NMJ)
- NMJ is located midway along the muscle fiber’s length
What are the three components that make up a neuromuscular junction?
- axon terminal - ends of a motor neuron
- motor end plate - where the myofiber is interacting with the neuron
- specialized region of the sarcolemma - synaptic cleft - space betwen the motor and the myofiber (membrane)
What is a motor neuron?
a nerve cell that propagates an electrical impulse from the nervous system to skeletal muscle fibers
What are myofibrils and their components?
Myofibrils are cylindrical structures making up muscle fibers
- assemblies of special protein filaments that allow muscles to contract
- thin and thick filaments
- organelles such as T-tubules
- surrounded by sacroplasmic reticulum
What is sarcolemma?
cell membrane surrounding muscle fibers
What is sarcoplasm?
the cytoplasm of muscle fibers
What is sarcoplasmic reticulum?
the endoplasmic reticulum of muscle fibers
- surrounds myofibrils
- stores and releases calcium
What are T-tubules? (Transverse tubules)
- infoldings of the sarcolemma
- connection or extension of the cell membrane bringing the cell membrane within the cell
What are triads
segments where T-tubules and sarcoplasmic reticulum connect(?)
What is excitation?
- the sarcolemma generating an electrical signal that precedes contraction
- occurs at the NMJ
- communication of signals that gets the cells excited
What is contraction?
- myofibres/myofibrils becoming shorter or producing tension
- actual process leading to the physical movement of the structure, referring to the muscle fibers
What is E-C coupling?
Mechanisms by which excitation triggers contraction
What are the three main steps of excitation of a muscle cell?
- Electrochemical signal from the motor neuron
- electrochemical signals is sent down the motor neuron to the axon terminal - Electrical excitation of the NMJ
- excitation occurs from the axon terminal across the synaptic cleft to the myofibre at the NMJ - Propagation of the electrical signal
- the message is propagated down
What triggers contraction of a skeletal muscle fibre?
Contraction of a skeletal muscle fibre is only triggered when a command is received from the nervous system, activating the motor neuron, which excites the myofibre
- a myofibre has to be EXCITED by a motor neuron/signal before it can trigger a CONTRACTION!
Why/how does excitation occur?
- transmembrane potential exists - difference in ratio of the ions inside (K+) and outside (Na+)
- excitable cells (muscle cells and neurons) can change their transmembrane potential, allowing it to send and receive signals
- at rest, there is a slight NEGATIVE membrane potential inside
- when the cell is excited, the transmembrane potential changes from slightly negative to slightly more positive/less negative
- channels are proteins on the membrane of the myofibre that allow for passage of ions across the membrane
- movement of the positively charged ions into the cell can change the transmembrane potential of the cell
What are the conditions of the cell at rest?
At rest, (-70mV) sodium cannot move through ion channels in the sarcolemma BUT potassium can
- sodium cannot enter the cell at rest
- sodium stay outside the cell
- potassium can move into the cell at rest
- potassium can move
What are two examples of excitable cells?
Muscle cells and neurons
What are excitable cells?
Cells that can rapidly vary their transmembrane potential to send and receive signals
Why does membrane potential exist?
There is an unequal distribution of ions and unequal permeability of the membrane
- ions move through ion channels
- at rest, sodium cannot move through ion channels in the sarcolemma (though K+ can)
- there is an abundance of K+ INSIDE the membrane
- there is an abundance of Na+ OUTSIDE the membrane
There is an abundance of _____ ions INSIDE the membrane
K+
There is an abundance of ______ ions OUTSIDE the membrane
Na+ and Cl-
Describe the process of excitation in a muscle cell/NMJ (up until propagation)
- the myofibre is at rest (-70mV, Na+ OUTSIDE and K+ INSIDE)
- an electrochemical signal is sent down the motor neuron to the axon terminal (carrying ACh neurotransmitter)
- neurotransmitter is released at the axon terminal into the synaptic cleft
- neurotransmitter binds to the ACh-receptors (protein ion channels/gates) on the membrane of the myofibre (sarcolemma)
- NMJ excitation occurs - sodium channels open and rushes in
- spread of excitation - enzymatic removal of the neurotransmitter to stop the excitation
- propagation down the muscle fibre…
What is Ligand? Give an example of one
A neurotransmitter is a Ligand
Ex: Acetylcholine (ACh)
- have the ability to attach to different proteins
- bind to receptors
What is a Ligand-gated ion channel?
A Ligand-gated ion channel is a receptor
Ex: AChR
- provide a path for sodium ions to cross the cell membrane
- AChR proteins allow sodium ions to cross the cell membrane if a ligand is attached to it
What is the function of neurotransmitters
motor neurons release a chemical neurotransmitter which causes the opening of ion channels (Receptors) that allow sodium into the myofibre
If sodium is able to cross the cell membrane of a cell, which way will it tend to move and why?
- sodium will move INWARD because it will be ATTRACTED by the more NEGATIVE environment inside the cell
- sodium will move INWARD because it will diffuse down its concentration gradient (high outside, low inside)
Curare is a drug that prevents ACh binding to ACh receptors: How will this drug affect the overall excitation (and thus contraction) of a normal muscle?
