Skeletal muscle on NS control Flashcards
What does the microscopic anatomy of skeletal muscle consist of?
Myofibrils, myofilaments, myofibers, sarcomere, sarcolemma, sarcoplasm, sarcoplasmic reticulum, triads and t-tubules
Define the structure and function of: Myofiber, Myofilaments & Myofibrils
Myofiber-Surrounded via endomysium, striated and multinucleated
Myofilament- composed of thick filaments of myosin and thin filaments of actin
Myofibrils- found within muscle fibers and run through them, composed of sarcomeres, made up of actin & myosin
Define the structure and function of: sarcolemma, sarcoplasm, sarcomere & sarcoplasmic reticulum
Sarcolemma: the cell membrane of the muscle fiber, it conducts and transmits AP’s to muscle fibers
Sarcoplasm: cell cytoplasm of sarcolemma, contains lots of mitochondria to produce ATP
Sarcomere: contract unit of a muscle, Z-lines (Z discs): These define the boundaries of a sarcomere.A-band: The dark band where thick myosin filaments overlap with thin actin filaments. I-band: The light band that contains only thin actin filaments. H-zone: The central region of the A-band where thick filaments do not overlap with thin filaments. M-line: The center of the H-zone, where thick filaments are anchored.
SR: stores and regulates release of calcium during muscle contraction, contains terminal cisternae close to t-tubules
Define the structure and function of: T-tubules & triads
T-tubules: tubes penetrate muscle fibers, allow transmission deep into muscle fiber ensuring they contract simultaneously
Triad: A structure formed by a t-tubule & 2 cisternae of SR, arrangement is needed for release of calcium ions
Describe the sliding filament model
Resting State: In a relaxed muscle, the myosin heads are in a “cocked” position, energized by the hydrolysis of ATP.The actin filaments are blocked by tropomyosin, a protein that covers the myosin-binding sites on actin. Troponin, another regulatory protein, is bound to tropomyosin, and in the relaxed state, it prevents the interaction between actin and myosin.
Signal to Contract (Excitation-Contraction Coupling): When a muscle is stimulated by a nerve impulse, it triggers an action potential that travels along the sarcolemma and into the muscle fiber via the T-tubules.The action potential reaches the sarcoplasmic reticulum (SR), causing the release of calcium ions (Ca²⁺) into the sarcoplasm.
Calcium Release and Binding to Troponin: The increase in calcium ion concentration in the sarcoplasm causes calcium to bind to troponin on the thin filaments (actin). When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the actin’s myosin-binding sites.
Cross-Bridge Formation: With the binding sites exposed, the energized myosin heads bind to the actin filaments, forming cross-bridges between the myosin and actin.The ATP that was used to “cock” the myosin head earlier is now hydrolyzed into ADP and inorganic phosphate (Pi). This energy release drives the power stroke.
Power Stroke: The myosin head pivots, pulling the actin filament toward the center of the sarcomere. This movement is called the power stroke. As the myosin heads pull on the actin, the sarcomere shortens, causing the entire muscle to contract. The actin filaments slide past the myosin filaments, but the length of the filaments themselves does not change.
Detachment and Resetting: After the power stroke, the myosin head remains attached to actin until a new molecule of ATP binds to it. The binding of ATP causes the myosin head to detach from actin and reset to its “cocked” position. This cycle repeats many times (with thousands of cross-bridges forming and breaking) as long as calcium ions are present and ATP is available.
Relaxation: When the nerve signal stops, calcium ions are actively pumped back into the sarcoplasmic reticulum (using ATP), reducing the calcium concentration in the sarcoplasm. As calcium is removed from troponin, tropomyosin returns to its original position, blocking the myosin-binding sites on actin, and muscle contraction ceases
Describe the structural changes during muscle contraction to the sarcomere
. According to this model, muscle contraction is the result of the sliding of thin filaments (actin) past thick filaments (myosin) within the sarcomeres of muscle fibers, leading to a shortening of the entire muscle.
The I-band (the area with only thin filaments) shortens during contraction because the actin filaments are pulled inward.
The A-band (the area with both thick and thin filaments) remains the same length, but the overlap between thick and thin filaments increases.
The H-zone (the region with only thick filaments) becomes smaller as the actin filaments slide past the myosin filaments.
Describe excitation coupling stages
Excitation–contraction coupling is the physiological process of converting an electrical stimulus to a mechanical response. It is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction.
Describe the structure of an NMJ
Motor Neuron Terminal: The motor neuron, which originates in the spinal cord or brainstem, has an axon that branches and terminates at the neuromuscular junction. The axon terminal (also called the presynaptic terminal) contains vesicles filled with acetylcholine (ACh), a neurotransmitter that transmits the signal from the nerve to the muscle.
