Nervous System and Muscle Physiology: Exam Review Study Notes

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Neurons are specialized cells of the nervous system responsible for receiving, integrating, and transmitting information throughout the body.

Signal Integration: Neurons receive and integrate signals from other neurons to facilitate communication within neural circuits.
Action Potentials: They generate electrical impulses (action potentials) that travel along their axons to communicate with other cells.
Neurotransmitter Release: At synapses, neurons release chemical messengers called neurotransmitters to transmit signals to neighboring cells.
Example: Sensory neurons receive information from sensory receptors and transmit it to the central nervous system for processing.

Neurons are classified based on the number and arrangement of their processes (axons and dendrites):

Multipolar Neurons: The most common type, typically have one axon and multiple dendrites, facilitating complex signal integration.
Bipolar Neurons: Have two main processes (one axon, one dendrite); found in sensory organs such as the retina.
Unipolar (Pseudounipolar) Neurons: Characterized by a single process that splits into two branches; commonly found in sensory ganglia.
Example: Motor neurons are multipolar, while retinal neurons are bipolar.

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest.

Key Factor: The selective permeability of the membrane to potassium ions (K+) is the primary contributor.
Ion Distribution: The concentration gradients of K+ and Na+ across the membrane, maintained by the sodium-potassium pump, are essential.
Equation: The Nernst equation can be used to calculate the equilibrium potential for a particular ion:
Example: At rest, the membrane is much more permeable to K+ than to Na+, resulting in a negative resting potential inside the cell.

Voltage-gated ion channels are essential for the generation and propagation of action potentials in excitable cells (neurons and muscle cells).

Function: These channels open or close in response to changes in membrane potential, allowing selective ion flow that contributes to action potentials.
Types: Common types include voltage-gated sodium (Na+), potassium (K+), and calcium (Ca2+) channels.
Example: During an action potential, voltage-gated Na+ channels open rapidly, causing depolarization.

Neurotransmitters are chemicals that transmit signals across synapses from one neuron to another or to a target cell.

GABA (gamma-aminobutyric acid): Primarily acts as an inhibitory neurotransmitter, reducing neuronal excitability.
Serotonin: Involved in sleep regulation and circadian rhythms.
Substance P: Enhances pain perception and inflammation.
Acetylcholine: Responsible for muscle contraction and also acts in the central nervous system.
Example: GABAergic neurons in the brain help prevent overexcitation and seizures.

Communication at chemical synapses involves the release of neurotransmitters in response to an action potential.

Process: Action potentials trigger the release of neurotransmitters, which bind to receptors on the postsynaptic membrane, leading to a rapid change in membrane potential.
Steps:
1. Action potential arrives at the presynaptic terminal.
2. Voltage-gated Ca2+ channels open, allowing Ca2+ influx.
3. Neurotransmitter vesicles fuse with the presynaptic membrane and release contents into the synaptic cleft.
4. Neurotransmitters bind to postsynaptic receptors, causing ion channels to open or close.
Example: At the neuromuscular junction, acetylcholine is released to stimulate muscle contraction.

Sensory receptors can be classified based on their adaptation to stimuli:

Phasic Receptors: Adapt quickly to stimuli and are best for detecting changes, such as feeling the weight of a backpack when first put on.
Tonic Receptors: Provide continuous information about a stimulus, such as the ongoing stretch of muscles during movement.
Example: Muscle spindles are tonic receptors, while Pacinian corpuscles are phasic.

Lateral inhibition is a neural mechanism that enhances sensory perception and contrast.

Definition: Occurs when excited neurons inhibit their neighboring neurons, sharpening the perception of a stimulus.
Application: Important in the visual system for edge detection and contrast enhancement.
Example: Lateral inhibition helps distinguish the boundaries of objects in the visual field.

The human auditory system distinguishes pitch and loudness based on sound wave properties.

Pitch: Determined by the frequency of sound waves; higher frequencies correspond to higher pitches.
Loudness: Determined by the amplitude of sound waves; greater amplitude means louder sound.
Example: A high-pitched whistle has a high frequency, while a loud drumbeat has a high amplitude.

Rods and cones are the two main types of photoreceptor cells in the retina, each with distinct functions.

Rods: More numerous, highly sensitive to dim light, responsible for night vision and peripheral vision.
Cones: Fewer in number, sensitive to bright light, responsible for color vision and high visual acuity.
Example: Reading in bright light uses cones, while seeing in a dark room relies on rods.

Neurons communicate with their targets using different types of signaling molecules:

Neurotransmitters: Released from the presynaptic neuron to bind to receptors on the postsynaptic neuron or target cell for rapid, localized signaling.
Hormones: Released into the bloodstream to signal long distances to their targets.
Example: Acetylcholine acts as a neurotransmitter at neuromuscular junctions, while adrenaline acts as a hormone.

Tropomyosin is a regulatory protein involved in the control of muscle contraction.

Function: Tropomyosin covers myosin-binding sites on actin filaments, preventing cross-bridge formation in resting muscle.
Regulation: In response to Ca2+ binding to troponin, tropomyosin shifts position, exposing the myosin-binding sites and allowing contraction.
Example: During muscle activation, Ca2+ influx leads to tropomyosin movement and muscle contraction.

If tropomyosin fails to function properly, muscle contraction becomes unregulated.

Continuous Contraction: Myosin-binding sites on actin would remain exposed at all times, causing continuous and unregulated muscle contraction.
Clinical Relevance: Such dysfunction could lead to muscle rigidity or spasticity.