Lower excitation because the sodium channels cannot open and thus excitation cannot occur
- causes flaccid paralysis
- muscles cannot turn on
- no contractions/movement
Novichok blocks the enzyme that breaks down ACh in the synaptic cleft: How will this drug affect the overall excitation (and thus contraction) of a normal muscle?
Higher excitation because the sodium channels never close so the excitations keep happening
- spastic paralysis
- prevents breakdown of ACh
- no control of the muscles
- never turn off
- constant excitation
What is the name of the enzyme that is in charge of removing the neurotransmitter and stopping excitation
AChE
- acetylcholinesterase
What is depolarization?
AKA excitation
- movement of particles across the membrane when the gated ion channels open
- changes the transmembrane potential
- goes from slightly more negative inside to slightly more positive
What allows propagation to occur?
- propagation requires voltage-gated ion channels
- voltage-gated ion channels become activated by the change in transmembrane potential
- ion channels open when the potential/voltage changes
What is propagation?
spreading of excitation through the myofibre from the NMJ
- due to voltage-gated ion channels
What is an action potential?
A spread of depolarization/excitation can cause extra (voltage-gated) ion channels to open - this is what creates an action potential
- action potentials propagate through the entire length of the myofibre’s sarcolemma
- can be very very rapid
What role do T-tubules play in propagation?
T-tubules bring the plasma membrane’s action potential into the vicinity of the sarcoplasmic reticulum
- because the T-tubules are infoldings of the sarcolemma
- triggers a CALCIUM release
- two membrane proteins physically and physiologically connect electrical signals in the sarcolemma to calcium channels in the SR
Describe the process of excitation in a muscle cell/NMJ (after propagation)
- AChE breaks down ACh and stops excitation in one region of the sarcolemma
- the change in transmembrane potential causes voltage-gated ion channels to open (action potential)
- excitation continues as ions move into/out of the membrane
- T-tubules carry the action potential into the muscle cell to the sarcoplasmic reticulum and causes a calcium release
- calcium release leads to contraction (more about this in the next lecture)
Muscles are one of the four basic tissue types, what are the three distinct subtypes of muscle tissue?
skeletal
cardiac
smooth
Skeletal muscles are triggered to contract only through _________________________________ that begin in nervous tissue. A chemical ________________ released at the _________________________ triggers the opening of ion channels that change the membrane potential of the myofibre, causing an ___________________.
- electrical and chemical signals
- neurotransmitter
- neuromuscular junction (NMJ)
- action potential
A muscle action potential spreads throughout the length of the myofibre, and triggers the release of _____________________from the sarcoplasmic reticulum.
CALCIUM IONS
- calcium ions being released from the sarcoplasmic reticulum lead to contractions
Define the properties of muscle tissue and the subtypes
Muscle tissue is able to contract and produce mechanical force
(striated muscle)
- skeletal muscle tissue
- cardiac muscle tissue
(smooth muscle tissue)
- smooth muscle
Describe the organization of a skeletal muscle and the tissue types that it contains
Muscle > Fascicle > Myofibre > Myofibril
Exterior:
1. Epimysium - dense irregular connective tissue that surrounds the whole muscle, connecting the muscle to tendon
Interior:
2. Blood vessels - endothelium of the blood vessels are made of epithelia
3. Nerve fibres - activate and monitor muscle contractions, made of nervous tissue
4. Fascicle - bundles of myofibers surrounded by perimysium
5. Perimysium - dense elastic connective tissue surrounding fascicles
6. Myofiber - bundles of myofibrils and other cell organelles (aka muscle cell) surrounded by endomysium
7. Endomysium - loose/areolar connective tissue that surrounds each muscle fibre/myofibre
Identify the features of a myofibre
Exterior:
1. Endomysium - loose/areolar connective tissue that surrounds each muscle fibre
Interior
2. Myofibrils - specialized proteins that make up myofibres
3. Myosatellite cells - stem cells for muscle tissue found within endomysium
4. Blood vessels
Identify the specialized parts of myofibres (ex: NMJ, and three key organelles in a skeletal muscle triad) by structure and function
NMJ = neuromuscular junction
1. axon terminal
2. synaptic cleft
3. motor end plate
T-tubules (x1)
Sarcoplasmic reticulum (x2)
Explain the difference between excitation and contraction in a skeletal muscle cell
Excitation -> Contraction
Excitation - generation of an electrical signal down the muscle
Contraction - release of calcium ions that causes myofibres/myofibrils to become shorter or produce tension
Explain the key steps that occur across a NMJ and within a myofibre during excitation
- myofibre at rest
- electrical signal comes down motor neuron to axon terminal of NMJ
- release of neurotransmitter
- neurotransmitter binding and NMJ excitation occurs
- movement of ions causes a change in transmembrane potential
- change in transmembrane potential results in action potential
- voltage-channels open and excitation spreads
- breakdown of neurotransmitter, enzymatic removal
- propagation of excitation
