Week 8 (Neuromuscular System) Flashcards
(32 cards)
What is the resting membrane potential (RMP) in skeletal muscle?
90mV
What are the types of skeletal muscle fibres?
Type I, slow twitch
Type II, fast twitch
Structure of a Typical Neuron
A neuron consists of a cell body, dendrites, and an axon.
Classification of Neurons
Based on Number of Axons and Dendrites (Structural Classification)
Multipolar neurons – multiple processes (one axon, multiple dendrites).
Bipolar neurons – two processes (one axon, one dendrite).
Unipolar neurons – single process that splits into two branches.
Based on Presence or Absence of Myelin
Myelinated neurons – have a myelin sheath.
Unmyelinated neurons – lack a myelin sheath.
Based on Function
Sensory (afferent) neurons – transmit impulses from sensory receptors to the central nervous system (CNS).
Interneurons (association/relay neurons) – connect sensory and motor neurons within the CNS.
Motor (efferent) neurons – transmit impulses from the CNS to muscles or glands.
Based on Velocity of Nerve Impulse Transmission
Classification of nerve fibers based on conduction speed.
Synapse
The point at which one neuron communicates with another.
Presynaptic neuron: Neuron transmitting the signal through its axon.
Postsynaptic neuron: Neuron receiving the signal through its dendrites.
Functions of Muscle Tissue
Produces Skeletal Movement
Movement of the whole body.
Localized movement.
Maintains Posture and Body Positions
Skeletal muscle contractions help maintain posture.
Stabilizes Joints
Generates Heat (Thermogenesis)
Muscle contractions produce heat.
Stores and Moves Substances Within the Body
Provides nutrient reserves.
Cardiac muscle pumps blood.
Skeletal muscle promotes lymph and blood flow.
Smooth muscle propels substances through organs.
Guards Entrances and Exits
Digestive and urinary tract openings controlled by sphincters.
Other Functions
Protects organs, dilates pupils, causes piloerection, regulates blood pressure.
Characteristics of Muscle Tissue
Excitability: Ability to respond to stimuli by generating electrical signals.
Contractility: Ability to contract forcefully when stimulated.
Extensibility: Ability to stretch without damage.
Elasticity: Ability to return to original shape after contraction or extension.
Classification of Muscle Tissue
By Location/Function
Skeletal muscle
Cardiac muscle
Smooth muscle
By Structure (Striations)
Striated muscle
Unstriated muscle
By Control of Contraction
Voluntary muscle
Involuntary muscle
Structure of Skeletal Muscle
Muscle Fiber Size
Diameter: 10-100 μm
Length: Up to 30 cm
Sarcolemma – Plasma membrane of a muscle fiber.
Sarcoplasm – Cytoplasm of a muscle cell.
Contains organelles, glycogen, myoglobin, ions, and mitochondria.
Myofibril – Rodlike organelle with alternating light (I) and dark (A) bands.
Sarcomere – smallest contractile unit of muscle fibre & functional unit of muscle fiber.
Composed of thick and thin filaments (myosin, actin, tropomyosin, troponin).
Sarcoplasmic Reticulum (SR) – Stores and releases calcium (Ca²⁺).
T-Tubules – Conduct nerve impulses and trigger Ca²⁺ release.
Classification of Skeletal Muscle Fibers
Type I (Slow-Twitch / Slow Oxidative / Red Fibers)
High myoglobin, small size, many mitochondria, fatigue-resistant.
Type II (Fast-Twitch / Fast Glycolytic / White Fibers)
Low myoglobin, large size, high glycolytic activity, quick fatigue.
Type IIa – Intermediate properties.
Type IIb – Classic fast-twitch fibers, low aerobic potential.
Type IIc – Rare (~1%).
Skeletal Muscle Hypertrophy & Hyperplasia
Hypertrophy: Increase in muscle size due to larger muscle fibers.
