Core Concepts in Neurophysiology, Sensory Systems, and Muscle Biology

Synaptic Transmission Fundamentals

• Synapse: The gap between neurons (synaptic cleft).
• Synaptic Knob: The terminal structure containing vesicles.
• Neurotransmitters: Chemicals that transmit signals.
  • Acetylcholine: Activates muscles.
  • Monoamines: Involved in cognitive processes (emotion, arousal, memory).
  • Amino Acids, Peptides, and Estrogen.
• Vesicles: Store neurotransmitters.
• Axon Hillock: Essential for initiating the firing of a neuron (action potential generation).

Postsynaptic Potentials (EPSP and IPSP)

• EPSP (Excitatory Postsynaptic Potential): Moves the membrane potential toward the threshold.
• IPSP (Inhibitory Postsynaptic Potential): Moves the membrane potential away from the threshold.

Vision and Refractive Errors

• Nearsightedness (Myopia):
  • The eyeball is too long.
  • Focal point falls in front of the retina.
  • Corrected with a divergent lens.
• Farsightedness (Hyperopia):
  • The eyeball is too short.
  • Focal point falls behind the retina.
  • Corrected with a convergent lens.
• Astigmatism: Caused by irregular curvature or density changes in parts of the eye (e.g., cornea or lens).

Photoreceptors: Rods and Cones

The pigmented membrane (Retinal Pigment Epithelium) supports photoreceptor function through:

• Light absorption
• Epithelial transport
• Spatial ion buffering
• Visual cycle (regeneration of photopigments)
• Phagocytosis (of shed outer segments)
• Secretion and immune modulation

Rods vs. Cones

• Rods: Function in low light (scotopic vision), perceive shades of gray.
• Cones: Require moderate light (photopic vision), responsible for color vision; do not function well in low light.

Action Potential Generation in a Rod Photoreceptor

When light hits the rod, the cell hyperpolarizes, stopping the release of inhibitory neurotransmitters. The mechanism involves:

1. Rhodopsin absorbs light and decomposes.
2. This decomposition leads to the breakdown of cGMP.
3. Loss of cGMP causes Na+ channels (which were open in the dark) to close.
4. The cell hyperpolarizes (K+ moves out), generating the signal.

Rhodopsin is decomposed by light and regenerated from scotopsin and retinal.

Muscle Physiology and Sarcomere Structure

Sarcomere Components

• A-Band: The entire length of the thick (myosin) filament.
• H-Zone: The middle region of the A-band where only thick filaments are present.
• M-Line: The center line within the H-zone, anchoring the thick filaments.
• Z-Line (Z-Disc): Anchors the thin filaments; defines the boundary of the sarcomere.
• I-Band: The region containing only thin (actin) filaments, located between sarcomeres.

Muscle Contraction Initiation

Calcium ions (Ca²⁺) necessary for contraction are stored in the Sarcoplasmic Reticulum (SR) and are released into the sarcoplasm upon arrival of an action potential.

Muscle Energy Sources and Fiber Types

Energy Sources for Muscle Contraction

• ATP (Adenosine Triphosphate)
• Creatine Phosphate
• Cellular Respiration (Aerobic Metabolism)
• Oxygen Debt (EPOC - Excess Post-exercise Oxygen Consumption)

Muscle Fiber Classification

• Slow Twitch (Type I): Slow oxidative fibers.
• Fast Twitch (Type IIa): Fast oxidative-glycolytic (Fast Red).
• Fast Twitch (Type IIb/IIx): Fast glycolytic (Fast White).

Neuronal Refractory Periods

• Absolute Refractory Period: The neuron cannot generate another action potential, regardless of stimulus strength.
• Relative Refractory Period: The neuron can generate another action potential, but only if the stimulus is stronger than normal (occurs during repolarization).

These periods ensure unidirectional impulse propagation.

Myelination and Conduction Velocity

• Schwann Cells: Glial cells that wrap around axons, forming the myelin sheath, which significantly increases impulse speed.
• Node of Ranvier: Gaps or openings in the myelin sheath.
• Saltatory Conduction: The process where action potentials jump from one Node of Ranvier to the next, maximizing conduction velocity along myelinated axons.

Divisions of the Nervous System

Afferent (Input) Pathways (Sensory)

• Somatic Senses: Vision, hearing, and balance.
• Special Senses: Taste and Olfaction.
• Visceral Senses: Internal senses (e.g., pain, reflexes related to internal organs).

Efferent (Output) Pathways (Motor)

• Somatic Nervous System: Controls skeletal muscle (voluntary movement).
• Autonomic Nervous System (ANS): Controls involuntary functions (cardiac muscle, smooth muscle, glands).
  • Sympathetic Division: "Fight or flight."
  • Parasympathetic Division: "Rest and digest."
  • Enteric Nervous System (ENS): Controls the gastrointestinal tract.

Classification of Sensory Receptors

• Chemoreceptors: Detect chemical changes (e.g., taste and olfaction/smell).
• Nociceptors: Detect pain stimuli.
• Thermoreceptors: Detect temperature changes (heat and cold). Note: They often exhibit habituation.
• Mechanoreceptors: Respond to mechanical forces (pressure, stretch, movement).
  1. Proprioceptors: Sense body position and movement.
  2. Pressoreceptors (Tactile Receptors): Sense touch and pressure.
     • Meissner's Corpuscles: Detect light touch.
     • Pacinian Corpuscles: Detect deep pressure and vibration.
  3. Stretch Receptors: Monitor muscle length and tension.
• Photoreceptors: Detect light changes (found primarily in the retina of the eye and the pineal gland).

Functions of Sensory Information

1. Formulate efferent (motor) responses.
2. Maintain awareness of surroundings.
3. Cortical stimulation (conscious perception).
4. Storage of information (memory formation).

Receptor Potential Structures

1. Generator Potential: Produced by specialized nerve endings (the receptor is the sensory neuron itself).
2. Receptor Potential: Produced by a separate, specialized receptor cell that then communicates with an associated sensory neuron.