Structure and Physiology of muscle Flashcards
What is similarity between nerve cells and muscle cells
Share many properties
e.g. muscle cells also conduct action potentials
What is the target of many nerve fibres
Muscles
what are skeletal muscle?
. Attached to a skeleton . Voluntary ( somatic ) . Striated . Movement . Most common
what are cardiac muscles?
. Muscles of the heart
. Striated
. Voluntary ( somatic )
. Forceful and quick
what are smooth muscles?
. Found in hollow organs e.g. blood vessel , digestive tract, inside eye, respiratory system
. Non -striated
. Involuntary
What are muscle cells known as and why
Fibres - individual cells
They are elongated in one direction
Measurements of muscle cells
Up to 3-4cm long
Diameter of 10-150 micrometres
What are fibres
Individual cells - muscle cells - long and thin cell
What is a muscle made up of
1000s of such muscle fibres ( several fascicles )
What are fibres grouped into
Fascicles
What are fascicles surrounded by
Perimysium connective tissue
What make up the whole muscle cell
Several fascicles
What surrounds the whole muscle
Epimysium connective tissue
What is each fascicle made up of
Several cells
What separates muscle fibres
Endomysium
How do skeletal muscle attach to the bone?
Skeletal muscle is attached to bone by tendons
What is the outer membrane of a muscle cell called
Sarcolemma
What does each skeletal muscle fibre contain and why
Several nuclei
Because muscle fibres are long and thin so many more
Where is nuclei of skeletal muscle located and why
Periphery ( edge ) of cell
Because most of cell is made of striations
What is cell filled with and what is it like
Sarcoplasm
Like cytoplasm - contains usual organelles especially mitochondria
What does skeletal muscle appearance depend on
Plane of section - how you’re looking at it - look different when on different sides
How does skeletal muscle appear in longitudinal section?
Long and thin
How does skeletal muscle appear in transverse section?
Round - circles
What do skeletal muscle contain ?
Nerve tissue
Blood vessels
Why do skeletal muscles contain blood vessels?
Skeletal muscles are very metabolically active therefore it needs to be supplied with oxygen and nutrients and take away waste products( needs blood supply )
What is the most prominent feature of skeletal muscle fibre?
Striations - light and dark stripes
Organisation of a skeletal muscle
. Muscle is made up of several fascicles
. Each fasicle is made up of several muscle fibres
. Every fibre contains several myofibrils
What are myofibrils
2 rod like structures inside muscle cell - made of 2 proteins ( myofilaments ) - actin and myosin
What is each myofibril made up of
Series of sarcomeres composed of the myofilaments actin and myosin
What is a sarcomere
Distance between two z lines ( connective tissue )
What are actin filaments attached to
Z lines
What are between the actin filaments
Myosin filaments
Actin
Thin
Light
I band
Attached to z line
Myosin
Thick
Dark
A band
What is the A band
Area of overlapping actin and myosin
What is the region of only myosin - no overlap
H zone
middle of A band
What makes the sarcromere
Actin and myosin overlap - arranged in a regular way
What are Z lines
Disks of connective tissue
What is each myofibril made up of
Series of end to end sarcomeres
What is muscle fibre surrounded by and what is inside
Plasma membrane
Several myofibrils
What is small bundle of myofibrils surrounded by?
Sarcoplasmic reticulum
What is the system of tubes running right angles to surface of fibre
Transverse tubular system - continuation of sarcolemma ( membrane )
Where does the transverse tubular system end ?
Along the terminal cisternae of the SR , on each sarcomere forming a triad
What is Sarcoplasmic reticulum
Modified ER - network/series of hollow tubes enclosed in a membrane - act as calcium store
What are the 2 series of tubes surrounding myofibrils
Transverse tubules
Sarcoplasmic reticulum
What happens to membrane of sarcolemma
Continues down into muscle cell forming transverse tubular system
Summary of skeletal muscle
. Muscles are made of bundles of fascicles
.Fascicles are made of fibres
. Fibres contain myofibril
. Myofibril are made of actin and myosin
. myofibrils are surrounded by sacroplasmic reticulum
.Transverse tubular system is a continuation of the sarcolemma
What do the lumps on myosin represent?
