Topic 10: Skeletal Muscle Flashcards
What are 4 characteristics of muscles?
EXCITABLE: respond to stimuli
CONTRACTILE: can shorten, thicken
EXTENSIBLE: stretch when pulled
ELASTIC: return to original shape
What are 4 muscle functions?
Movement
Posture, facial expression
Heat production (37C)
Production of viscera (body wall)
True or false: multiple neurons can innervate a muscle fibre
False. In fact 1 neuron can innervate multiple fibres at a time
What is a motor unit?
A single motor neuron and all the muscle fibres it innervates
What is the structure of a motor unit?
A presynaptic cell (neuron) with axon terminal filled with vesicles that contain neurotransmitter ACh
A postsynaptic cell membrane is specialized region of sarcolemma of a fibre, called motor end plate
These are separated by a synaptic cleft
What are the steps for stimulating a skeletal muscle fibre?
—AP reaches axon terminal & synaptic end bulb
—Voltage gates Ca++ channels open, which causes exocytosis of ACh
—ACh binds to receptors on motor end plate
—Chemically gated channels open and Na+ enters muscle fibres creating an end plate potential
—EPP causes opening of Na+ voltage gated channels, which results in AP
—AP propagates along sarcolemma
True or false: 1 action potential = 1 end plate potential on the sarcolemma
True
What are the basic steps of skeletal muscle contraction?
Excitation of muscle fibre (electrical event)
Excitation-contraction coupling (converts AP into mechanical event of contraction)
Contraction = sliding filament mechanism
What are the steps to the mechanical event of contraction?
-Activated myosin heads attach to binding sites on actin
-Energy stored in myosin is released, myosin head pivots (= power stroke), ADP and P are released, actin slides over myosin to M line
-ATP attaches to myosin head, causing its release for actin and unpivots (= recovery stroke)
-Myosin heads attach reactivates
-If Ca++ in cytosol remains high, the steps repeat
What are the steps of the excitation of the muscle fibre?
-Sarcomere depolarized: End Plate Potential (EPP) triggers an AP
-AP propagates down T-tubules to deep within fibre
What are the steps of the excitation-contraction coupling (electrical ~> mechanical event)?
-AP in T-tubules cause release Ca++ (coupling agent) from terminal cisternae of SR via mechanically-gated channels
-Ca++ binds to troponin
-Troponin-tropomyosin complex moves, exposing myosin binding sites on actin
What are the steps of muscle fibre relaxation?
-ACh is broken down by acetylcholinesterase (AChE) on the motor end plate (facing synaptic cleft)
-SR actively takes up Ca++
-ATP binds to and releases myosin heads
-Tropomyosin moves back to cover myosin binding sites on actin
In terms of muscle fibre relaxation, what is ATP necessary for?
-Cross bridge release (ATP not broken down)
-Activation of myosin (ATP ~> ADP + P)
-Pump Ca++ into SR
-Fibre Na+/K+-ATPase activity
Explain botulism
-Improper canning: clostridium botulinum
-Prevents exocytosis of ACh: flaccid paralysis
-Medical: treat uncontrolled blinking, crossed eyes
-Cosmetic: Botox (wrinkles etc)
Explain Rigor Mortis (“stiffness of death”)
-Ca++ concentration inside cells increases (enters from ECF): exposes binding sites on actin and allows crossbridges to form—myosin cannot be released without ATP
-starts ~3 hr after death to ~12 hr
-gradually subsides over days while cells break down
What is curare poisoning?
Prevents ACh from binding to receptors
Flaccid paralysis: historically used to prevent people moving during surgery
What affects muscle tension in a fibre?
Frequency of stimulation
Fibre length
Size of fibre
Fatigue
A single stimulus produces a muscle twitch (weak contraction and relaxation). Explain
i. Single stimulus ~> 1 AP
ii. Latent period: excitation-contraction coupling occurring
iii. Contraction period (+ tension): cross bridge attachment & sliding filaments, not all myosin heads attach~>does not reach max tension
iv. Relaxation (— tension)
How does a second stimulus affect muscle tension?
It causes the release of more Ca++, adding to what’s already there. This produces a 2nd contraction with higher tension = wave summation
(Contraction has no refractory period)
How does a rapid sequence of stimulus affect muscle tension?
Tension increases further (more Ca++ availability ~> wave summation)
Partial relaxation between contractions produces quivering = incomplete tetanus
How does a high frequency of stimuli affect muscle tension?
No relaxation between contractions: complete tetanus
All troponin is saturated with Ca++, and the fibres warm, so it works faster
Occurs normally in body
Why is the resting fibre length optimum for muscle tension?
It allows for a max # of cross bridges to form upon stimulation
Does the thickness of a muscle fibre increase or decrease tension?
Increase
What is the difference between red and white muscle fibres?
TYPE 1
Red: slow twitch (more myoglobin)
TYPE 2
White: fast twitch (less myoglobin)
