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Questions and Answers
What is the primary function of muscle spindles in stretch reflexes?
What is the primary function of muscle spindles in stretch reflexes?
Inhibition of motoneurons is achieved through the release of excitatory neurotransmitters.
Inhibition of motoneurons is achieved through the release of excitatory neurotransmitters.
False
What type of reflex does the knee jerk reflex represent?
What type of reflex does the knee jerk reflex represent?
Stretch reflex
The ______ reflex is useful in postural control and involves the contraction of agonist muscles.
The ______ reflex is useful in postural control and involves the contraction of agonist muscles.
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Match the following components of the stretch reflex with their functions:
Match the following components of the stretch reflex with their functions:
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What occurs following the activation of muscle spindles during a stretch reflex?
What occurs following the activation of muscle spindles during a stretch reflex?
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The sensory neuron in a stretch reflex connects only to a motor neuron.
The sensory neuron in a stretch reflex connects only to a motor neuron.
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Which neurotransmitter is released to excite motoneurons during muscle contraction?
Which neurotransmitter is released to excite motoneurons during muscle contraction?
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What is the primary characteristic of a fast twitch muscle fibre?
What is the primary characteristic of a fast twitch muscle fibre?
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All muscle fibres in a single motor unit can have different physiological profiles.
All muscle fibres in a single motor unit can have different physiological profiles.
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What happens to muscle contraction when the frequency of twitches is increased?
What happens to muscle contraction when the frequency of twitches is increased?
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A brief contraction of a muscle due to electrical stimulation is known as a __________.
A brief contraction of a muscle due to electrical stimulation is known as a __________.
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Which type of muscle fibre is characterized by a slow speed of force development and a slow fatigue rate?
Which type of muscle fibre is characterized by a slow speed of force development and a slow fatigue rate?
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Match the following muscle fibre types with their characteristics:
Match the following muscle fibre types with their characteristics:
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Muscle fibres of a motor unit can have different myosin fibre types.
Muscle fibres of a motor unit can have different myosin fibre types.
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What is required for a motor unit to contract uniformly throughout the muscle?
What is required for a motor unit to contract uniformly throughout the muscle?
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What is the primary role of muscle spindles in the body?
What is the primary role of muscle spindles in the body?
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Muscle spindles contain interneurons to process sensory input.
Muscle spindles contain interneurons to process sensory input.
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What type of reflex is associated with muscle spindles?
What type of reflex is associated with muscle spindles?
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Muscle spindles are classified as __________ that detect changes in muscle length.
Muscle spindles are classified as __________ that detect changes in muscle length.
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Match the component of the reflex arc with its corresponding function:
Match the component of the reflex arc with its corresponding function:
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What initiates an action potential in a muscle spindle?
What initiates an action potential in a muscle spindle?
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Muscle spindle reflexes are examples of involuntary responses.
Muscle spindle reflexes are examples of involuntary responses.
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Name one protective function of reflexes involving muscle spindles.
Name one protective function of reflexes involving muscle spindles.
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Study Notes
Membrane Physiology
- The plasma membrane is a bilayer of phospholipid molecules.
- Phosphate heads face outwards, fatty acid tails inwards.
- The membrane is impermeable to polar molecules.
- Contains proteins like ion channels (voltage-gated, ligand-gated, mechanically-gated, non-gated, leak).
- Creates an intracellular environment different from the external environment.
- Defines cell boundaries, encloses organelles (nucleus, nissl substance, mitochondria).
- Enables cell function by creating an internal environment different from the external one.
- The membrane can influence intracellular but not extracellular compartment composition.
Electrochemical Gradients
- Ions accumulate in different concentrations inside/outside cells, creating:
- Chemical gradient (concentration difference)
- Electrical gradient (polarity difference)
- The combined effect is the electrochemical gradient.
- Main ions involved in the membrane potential are Na+, K+, Cl−, and Ca2+.
- Each ion has its own equilibrium potential (no net movement).
Equilibrium Potential (K+)
- Equilibrium potential is the membrane potential where there's no net movement of a particular ion.
- The inside of the cell is more negative than the outside.
- Chemical force pushes K+ ions outwards.
- Electrical force pulls K+ ions inwards.
- Equilibrium is reached when chemical force = electrical force.
Nernst Equation (Equilibrium Potential Calculation)
- Used to calculate the equilibrium potential for an ion given its concentrations inside and outside the cell, also the ion charge.
- Note: This calculation does not require memorization.
Interaction of Ions
- Nernst equation calculated for K+, now Na+ is considered
- More Na+ ions outside than inside, chemical force driving Na+ into cell
- Electrical force also driving Na+ ions into the cell (inside is negative).
- This combination creates a strong driving force for Na+ into the cell.