Cross bridges
Myosin sticks out - attaches to actin
What do cross bridges enable?
Attachment of myosin to actin
Structure of myosin molecule
. Elongated molecules composed of….
- a heavy tail ( dense protein)
- 2 heads ( light protein) ( less dense )
.The junction of the heads and tail is known as a hinge
.The heads form part of cross bridge that attach to actin
What is the hinge region of myosin
Where head and tail join
What is a thick myosin filament made of?
Many individual myosin molecules
Many mysoin molecules get together to from one of dark myosin filaments
How are cross bridges of individual myosin molecules arranged?
Point in opposite directions at the 2 ends of a thick myosin filament
Myosin heads
Point in different directions in the 2 halves of the myosin filaments
Stick out - forming crossbridges which attach to actin
What is structure of actin filaments
3 components…..
. Helix of two strands of globular actin with active sites
.Thin rod like tropomyosin molecule in a helix
. troponin at regular intervals along tropomyosin
What is tropomyosin
Lie in the groove formed by twisted actin ( cover active sites in resting state )
Rod like, thin
What is troponin
Complex of three proteins one which has high affinity for calcium ions
Attach to tropomyosin
What attaches to active site of actin monomer
Myosin crossbridge
why cant cross bridge attach to active site on each actin monomer?
Active site is covered by tropomyosin
What happened in 1954
- Worked out how muscle contracted
- Invented electron microscope - looked at skeletal muscle under microscope
What assumption was made prior to the EM and why
Filaments folded in some undefined way
Arrangement of skeletal muscle into sarcomeres was not known or contraction mechanism
Assumed that actin and myosin is randomly distributed in muscle fibre
What happens during resting muscle
Cross bridge can’t attach to actin because tropomyosin is blocking active site ( attached itself ) so myosin can’t attach to myosin
Troponin is bound to tropomyosin
What happens during muscle contraction
EM showed
. width of A band( actin + myosin overlapping ) is unchanged
. width of I band ( actin only ) decreased
Actin and myosin slide over each other
Why does width of I band decrease
Due to sliding the actin and myosin over each other
What eventually happens to I band
Eventually disappears - only happens if actin and myosin slid over each other
What is sliding filament theory
I band shortening due to actin and myosin sliding over each other
What are the early stages of excitation-contraction coupling?
- action potential in motor neuron - gets to synapse
- synaptic events lead to ACH release
- binding of ACH to post synaptic receptors = opens Na+ channels
- Small depolarisation of muscle fibre (epp)
- surface propagation of action potential
Where does action potential travel
Along surface of muscle cell on sarcolemma
What are the first middle stages of excitation-contraction coupling?
- action potential goes down t-tubule system to reach myofibril
- depolarises terminal cisternae of SR
- SR releases calcium
What is inside of muscle cell bathed with
Calcium
Structure of t- tubule system
- Ends close to SR
- Straight down it to individual myofibrils - indentation of surface of muscle cell
What happens when action potential goes right down into centre of muscle cell
Opens Ca ion channels in SR = Ca out of SR into muscle
What are the second middle stages of excitation-contraction coupling?
- calcium released from the sr binds onto troponin - part of troponin has high affinity for Ca
- conformational change in troponin = changes shape
- pulls tropomyosin from actin
- uncovers active site on actin
- myosin cross cross bridges attach
What is the effect of calcium binding to troponin?
Uncovers active site on actin , allowing myosin to form cross bridges as it attaches
Troponin changes shape and pulls tropomyosin away from active site = actin and myosin join to each other
What are the later stages of excitation-contraction coupling?
- Myosin cross bridges attaches to actin
- myosin head tilts - hinge bends
- myosin cross bridge detached
- cross bridge attaches to next active site
- actin is pulled toward centre of sarcomere and sarcomere shortens
What happens at the end of muscle contraction?