(result of repetitive forceful contractions of muscle fibres)
Hyperplasia: Increase in muscle size due to more muscle fibers
(controversial; mixed evidence in human studies under rare condition of extreme muscle force generation, actual number of muscle fibres have been observed to increase)
MEMBRANE POTENTIAL
Difference in electrical potential across the cell membrane
Voltage difference due to a slight excess of cations (positive ions) on one side and anions (negative ions) on the other
Occurs in most cells
Resting Membrane Potential (RMP)
Potential difference across the cell membrane when the cell is at rest
Generated by different concentrations of Na⁺ (sodium), K⁺ (potassium), Cl⁻ (chloride), and protein anions
Inside of the cell is more negative compared to the outside
Inside: More negative
Outside: More positive
Ionic differences due to:
Different permeabilities of the membrane to Na⁺, K⁺, Cl⁻, and protein anions
Operation of Na⁺/K⁺ pump
Main Contributor to RMP: K⁺ Outflux
Leak channels (~100x more permeable to K⁺ than Na⁺)
More K⁺ leak channels than Na⁺ leak channels
RMP must exist for an action potential to occur
Key role in the excitability of nerve and muscle cells
ACTION POTENTIAL (AP)
changes in electrical potential occurring at the surface of nerve tissue or muscle tissue at the moment of excitation
rapid change in membrane potential followed by a return to resting membrane potential
occurs when a neuron is conducting a nerve impulse
propagated with the same size along the entire length of a nerve axon or muscle cell
Phases of an Action Potential (2 phases: depolarisation and repolarisation)
1) Resting State
2) Depolarization Phase
Charge inside the cell becomes less negative (more positive)
Charge outside becomes less positive (more negative)
3) Repolarization Phase
Charge inside the cell becomes more negative (less positive)
Returns toward resting membrane potential
4) Hyperpolarization Phase
5) Resting State
ALL-OR-NONE PRINCIPLE
Either an action potential fires or it does not
AP only occurs if the threshold level is reached
Stronger stimulus does not create a larger AP
Depolarization wave is the same size for all action potentials
ACTION POTENTIAL PROPAGATION
Unmyelinated Axon
Continuous propagation of nerve impulse
Action potentials occur over the entire axon membrane
Myelinated Axon
Saltatory conduction (“to jump/leap”)
Myelin sheath absent at Nodes of Ranvier
Action potential jumps from node to node
Voltage-gated Na⁺ channels only at nodes
Depolarization occurs only at Nodes of Ranvier
Saltatory conduction increases conduction velocity
Myelinated neurons conduct faster than unmyelinated neurons
Neuromuscular Junction (NMJ) & Muscle Contraction Neuromuscular Junction (NMJ)
Synapse between motor neuron and skeletal muscle fiber.
each axon ending forms a NMJ with a single muscle fibre usually only one per muscle fibre
muscle fibre contracts only under control from the NMJ chemical synapse
Components:
Synaptic terminal – Releases acetylcholine (ACh).
Motor end plate – Contains ACh receptors.
Synaptic cleft – Contains AChE (enzyme breaking down ACh).
Neuromuscular Transmission
Neuromuscular transmission is the process by which impulses are transmitted from a motor neuron to the muscle fiber through the neuromuscular junction (NMJ).
Steps of Neuromuscular Transmission
1) Action potential arrives at the axon terminal of the motor neuron.
2) Calcium ions (Ca²⁺) enter the axon terminal.
3) Ca²⁺ entry triggers the release of acetylcholine
(ACh) from synaptic vesicles via exocytosis.
4) ACh diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma.
5) Local depolarization occurs at the motor end plate.
6) Action potential propagates along the sarcolemma.
7) ACh is removed from the synaptic cleft by:
Acetylcholinesterase (AChE), which breaks it down.
Diffusion away from the synapse.
Motor Unit
A motor unit is the nerve-muscle functional
unit, essential for normal skeletal muscle function.
It consists of a motor neuron and all the muscle fibers it innervates.
A whole muscle is controlled by multiple motor neurons.
Each motor neuron branches as it enters the muscle.
Each branch forms a neuromuscular junction (NMJ) with a single muscle fiber.
Skeletal Muscle Contraction
During Contraction:
Overlapping thin filaments (actin) and thick filaments (myosin) slide past each other.
Thin filaments slide past thick filaments, increasing overlap.
Actin and myosin do not change length during contraction.
Sliding occurs in all sarcomeres of a myofibril.
Myofibril shortens → Muscle fibre (muscle cell) shortens → Muscle shortens.
Shortening sarcomeres are responsible for skeletal muscle contraction.
During Relaxation:
Sarcomeres lengthen.
In a relaxed state, thick and thin filaments overlap only slightly.
Mechanism of Sliding Filament Model
Upon stimulation, myosin heads bind to actin, initiating sliding.
Myosin heads latch onto active sites on actin, forming a cross bridge.
Thin filaments move due to mechanical forces from the interaction of cross bridges with thin filaments.
Each myosin head binds and detaches multiple times, acting like a ratchet to generate tension and propel thin filaments toward the center of the sarcomere.
As this occurs across all sarcomeres, the muscle fibre shortens, leading to overall muscle contraction.