Resting Membrane Potential
- Em = -60 to -70 mV
- Indicates an excess of negative charge inside the cell.
- Voltage is stable due to equal movement of ions across the membrane.
Permeability of Membrane
- How easily ions cross the membrane.
- The membrane has permeability to each ion (PK, PNa).
- The number of leak channels for each ion factors into permeability.
Factors Influencing Ion Movement (Flux)
- Chemical gradient: Unequal distribution drives ions. K+ higher inside, Na+ higher outside.
- Electrical force: Opposite charges attract, ions move towards inside cell at negative Em.
Goldman Equation
- Used to estimate resting membrane potential.
- Takes into account concentrations of ions and their relative permeability.
- Can consider three or four different ions to approximate membrane potential.
Na+/K+ Pump
- Maintains resting potential.
- Pumps 3 Na+ ions out and 2 K+ ions in.
- Does not establish resting potential, only maintains it by compensating for the net outward leak of positive ions.
Neuronal Excitability
- Electrical signal for communication between cells.
- Initiated by a stimulus, influx of Na+.
- Threshold potential needed to start action potential generation.
Stages of the Action Potential
- Threshold reached = voltage gated Na+ channels open, Na+ influx, membrane potential more positive.
- Depolarization = More Na+ influx, positive feedback loop.
- Peak = Voltage gated Na+ channels close, Voltage gated K+ channels open, K+ efflux, potential drops.
- Repolarization = K+ leave membrane potential drops to RMP, voltage gated K+ channels close.
- Hyperpolarization = Excessive K+ efflux, MP gets more negative but is then restored.
- Na+/K+ pump maintains normal RMP.
Refractory Periods
- Absolute refractory period: No new action potential possible.
- Relative refractory period: Possible to initiate another action potential, but requires stronger stimulus.
Conduction of Action Potentials
- Myelinated vs unmyelinated neurons:
- Myelinated conduction is much faster (saltatory conduction).
- Unmyelinated is relatively slower.
- Axon diameter influences conduction speed. Larger diameter = faster.
Somatic Nervous System
- Part of the nervous system that is under voluntary control.
- Provides awareness of the world around us, allows for localization of sensations throughout the body, and provides awareness of body position and movement (proprioception).
The Nervous System
- CNS: Brain, spinal cord.
- PNS: Everything else, carries information from receptors to the CNS and from the CNS to effectors.
- Afferent: toward CNS
- Efferent: away from CNS
- Sensory information - via afferent neurons
- Motor commands - via efferent neurons
The Neuron (Nerve Cell)
- Specialized functional unit of the nervous system.
- Cell body (perikaryon)
- Axon
- Dendrites
Anatomy of a Neuron
- Axon hillock: initiating action potentials
- Myelin sheath: Insulation for faster transmission
- Nodes of Ranvier: Gaps in myelin sheath, action potential jumps.
- Schwann cells: PNS myelin
- Oligodendrocytes: CNS myelin
- Nodal membrane: Membrane at the node of Ranvier
- Intranodal membrane: Membrane at the regions that are myelinated
Motor Neurones
- Efferent neurons that supply skeletal muscles (skeletal muscle is the effector).
- Bring about muscle movement/limb displacement.
- Set muscle tone.
Classes of Muscle
- Striated Muscle
- Skeletal Muscle
- Cardiac Muscle
- Non-striated Muscle
- Smooth Muscle
Anatomy of the Neuromuscular Junction
- Specific area of muscle where a motor neuron connects.
- Confined to a neurovascular hilum (nerve entry point).
Myelination
- Variation in thickness across nerve types.
- More myelination = faster conduction speed.
- Diseases like MS and diabetes can demyelinate axons.
- Myelin provides insulation.
Neuroglia/Glial Cells (PNS & CNS)
- Support cells in the nervous system:
- Schwann cells (PNS): myelination and support
- Satellite cells (PNS): support neurons
- Microglia (CNS): immune function
- Oligodendrocytes (CNS): myelination
Membranous Envelopes and Nerve Organization
- Connective tissue that encases nerves:
- Epineurium: Outermost layer
- Perineurium: Surrounds fascicles (bundles of axons)
- Endoneurium: Covers individual axons
Synapses and Neurotransmitters
- Site of communication between neurons or neurons and muscles.
- Electrical or chemical synapses:
- Electrical: Direct physical contact, faster communication. Less common in the mature CNS.
- Chemical: Requires neurotransmitter release, slower but more complex and versatile communication. More common in the mature CNS.
Chemical Synapse Characteristics
- Uni-directional information transfer from pre-synaptic to post-synaptic cell.
- Plenty of vesicles to transfer neurotransmitters. Synaptic cleft separates neurons.
- Neurotransmitters diffuse across the synaptic cleft.