Calcium is pumped back into the SR and muscle relaxes
What happens when muscle contracts - sliding filament theory of muscle contraction
Actin and myosin slide over each other
Sacromere shortens when it tilts
Myosin crossbridge attaches to actin - tilts - hinge bends - cross bridge attaches to next active site
What is each tilt
Power stroke
Where does energy come from in muscle contraction
ATP
Summary of excitation - contraction coupling
- Action potential along surface of muscle cell in motor neuron
- Causes vesicles containing AcH to go to pre - synaptic neuron and bind
- Releases AcH and diffuses to post - synaptic neuron
- Opens Na+ channels - small depolarisation - EPP
- Action potential down transverse tubules in tubular system- reaches SR
- Opens Ca channels - Ca moves out - binds to tropomyosin - changes shape - pulls tropomyosin away from active site.
What is slow muscle?
- Active for long time(e.g maintain posture)
- Uses lots of energy therefore have mitochondria
- Myoglobin ( bind to O2) for oxygen storage
- Red colour
- Big muscles
What is fast muscle?
- Breifly active - shorts amount of time
- Large sarcoplasmic reticulum for rapid release and uptake of calcium
- Little myoglobin - white colour
- Often small muscles
Example of fast muscle
Extraocular muscles ( inferior rectus ) - fast for rapid eye movements called SACCADE
Examples of slow muscle
Gastrocnemius
Soleus
What is Gastrocnemius
Muscle at back of lower leg = lift leg up and walking
Example of fish for slow and fast muscles
Different fish have different colours
Tuna - slow muscles - spends all its time cruising through ocean - can of tuna is red because lots of myoglobin
- Dover sole fish - live on ocean floor and ocassionally go up and back on ocean floor, don’t swim for long times = quick = white - fast muscle = lighter in colour
Why do cows have red meat
Cows came from animals that lived in planes and had to always be on the move,running away = need energy
What are the 2 ways you can control the size of muscle contraction ( when and how much we contract it)
Spatial
temporal
Define spatial summation
Motor unit recruitment
Varying the number of active motor units/fibres = vary muscle contraction size
What is a motor unit
A single neuron and all the muscle fibres it innervates
What is muscle made up of
Hundreds of different motor units
When do we get a big contraction ( more cells shortening )
When LOTS of motor units active
When do we get a small contraction
ONE/FEW motor units active - only few muscle cells shorten and soft contraction
What is motor unit recruitment
Each motor neuron supplies a few muscle fibres
Why do different muscles contract at different rates
It’s to do with their….
- Size
- Whether they have myoglobin or not
- How deep their SR is
What does the degree of fine control of a muscle depend on
Size of motor units
Fine control of muscles - dictate whether small or big contraction
Muscles that require a fine degree of control have numerous small motor units
What would the smallest motor unit be
ONE motor neuron contacting a SINGLE muscle fibre
Link between number of motor units and degrees of contraction
The number of motor units is the same as the number of different degrees of contract e.g. 10 motor units = 10 different degrees of contraction
What does muscle of larynx in sounds produce
Small motor units - 2/3 muscle fibres
How many muscle units do eye muscles have
3 - 10
What do small delicate muscles with fine degree of control contain
Motor units containing only a few muscle fibres
How many muscle fibres in larger muscles
Units of about 150 - 500 muscle fibres
E.g. those in movement - gastoral
What is temporal summation
Varying the frequency of stimuli - how frequently you stimulate muscle
What is the response of a muscle to a single stimulus?
A muscle twitch
What are the 3 phases?
- Latent period: interval between stimulus and beginning of contraction - before muscle contracts
- Contraction period- when the muscle is shortening
- Relaxation Period- during which tension declines
How long is average muscle twitch
100ms
What does length of contraction period depend on
Whether its fast or slow muscle
Explain the steps of stimulating muscle cell twice with different intervals between stimulation
- When the time between stimuli is far apart, so observe seperate twitches
- When the time between stimuli is decreased, the 2nd contraction starts before the previous one is over- the amplitude of contraction is bigger
- the largest contraction- when the 2nd contraction begins before the muscle has relaxed from previous stimulus
When is the electrical event leading to muscle contraction over and what can this allow
Before muscle twitch begins
Stimulate muscle again before contraction caused by first stimulus is over because action potential that triggered contraction is so brief (1ms or 2)
How does stimuli closeness affect contraction size
The closer the stimuli are together, the bigger the contraction
What happens if you stimulate muscle cell twice but LONG interval between stimuli
individual small contractions
contracts then relaxes
What happens if you stimulate muscle cell twice but stimuli CLOSER together
Before 1st contraction fully relaxes, there’s another contraction
2nd contraction starts before 1st one is finished so final contraction is bigger
What is a tetanus?