Synaptic Cleft: The synaptic cleft is the tiny gap (about 20-30 nanometers wide) between the motor neuron terminal and the muscle fiber (sarcolemma). This gap allows for the diffusion of neurotransmitters from the presynaptic neuron to the postsynaptic muscle cell.
Motor End Plate: The motor end plate is a specialized region of the muscle fiber’s sarcolemma that lies directly across from the motor neuron terminal. The motor end plate contains high concentrations of acetylcholine receptors (AChRs), which are integral proteins that bind acetylcholine.
Describe the function of an NMJ
Action Potential Reaches the Neuron: A motor neuron action potential (electrical impulse) travels along the axon to the motor neuron terminal at the neuromuscular junction.
Release of Acetylcholine (ACh): When the action potential reaches the motor neuron terminal, it triggers the opening of voltage-gated calcium (Ca²⁺) channels in the presynaptic membrane. The influx of calcium ions causes the synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane and release ACh into the synaptic cleft via exocytosis.
Binding of ACh to Receptors: Acetylcholine diffuses across the synaptic cleft and binds to ACh receptors located on the motor end plate of the muscle fiber. These receptors are nicotinic acetylcholine receptors, which are ligand-gated ion channels. When ACh binds to these receptors, they open and allow sodium (Na⁺) ions to flow into the muscle cell and potassium (K⁺) ions to flow out, leading to a change in the muscle fiber’s membrane potential.
Depolarization and Action Potential in Muscle Fiber: The influx of sodium ions depolarizes the muscle membrane, leading to the generation of an action potential in the muscle fiber’s sarcolemma.The action potential spreads along the sarcolemma and into the muscle fiber through the T-tubules, ultimately reaching the sarcoplasmic reticulum (SR) and triggering the release of calcium ions (Ca²⁺), which initiate muscle contraction.
Acetylcholine Breakdown: After acetylcholine binds to the receptors, it is quickly broken down by the enzyme acetylcholinesterase (AChE), which is located in the synaptic cleft. The breakdown of acetylcholine ensures that the signal is brief and does not persist, allowing the muscle to relax once the action potential stops.
Recycling of ACh: The choline produced by the breakdown of acetylcholine is reabsorbed by the motor neuron and used to synthesize new acetylcholine, readying the neuron for the next action potential.
Describe the gross anatomy of the skeletal muscle
Consists of muscle fibers bundled into fascicles,
epimysium surrounds whole muscle, perimysium surrounds muscle fascicle and endomysium surrounds individual muscle fibers.
Tendons attached to bones are made of connective tissue
Muscles have a great BV & nerve supply. Each muscle has an origin and a insertion which determines it’s function
Describe the function of a motor unit
- muscle contraction: when MN is stimulated by AP, ACH is released at NMJ, causes motor unit to contract, number of motor units activated facilitates strength of contraction
- All or nothing response: when a MN fires all of the muscle fibers within contract fully
- Recruitment of MO: More motor units the greater the force of contraction
- Controlled by frequency coding which adjust the muscle contraction based on incoming AP’s
Describe the components of a motor unit
Consists of motor neuron & muscle fiber
MN transmit signals from CNS to effectors, each MN has an axon that extends from SC to muscle fiber
MF are innervated by MN, can vary in number of muscle fibres in a motor unit.
small motor units control fine movements & have fewer MF’s
large motor units include limb movements & have many MF’s
Describe the purpose of motor units
A motor unit consists of a motor neuron and the muscle fibers it controls. The motor unit functions as the basic unit for muscle contraction and plays a crucial role in the control of movement. Motor unit recruitment allows for graded control of muscle force, where smaller units are recruited for fine movements and larger units for powerful movements. The all-or-none principle ensures that once a motor neuron fires, all muscle fibers it innervates contract maximally, contributing to the smooth coordination of muscle action.
Define frequency coding
FC enables adjustment of muscle contractions based on incoming APs, allows the muscle to form a wide range of movements
Describe the 5 stages in frequency coding
- AP frequency: AP is delivered to muscle fibers, AP frequency facilitated how muscle contracts. E.g., a low frequencies muscle fibers contract individually and can relax between stimuli. At high frequencies the muscle fibers do not have the time to relax before the next stimulus arrives fully
- Twitch contraction: single AP causes muscle twitch, force given by twitch id dependent on number of muscle fibres activated
- Summation: All APs are summed, if F is high leads to temporal summation as the muscle cannot relax
- Tetanus: sustained contraction, no relaxation, 2 types: fused & unfused
unfused: partial relaxation
fused: continous smooth contractions - Recruitment VS frequency coding: recruitment refers to using a motor unit to increase force output. Frequency coding refers to adjusting state due to APs to modulate force
Define rigor mortis
It is the postmortem stiffening of muscles after death, resulting from biochemical changes in muscle fibers. A key indicator of time of death
Describe the stages in rigor mortis
- Onset (2-6hrs): Body stops producing ATP, no ATP results in muscle fibers being unable to detach from each other from muscle contraction, calcium ions leak out of SR causing muscle contraction. Body stiffens and cannot relax
- Full development (12-24hrs) : RM reaches max intensity, cross bridges remain and muscles stay contracted
- Resolution of RM (24-48hrs): proteases begin to breakdown muscle, muscles become less stiff as cross bridges are broken down, this can vary on temperature and health of a person
- Complete resolution (48-72hrs): decomposition occurs to breakdown muscle tissue, body undergoes further breakdown, muscles remain relaxed
What factors influence the timing in rigor mortis?