- Neurotransmitters bind to receptors
Critical Processes of Neurotransmission
- Action potential invades nerve terminal → depolarization triggers Ca2+ channel opening → Ca2+ influx into nerve terminal → neurotransmitter release by exocytosis
Fate of Neurotransmitters
- Neurotransmitter removal mechanism - Either re-uptake or enzymatic breakdown.
Major Neurotransmitters and Receptors
- Acetylcholine (nicotinic) at neuromuscular junctions
- GABA and glycine, other neurotransmitters and receptors.
Receptor Signaling Mechanisms
- Ionotropic (direct): Receptor operates gated ion channels. Excitatory (Na+ influx) or Inhibitory (Cl- influx).
- Metabotropic (indirect): Receptor activates G proteins that influence other channels or intracellular signaling cascades. Slower, longer lasting effects.
Synaptic Integration
- Combining multiple synaptic inputs to determine neuron firing.
- Spatial summation (combining inputs from different synapses)
- Temporal summation (combining inputs from the same synapse over time).
Mixed Synapses
- Synapses that receive both excitatory and inhibitory inputs.
- Overall effect depends on the balance of excitation and inhibition.
Motor Units
- Functional unit of the motor system, composed of a motor neuron and all the muscle fibers it innervates.
- Varied innervation ratios, depending on the function and control needed. Higher ratio = less precise movement.
Characteristics of Motor Units
- Motor properties are determined by the motor neuron.
- Specific muscle fiber types (twitch characteristics - fast, intermediate, slow)
Types of Skeletal Muscle
- Convergent/triangular, circular/sphincteric, parallel/strap, fusiform and pennate.
- Pennate muscles have multiple fibers running at angles to the tendon, leading to powerful contractions.
- Other types include unipennate, bipennate, and multipennate.
How Muscle Contraction Occurs (Sliding Filament Model)
- Neuromuscular junctions ensure simultaneous contraction.
- Action Potential → ACh release → depolarization → calcium release → muscle contraction.
Role of Ca2+ (in excitation-contraction coupling)
- Ca2+ concentration higher inside than outside.
- Action potential opens voltage-gated Ca2+ channels in presynaptic terminal, Ca2+ influx → vesicle fusion, Acetylcholine release.
- Ca2+ sensor (synaptotagmin) triggers vesicle fusion.
- Leads to release of neurotransmitters, resulting in muscle contraction.
The Neuromuscular Junction (NMJ)
- A specialized synapse between a motor neuron and a muscle fiber.
- Allows very fast, reliable contraction.
Generation of Action Potentials at NMJ
- End-plate potentials (EPP) are caused by ACh binding to receptors, resulting in Na+ influx and depolarization.
- EPP depolarization triggers opening of voltage-gated Na+ channels.
- This produces a large action potential, activating the muscle fiber.
Fate of ACh
- ACh is rapidly removed from the synaptic cleft.
- Hydrolyzed by acetylcholinesterase (AChE) into choline and acetate.
- Choline is reabsorbed by the presynaptic neuron to be reused.
Twitch Characteristics of Muscle Fibers
- Brief muscle contractions in response to stimulation.
- Classified into different types according to speed of contraction and fatigue resistance.
Summation of Twitches
- Combining individual twitches to produce sustained and stronger muscle contractions.
- Higher stimulation frequencies = stronger and smoother muscle contraction.
- Twitches summate to create tetanus (sustained contraction).
Disease Affecting NMJ
- Myasthenia gravis: Autoimmune disease attacking nicotinic ACh receptors, causing muscle weakness.
Degeneration and Regeneration of Peripheral Nerves
- Causes for degeneration (genetic, aging, injury, toxins).
- Damage to the nerve:
- Types of Damage (Superficial, Intermediate, Deep)
- Symptoms following damage (Neuropraxia, axonotmesis and neurotmesis)
- Reactions to nerve damage
- Fate of Proximal and Distal segments
- Regeneration after damage
- Switching of different muscle fiber types following injury
Reflexes
- Quick, involuntary responses to stimuli.
- Important for posture, protection and homeostasis (e.g., withdrawal, stretch and crossed-extensor reflexes).
- Components: Sensory receptor, afferent limb (sensory neuron), central component (interneurons), efferent limb (motor neuron), effector (muscle, gland, etc.)
Stretch Reflex (Myotatic Reflex)
- Monosynaptic or polysynaptic.
- The muscle spindle detects stretch → signal to sensory neuron → synapse with and excite motor neuron → muscle contraction.
Golgi Tendon Reflex
- Detects tension → signal to sensory neuron → synapse with and inhibit motor neuron → antagonists contract.
- Prevents over-stretching of muscles to protect from damage.
Withdrawal and Crossed-Extensor Reflexes
- Coordinated response to a painful stimulus.
- Withdrawal and compensatory extension of the opposite limb.
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