Muscle Tissue Lecture 3 Flashcards
NMJ Definition
- Area where somatic motor neuron terminal and skeletal muscle synapse/connect.
- Site for transmitting action potential from nerve to muscle.
Ca2+ Influx and AP
- Ca2+ influx into muscle fiber due to AP.
- AP causes membrane changes, opening/closing ion channels, releasing Ca2+.
Somatic Motor Neurons
- Extend from brain & spinal cord, responsible for somatic movements.
- Terminate at muscle fiber sarcolemma, forming NMJ.
Synaptic Cleft
- Gap between somatic neuron terminal and muscle.
- Allows communication between nerves.
Neurotransmitters
- NTs propagate signals across cleft.
- Excitatory/inhibitory effects depending on synapse.
- Acetylcholine (ACh) is primary neurotransmitter at NMJ.
ACh and Motor End Plate
- ACh stored in vesicles in pre-synaptic terminal.
- Motor end plate: Post-synaptic membrane with ACh receptors.
- ACh binding triggers ion channel opening.
Presynaptic vs. Postsynaptic
- Presynaptic membrane: Motor neuron terminal.
- Postsynaptic membrane: Muscle fiber (motor endplate).
Steps of AP Generation:
- Release of ACh
- Activation of ACh receptors
- Production of the Action Potential
- Termination of the Action Potential
Release of ACh
- Nervous impulse reaches axon terminal.
- Voltage-gated Ca2+ channels open.
- Ca2+ triggers exocytosis of ACh into cleft.
Activation of ACh Receptors
- ACh binds to ligand-gated Na+ channels at motor endplate.
- Opens channels, allowing Na+ influx into sarcoplasm.
Production of Action Potential
- Na+ influx causes cell depolarization.
- Opens more voltage-gated Na+ channels, generating AP.
- AP propagates along sarcolemma, triggering Ca2+ release from SR for muscle contraction.
Termination of Action Potential
- ACh must be removed once nerve signal stops.
- ACh removed by diffusion or broken down by acetylcholinesterase (AChE).
- End products recycled for new ACh synthesis.
- Some NTs removed via reuptake, but not ACh.
Characteristics of Cardiac Muscle
- Heart is hardest working muscle.
- Beats over 100,000 times daily.
- Average 75 beats per minute.
- Striated and involuntary.
- Uses Ca2+ ions from SR and interstitial fluid.
- Allows prolonged contraction (10-15x longer than skeletal muscle).
Auto-Rhythmicity of Cardiac Muscle
- Contracts in response to self-generated action potentials.
- Does not rely on nerve supply.
- Some cells are “self-excitable” (pacemaker cells).
- Generate rhythmic contractions to adjacent cells.
Pacemaker Cells
- Sinoatrial (SA) node
- Atrioventricular (AV) node
- Responsible for automatic rhythmic contractions of upper and lower heart portions.
Characteristics of Smooth Muscle
- Non-striated and involuntary.
- Can be autorhythmic, influenced by nervous system.
- Action potential in one fiber spreads to neighboring fibers, causing synchronized contraction.
Structural Features of Smooth Muscle
- Thin filaments attached to dense bodies (similar to Z-discs).
- Intermediate filaments also attach to dense bodies, transmitting tension from muscle contraction.
Caveolae in Smooth Muscle
- Pouchlike invaginations containing extracellular Ca2+.
- Less sarcoplasmic reticulum (SR) and no transverse tubules.
- Contraction relies more on extracellular calcium.
Role of Calmodulin
- Protein binding to Ca2+ in smooth muscle.
- Acts as regulatory protein like troponin in skeletal muscle.
- Activates myosin heads for contraction.
- Contraction involves pulling of actin and myosin on dense bodies, intermediate fibers, causing cell contraction.
Biological Energy Systems:
Anaerobic Processes:
* Occur without oxygen.
* Short bursts of high-intensity movements.
Aerobic Processes:
* Require oxygen.
* Long-duration, low-intensity exercises.
Types of Exercises
Anaerobic:
* Weight training, sprinting, interval training.
Aerobic:
* Running, walking, swimming, biking.
Anaerobic Glycolysis
- Breaks down glucose to yield ATP.
- Produces lactic acid in absence of oxygen.
- Provides ATP for moderate to high-intensity, short-term exercise.
Aerobic Respiration
- Occurs in presence of oxygen.
- Utilizes pyruvic acid from glycolysis to produce ATP.
- Primary source of ATP at rest and during low-intensity activities.
Muscle Fatigue
- Inability to maintain forceful contraction.
- Reasons include lack of oxygen, calcium, ATP, or buildup of waste products.
Oxygen Replenishment
- Postexercise oxygen uptake termed Oxygen Debt or EPOC.
- Used for resynthesis of ATP and creatine phosphate, glycogen, and tissue oxygenation.
Motor Unit
- Includes somatic motor neuron & all innervated muscle fibers.
- Can range from 1 fiber per neuron to over 3000 fibers per neuron.
Muscle Twitch
- Brief contraction of all fibers in response to a single action potential (AP).
- Lasts about 20-200 msec, longer than muscle AP.
Fasciculations
- Involuntary visible contractions of motor units.
- Can be benign or indicative of underlying pathologies.
Stages of Contraction
Latent Period: AP sweeps sarcolemma, no tension.
Contraction Period: Ca²⁺ binds, crossbridges form, contraction.
Relaxation Period: Crossbridges break, Ca²⁺ restored.
Refractory Period: Subsequent AP can’t generate contraction.
Frequency of Stimulation
- Peak tension depends on stimulation frequency.
- Myograms show effects like wave summation, unfused tetanus, fused tetanus.
Un-Fused Tetanus
- Sustained, wavering contraction at 20-30 stimuli/sec.
- Shows partial relaxation between stimuli.
Wave Summation
- Second stimulus before relaxation causes stronger contraction.
- Results in contractions up to 5x greater than single twitch.
Fused (Complete) Tetanus
- Sustained contraction at 80-100 stimuli/sec.
- No time for relaxation, sustains powerful contractions.
Motor Unit Recruitment
- Increases number of active motor units for smooth movement.
- Smaller fibers recruited first, larger fibers as force requirement increases.
Muscle Tone:
Amount of tension in a muscle at rest due to weak, involuntary contractions.
Caused by constant innervation from spinal cord and higher brain centers.
Flaccid Paralysis
- Loss of muscle tone.
- Reduced reflexes, muscle atrophy.
- Associated with lower motor neuron injuries like trauma, ALS, Guillain-Barre Syndrome, Polio, nerve compression, and Myasthenia gravis (MG).
Spastic Paralysis
- Increased muscle tone, hyperreflexia.
- Associated with upper motor neuron injuries like stroke, multiple sclerosis, traumatic brain injury, spinal cord injury, and cerebral palsy.
Rigidity
- Increase in muscle tone with no reflex effect, inability to relax.
- Can occur with tetanus (Clostridium tetani infection).
- Tetanus may also cause tetany.
