Anatomy & Physiology Study Guide: Nervous System and Sensory Physiology (Chapters 8-10)

Chapter 8: Neural Signaling and Synaptic Transmission

Events Occurring in an Action Potential

An action potential is a rapid, temporary change in a cell's membrane potential, essential for neural communication.

• Changes in Ion Permeability and Ion Flow: During an action potential, voltage-gated Na+ channels open, allowing Na+ influx (depolarization), followed by K+ efflux (repolarization).

• Voltage-Gated Na+ Channel Function: These channels open rapidly in response to threshold depolarization, then inactivate, ensuring unidirectional propagation.

• Depolarization Progression: Depolarization moves along the axon as local currents depolarize adjacent membrane segments, triggering further action potentials.

• Conduction in Unmyelinated vs. Myelinated Axons:

  • Unmyelinated: Action potentials propagate continuously along the axon.

  • Myelinated: Action potentials "jump" between nodes of Ranvier (saltatory conduction), increasing speed.

• Absolute vs. Relative Refractory Periods:

  • Absolute: No new action potential can be initiated (Na+ channels inactivated).

  • Relative: A stronger-than-normal stimulus can initiate another action potential (some Na+ channels reset, K+ channels still open).

Example: The rapid conduction in myelinated axons allows for quick reflexes, such as pulling your hand away from a hot surface.

Chemical Synapses and Neurotransmitter Release

• Neurotransmitter (NT) Release: Triggered by Ca2+ influx when an action potential reaches the axon terminal, causing vesicles to fuse with the membrane and release NTs into the synaptic cleft.

• Amount of NT Released: Proportional to the frequency and strength of action potentials; stronger stimuli release more NT.

• Postsynaptic Effects: NTs bind to receptors, causing either depolarization (excitatory postsynaptic potential, EPSP) or hyperpolarization (inhibitory postsynaptic potential, IPSP).

Example: Acetylcholine at the neuromuscular junction causes muscle contraction by depolarizing the muscle cell membrane.

Chapter 9: Central Nervous System Structure and Function

Gray Matter, White Matter, Tracts, and Nuclei

The central nervous system (CNS) is organized into gray matter (cell bodies, dendrites) and white matter (myelinated axons).

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; forms the cortex and nuclei.

• White Matter: Composed of myelinated axons; forms tracts that connect different CNS regions.

• 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: A clear fluid produced by the choroid plexus; cushions the brain and spinal cord, removes waste, and provides nutrients.

• Blood-Brain Barrier: A selective barrier formed by endothelial cells; restricts passage of substances from blood to CNS, protecting neural tissue.

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 Horns: Dorsal (sensory), lateral (autonomic), and ventral (motor) horns.

• White Matter Tracts: Ascending (sensory) and descending (motor) pathways.

Brainstem and Diencephalon Functions

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

• Pons: Relays signals between cerebrum and cerebellum; involved in sleep and respiration.

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

• Cerebellum: Coordinates voluntary movement and balance.

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

Hormones of the Pituitary Gland

• Anterior Pituitary: Secretes tropic hormones (stimulate other glands), e.g., ACTH, TSH, LH, FSH.

• Posterior Pituitary: Releases ADH (antidiuretic hormone) and oxytocin.

• Tropic Hormones: Hormones that regulate other endocrine glands.

• Hypertrophy vs. Atrophy: Hypertrophy is an increase in tissue size; atrophy is a decrease.

Functional Areas of the Cerebral Cortex

• Primary Motor Cortex: Controls voluntary muscle movements.

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

• Primary Visual Cortex: Processes visual information.

• Primary Auditory Cortex: Processes auditory information.

• 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) detected by a receptor.

• Neural Receptors: Can be myelinated (fast conduction) or unmyelinated (slow conduction).

• Nonneural Receptors: Specialized cells that release neurotransmitters to sensory neurons.

• Key Terms:

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

  • Threshold: Minimum stimulus intensity required to trigger 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 Sensory Receptors

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

• Thermoreceptors: Detect temperature changes.

• Chemoreceptors: Respond to chemical stimuli (taste, smell, blood pH).

• Photoreceptors: Detect light (in the retina).

Sensory Pathways and Coding

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

• Perceptual Thresholds: The level at which a stimulus is strong enough to be perceived.

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

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

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

• Adaptation: Tonic receptors adapt slowly; phasic receptors adapt quickly to sustained stimuli.

Somatic Sensation and Pain

• Somatic Sensation Pathway: Receptors → primary sensory neuron → secondary neuron (spinal cord/brainstem) → tertiary neuron (thalamus) → somatosensory cortex.

• Nociceptors: Pain receptors located throughout the body; respond to mechanical, thermal, or chemical stimuli.

• Fast vs. Slow Pain:

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

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

• Pain Modulation: Involves inhibitory interneurons and descending pathways; can be influenced by endorphins and other neurotransmitters.

• Gate Control Theory: Non-painful input (Aβ fibers) can inhibit pain transmission by activating inhibitory interneurons, reducing perception of pain from C fibers.

• Referred Pain: Pain perceived at a location other than the site of the stimulus, due to convergence of sensory pathways.

Receptor Type	Stimulus Detected	Example
Mechanoreceptor	Mechanical (touch, pressure)	Skin touch receptors
Thermoreceptor	Temperature	Warm/cold receptors in skin
Chemoreceptor	Chemicals	Taste buds, olfactory cells
Photoreceptor	Light	Rods and cones in retina

Additional info: The notes reference figures and diagrams (e.g., Figure 8.9, 8.13) that are not included; students should refer to their textbook for these visuals. Some questions prompt students to recall details from lecture, such as specific hormone names and targets.