Human Physiology: Study Guide for Nervous System and Sensory Physiology (Chapters 8–10)

Chapter 8: Neural Signaling and Synaptic Transmission

Events of an Action Potential

An action potential is a rapid, temporary change in a cell's membrane potential, essential for neural communication. Understanding its phases is crucial for grasping how neurons transmit signals.

• Resting Membrane Potential: The neuron maintains a stable voltage (typically around -70 mV) due to ion gradients and selective permeability.

• Depolarization: Triggered when voltage-gated Na+ channels open, allowing Na+ influx, making the inside more positive.

• Repolarization: Voltage-gated K+ channels open, K+ exits the cell, restoring negativity.

• Hyperpolarization: K+ channels remain open briefly, causing the membrane potential to become more negative than resting.

Key Equation:

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for K+)

Example: During depolarization, the membrane potential may rise from -70 mV to +30 mV.

Ion Permeability and Channel Function

• Voltage-Gated Na+ Channels: Open rapidly during depolarization, then inactivate.

• Voltage-Gated K+ Channels: Open more slowly, responsible for repolarization.

Additional info: The sequence of channel opening and closing ensures the unidirectional propagation of the action potential.

Propagation of Action Potentials

• Unmyelinated Axons: Action potentials propagate continuously along the membrane.

• Myelinated Axons: Action potentials "jump" between nodes of Ranvier in a process called saltatory conduction, increasing speed.

Comparison Table:

Feature	Unmyelinated Axon	Myelinated Axon
Conduction Speed	Slower	Faster
Energy Efficiency	Lower	Higher
Saltatory Conduction	No	Yes

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated, regardless of stimulus strength.

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential.

Chemical Synapses and Neurotransmitter Release

• Neurotransmitter (NT) Release: Triggered by Ca2+ influx when an action potential reaches the axon terminal.

• Steps: Action potential arrives → voltage-gated Ca2+ channels open → Ca2+ enters → vesicles fuse with membrane → NT released into synaptic cleft.

• Amount of NT Released: Increases with stronger or more frequent action potentials.

Postsynaptic Effects: NTs can cause depolarization (excitatory postsynaptic potential, EPSP) or hyperpolarization (inhibitory postsynaptic potential, IPSP).

Chapter 9: Central Nervous System Structure and Function

Gray Matter, White Matter, Tracts, and Nuclei

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons. Found in the cortex and central regions of the spinal cord.

• White Matter: Composed of myelinated axons forming tracts for signal transmission.

• Tracts: Bundles of axons in the CNS, classified as ascending (sensory) or descending (motor).

• Nuclei: Clusters of neuron cell bodies within the CNS.

Cerebrospinal Fluid (CSF) and Blood-Brain Barrier

• CSF: Clear fluid produced by the choroid plexus; cushions the brain and removes waste.

• Blood-Brain Barrier: Selective barrier formed by endothelial cells; restricts passage of substances from blood to brain.

Spinal Cord Organization

• Dorsal Roots: Carry sensory (afferent) information into the spinal cord.

• Ventral Roots: Carry motor (efferent) information out of the spinal cord.

• Gray Matter: Divided into dorsal, lateral, and ventral horns.

• White Matter: Contains ascending and descending tracts.

Brainstem and Diencephalon Functions

• Medulla: Controls vital autonomic functions (e.g., heart rate, breathing).

• Pons: Relays signals and regulates sleep and respiration.

• Midbrain: Involved in vision, hearing, and motor control.

• Cerebellum: Coordinates movement and balance.

• Diencephalon: Includes the thalamus (sensory relay), hypothalamus (homeostasis), and pituitary gland (hormone release).

Hormones of the Pituitary Gland

• Anterior Pituitary: Releases tropic hormones (stimulate other glands). Examples: ACTH, TSH, LH, FSH.

• Posterior Pituitary: Releases ADH and oxytocin.

• Tropic Hormones: Hormones that regulate other endocrine glands.

Functional Areas of the Cerebral Cortex

• Primary Motor Cortex: Controls voluntary movement.

• Primary Somatosensory Cortex: Processes sensory input from the body.

• Primary Visual Cortex: Processes visual information.

• Primary Auditory Cortex: Processes sound.

• Primary Olfactory Cortex: Processes smell.

• Primary Gustatory Cortex: Processes taste.

Chapter 10: Sensory Physiology

Transduction and Sensory Receptors

Transduction is the process by which sensory receptors convert stimuli into electrical signals (membrane potential changes).

• Modality: The type of stimulus (e.g., light, sound, pressure).

• Neural Receptors: Can be myelinated or unmyelinated; transmit sensory information.

• Nonneural Receptors: Specialized cells that synapse onto sensory neurons.

Key Terms

• Adequate Stimulus: The specific type of stimulus to which a receptor is most sensitive.

• Threshold: Minimum stimulus intensity required to generate a response.

• Receptor Potential: Graded potential produced in a receptor cell by a stimulus.

• Receptive Field: The area monitored by a single sensory neuron.

Major Groups of Receptors

• Mechanoreceptors: Respond to mechanical force (touch, pressure).

• Thermoreceptors: Respond to temperature changes.

• Chemoreceptors: Respond to chemical stimuli.

• Photoreceptors: Respond to light (in the eye).

Sensory Pathways and Coding

• Ascending Pathways: Sensory information travels to the brain via spinal tracts or cranial nerves.

• Perceptual Threshold: The level of stimulus intensity necessary for conscious perception.

• Habituation: Decreased response to a repeated, benign stimulus.

• Lateral Inhibition: Enhances contrast and spatial resolution by inhibiting neighboring neurons.

• Stimulus Intensity: Encoded by the number of receptors activated and frequency of action potentials.

Adaptation and Pain Modulation

• Tonic Receptors: Adapt slowly, maintain response to a stimulus.

• Phasic Receptors: Adapt rapidly, respond only to changes in stimulus.

• Nociceptors: Detect pain; located throughout the body.

• Fast Pain: Sharp, localized; transmitted by myelinated Aδ fibers.

• Slow Pain: Dull, diffuse; transmitted by unmyelinated C fibers.

• Referred Pain: Pain perceived at a location other than the site of the stimulus.

• Pain Modulation: Involves inhibitory interneurons and descending pathways (e.g., gate control theory).

Gate Control Theory: Non-painful input (Aβ fibers) can inhibit pain signals from C fibers, reducing pain perception.

Additional info: Understanding these mechanisms is essential for interpreting sensory deficits and pain management strategies in clinical practice.