Muscular Tissue Structure and Function Flashcards
Sarcomere
functional unit of striated muscle (skeletal & cardiac)
composed of actin(thin protein) and myosin(thick protein)
extends from z-line to z-line
structure of sarcomere;
- actin: Z-lines and I-bands
- myosin: M-line and H-band
- both: A-band
components of a neuro muscular junction
The point where a nerve fiber meets its target cell is called a synapse.
When the target cell is a muscle fiber, the synapse is called a neuromuscular junction (NMJ) or motor end plate.
Each terminal branch of the nerve fiber within the NMJ forms a separate synapse with the muscle fiber.
The Sarcolemma (cell membrane) of the NMJ is irregularly indented.
At each synapse, the nerve fiber ends in a bulbous swelling called a synaptic knob.
The synaptic knob does not directly touch the muscle fiber, but is separated from it by a narrow space called the synaptic cleft, which measures about 60 to 100 nm wide.
The synaptic knob contains synaptic vesicles, which are filled with a chemical (neurotransmitter) call Acetylcholine (Ach).
As the nerve impulse travels down the nerve fiber, it causes the causes the synaptic vesicles to undergo exocytosis, releasing Ach into the synaptic cleft.
steps in contraction phase
40msec
1.nerve impulse arrives at nerve ending
2.neurotransmitter is released (acetylcholine for skeletal; epinephrine for cardiac)
3.muscle cell membrane depolarizes (sodium enters muscle cell)
4.T-tubule system (holds sarcomeres together) depolarizes (sodium enters sarcomeres)
5.Calcium is released from the sarcoplasmic reticulum (smooth ER)
6.Calcium induces troponin-tropomyosin complex to shift and UNCOVER the active sites on the actin filament
7.Myosin cross-bridges attach to active sites
8.Power Stroke occurs; uses oxygen and ATP; myosin cross-bridges flex-bend-pull-and reattach to new active sites
9.Z-lines move toward each other
10. muscle contracts
DOESNT MATTER WHERE YOU START BECAUSE ITS A CYCLE
steps in relaxation phase
50msec
- nerve impulse stops
- re-uptake of neurotransmitter (NT gets taken up into cell)
- muscle cell membrane repolarizes (potassium exits muscle cell)
- T-tubule system repolarizes (potassium exits sarcomere)
- Calcium is stored in sarcoplasmic reticulum
- no binding of Calcium to troponin-tropomyosin complex; the complex will shift and COVER the active sites on the actin filament
- no attachment of Myosin cross-bridges
- no Power Stroke; replace oxygen and ATP
- Z-lines move away from each other
- muscle relaxes
steps in latent phase
10msec
1. activation of sodium-potassium pumps in muscle cell membrane
- 3 sodiums leave muscle cell; and 2 potassiums enter muscle cell for every pumping action of the pump
threshold, latent period, and twitch
Threshold is the minimum voltage needed to generate an action potential in the muscle fiber and produce a contraction.
At threshold or higher, a stimulus thus causes a quick cycle of contraction and relaxation known as a twitch.
There is a delay, or latent period, of about 2 milliseconds between the onset of the stimulus and the onset of the twitch; the force generated during this time is called internal tension; it is not visible on the myogram because it causes no shortening of the muscle.
contraction strength of twitches for stimulation frequency concentration of calcium muscle elasticity temperature pH
Stimulation frequency – stimuli arriving close together produce stronger twitches, then stimuli arriving at longer intervals
Concentration of Calcium in the Sarcoplasm – the higher the concentration, the stronger the twitch
Muscle elasticity – over stretched muscles produce weak twitches
Temperature – warmed up muscles contract more strongly
pH – low pH triggers weaker twitches (fatigue)
stimulus intensity (voltage) vs stimulus frequency
higher voltages excite more nerve fibers, which will stimulate more muscle contractions
high frequency stimulation produces stronger twitches
relationship between stimulus frequency and muscle tension
- twitch
- treppe
- wave summation/incomplete tetanus
- complete tetanus
Twitch – at low frequency, the muscle relaxes completely between stimuli and shows twitches of uniform strength
Treppe – at moderate frequency of stimulation, the muscle relaxes fully between contractions, but successive twitches are stronger
Wave Summation and Incomplete Tetanus – at a higher frequency, the muscle does not have time to relax completely between twitches and the force of each twitch builds on the previous one
Complete Tetanus – at high stimulus frequency, the muscle does not have time to relax at all between stimuli, and exhibits a state of continual contraction with about four times as much tension as a single twitch; eventually the muscle can fatigue
isometric contraction
muscle length remains constant; muscle tone changes; standing; sitting
not moving, skeletal muscles are contracting to help maintain posture
isotonic contraction
muscle length changes; muscle tone remains constant; walking; running
you can go from one contraction to another
contraction of smooth muscle
doesn’t have sarcomere
- involuntary muscle; innervated by the Autonomic Nervous System (visceral efferent fibers)
- cells do not have t-tubules & have very little sarcoplasmic reticulum
- cells do not contain sarcomeres (so are not striated) but are made up of thick & thin myofilaments.
- Thin filaments in smooth muscle do not contain troponin.
- calcium does not bind to troponin but, rather, to a protein called calmodulin. The calcium-calmodulin complex ‘activates’ myosin which then binds to actin & contraction (swivelling of cross-bridges) begins.
- have random arrangement of myosin and actin in the cytoplasm; when contraction is to begin, a nerve impulse causes each myosin to pair with an actin; as each pair occurs, that part of the cell contracts, to give a wave-like contraction known as peristalsis