- Temperature: high temps speed up the rate of decay, lower temps slow down RM
- Age/health: The overall health of a person affects the progression of RM
- Cause of death: toxins, blood loss etc can alter progression of RM
Describe the role of spinal reflexes
Provide quick responses that protect the body from danger/damage, provide movement and support motor functions whilst not needing conscious control
Postural Control: Many spinal reflexes contribute to maintaining balance and posture. For example, the stretch reflex, which helps maintain muscle tone, keeps the body stable by adjusting muscle tension when a muscle is stretched.
Motor Coordination: Reflexes assist in the smooth execution of movement. For instance, during walking, the alternating movements of the legs are coordinated through reflexes that ensure proper muscle contraction and relaxation
Describe an ipsilateral reflex
Where reflex occurs on the same side of the body the stimulus is detected on. SN send signal to SC which directly innervates motor neurons on the same side of the body
Describe a contralateral reflex
Where the reflex occurs on the opposite side of the body of the stimulus, it involves the SC
Describe the withdrawal reflex
WR helps body respond to harmful stimuli, automatic & ipsilateral response, process in SC giving fast response
1. stimulus is detected, by pain receptors triggering WR
2. Signal is passed to SN to the SC, travels to dorsal horn of SC, it synapses with relay neurons
3. relay neurons process & relay neurons to motor neurons, signal travels to muscle and initiates withdrawal movement
4. MN activates muscle that gets withdrawn
5. At same time. antagonist muscle is inhibited, to prevent interference during WR
Describe the crossed extension reflex
It works in response to withdrawal reflex, helps maintain balance during WR, a contralateral reflex & stabilizes the limb
1. stimulus is detected & activates pain receptors, WR occurs causing flexion (withdrawal)
2. The crossed extensor reflex is activated simultaneously, SN send signal to SC, relay neurons cross over to another side
3. relay neurons stimulate motor neurons, causing contralateral muscle to contract to stabilize and provide balance to the body
4. Maintaining balance: For example, when you step on something sharp with one foot and that leg withdraws (flexes), the other leg must extend to prevent the body from falling. The crossed extensor reflex ensures that the opposite leg straightens to bear the body’s weight, helping you remain stable.
Describe the myotatic reflex
It helps to maintain muscle tone & muscle posture, it is monosynaptic, gives a fast response, prevents overstretching of muscles e.g., knee jerk reflex
1. Stimulus- reflex occurs when muscle is stretched suddenly causing a change in length, detected by MS
2. Activation- When MS is stretched, signal is sent to SC, sensory neurons directly synapse with motor neurons in SC
3. Motor response- MN send a signal back to same muscle, causing contraction, this helps restore muscle to original length
4. Prevention- The myotatic reflex helps prevent muscles from overstretching, ensuring that the muscle maintains a constant tone and stabilizes joints. It helps maintain posture and allows for smooth, coordinated movements.
Describe the role of muscle spindles
Involved in the myotatic reflex, they detect stretch in a muscle, they are found embedded within muscle fibers. They are specialized sensory receptors
Describe the tendon reflex
It protects the muscle from excessive tension, to prevent overstrain, it is polysynaptic & gives slower response, has golgi organs that detect tension/strain, monitor & regulate it
1. tension is detected in the tendons triggering the reflex, detected by golgi organs found between muscles & joints
2. GO sends signals through sensory neurons to the spinal cord, increased tension causes activation of SN
3. Information about increased tension is carried to SC
4. Interneurons inhibit motor neurons, causes the muscle to relax to reduce tension, preventing injury by limiting force
5. Muscle is forced to relax, whilst antagonist contracts slightly
Define neuronal intergration
NI explains how the NS coordinates sensory input and processes motor output to provide correct response to environment.
It helps maintain homeostasis by responding to environmental changes
Describe the stages in neuronal integration
- Sensory input: receptors detect changes and sends signals to the CNS
- Processing in CNS: sensory info is processed in the brain/ spinal cord, they are intergrated, reflexes pass only to spine to allow quick response
- Motor output: Motor neurons carry signals from CNS to muscles to initate movement (muscle contraction
- Feedback: sensory feedback from muscles and joints help fine tune the response
Draw the neural circuit involved in each reflex