A large smooth contraction in response to a series of stimuli - stimulate muscle so frequently
What are the 2 types of contraction?
Isometric- muscle doesnt shorten when contracts- same length
Isotonic- muscle shorten
What are most contraction and example
Mixture of both muscle contractions
e.g. walking - lift leg isotonically but hit ground - tens muscle isometrically
What is the structure of cardiac muscle?
- Striated - therefore actin and myosin are arraned into sarcomeres
- Fibres branched
- Single central nucleus per fibre
- Intercalated discs act as gap junction ( electrical synapses between adjacent muscle cells )
Role of intercalated discs and branching
Rapid conduction of action potentials throughout heart and in many directions
What is the structure of smooth muscle?
- Composed of small spindle shaped cells (20-500 x 5 um)
- No striations
- One nucleus in centre
What are ocular structures which have smooth muscle?
Iris ( sphincter and dilator )
Ciliary body
What does no striations in smooth muscle mean
Actin and myosin cannot be arranged in sarcomeres; as there is little of them they are irregularly arranged
Can’t see light and dark bands
What is the structure of smooth muscle fibre?
- Not arranged in sarcomere
- Less actin and myosin than skeletal
ratio is 16:1 actin:myosin in smooth muscle however in skeletal it is (2:1)
-No z- lines ( actin not attached to z lines ) - actin and myosin are then connected to dense bodies or to the cell membrane
-Poorly developed SR- slow calcium released and pump very slow compared in the SR of skeletal muscle
-therefore contraction is slower and less powerful
Why is it good that smooth muscle contraction is slow and feeble?
- It uses less energy (which is good as smooth muscle is continuously active) - energy efficient
- Ones does not want to generate a lot of force
Why smooth muscle in gut
- In the gut there are contractions every 20s to mix food
so you need a lot of energy - Weaker and slower - don’t want powerful contractions of gut
What is skeletal muscle not good for
Digesting food in gut and altering blood vessel diameter
don’t want forceful
What happens if the muscle is too stretched in skeletal muscle and why
The sarcomere will not contract
If you stretch muscle sooner or later actin and myosin not going to overlap anymore - not form crossbridge - muscle cant contract
What does sarcomere arrangement of skeletal muscle ensure
Powerful and rapid contractions - due to actin and myosin sliding over each other
What happens if the muscle is too stretched in smooth muscle?
Actin and myosin irregularly arranged = there is always enough overlap of actin and myosin to generate tension
Actin and myosin always overlapping = still contract een when smooth muscle very stretched i.e. bladder FULL
Examples of smooth muscle stretching
Walls of bladder - grown by about 2/3 times normal size when its full after drinking and need to pee
Womb - Give birth - stretched by 8 times its normal size
What is the similarity between smooth muscle and skeletal muscle contraction?
actin and myosin both slide over each other
calcium is involved
ATP is required
What are the 2 ways smooth muscle can be made to contract?
- Depolarisation of the cell membrane
2. Chemically ( main way ) - release things by body that cause smooth muscle contraction
How does depolarisation of cell membrane cause smooth muscle contraction
- Smooth muscle innervated from ANS ( sympathetic and parasympathetic ) - innervation can be excitatory/stimulate or inhibitory
- Smooth muscle/nerve junctions - transmitter releases from varicosities
What are the two ways chemically contract smooth muscle
- Local tissue factors
- Hormones
How does local tissue factor contract smooth muscle
- Local build up of CO2 acts directly on smooth muscle - blood vessels dilate
- Low O2 or High CO2 levels act on vascular smooth muscle to cause vasodilation ( more O2 when dilation of blood vessel = contracts )
How do hormones contract smooth muscle
E.g. oxytocin stimulating uterine contraction during childbirth
What are two types of smooth muscle
- Single unit ( visceral ) - individual fibres joined by gap junctions - large sheet acts as a unit - often spontaneously active
- Multi unit - muscle fibres independent of one another - rich nerve supply - contraction graded e.g. large arteries
