Neural and Sensory Physiology: Key Concepts and Mechanisms

Neural Structure and Function

Axons and Neurons

Neurons are specialized cells of the nervous system responsible for transmitting electrical and chemical signals. Axons are one of the main structural components of neurons.

• Axon Structure: Axons are typically long, slender projections that conduct electrical impulses away from the neuron's cell body. They usually have a uniform width throughout their length and may branch at their terminal ends.

• Signal Transmission: Axons transmit signals to downstream neurons or effector cells, while dendrites receive incoming signals from upstream neurons.

• Example: The sciatic nerve contains axons that extend from the spinal cord to the lower limbs, transmitting motor and sensory information.

Resting Membrane Potential

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

• Definition: The inside of a resting neuron is more negative than the outside, typically around -70 mV.

• Key Ions: The difference is mainly due to the distribution of sodium (Na+), potassium (K+), and chloride (Cl-) ions.

• Equation: The Nernst equation can be used to calculate the equilibrium potential for a particular ion: $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Action Potentials and Refractory Periods

Action potentials are rapid changes in membrane potential that allow neurons to transmit signals. The refractory period ensures unidirectional propagation and limits firing frequency.

• Absolute Refractory Period: A period immediately following an action potential during which a neuron is completely insensitive to a second stimulus. This is due to inactivated sodium channels.

• Relative Refractory Period: Follows the absolute period; a stronger-than-normal stimulus is required to elicit another action potential.

• Depolarization: A shift in membrane potential toward a more positive value, typically due to Na+ influx.

• Key Ion: Sodium (Na+) is primarily responsible for the initiation of the action potential.

Endocrine System and Signal Transduction

Hormones and Immune Function

The endocrine system produces hormones that regulate various physiological processes, including immune system development.

• Thymosin: A hormone produced by the thymus gland, essential for the development and function of T-lymphocytes in the immune system.

Hormone-Receptor Complex and Gene Expression

Hormones can regulate gene expression by interacting with intracellular receptors.

• Mechanism: The receptor-hormone complex binds to specific DNA sequences, influencing the transcription of target genes.

• Example: Steroid hormones like cortisol bind to cytoplasmic receptors, then the complex enters the nucleus to regulate gene expression.

Second Messenger Systems

Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules.

• Phosphoinositide Pathway: Activation of phospholipase C (PLC) leads to the production of inositol trisphosphate (IP3) and diacylglycerol (DAG) as second messengers.

• Function: IP3 stimulates calcium release from the endoplasmic reticulum, while DAG activates protein kinase C.

Sensory Systems and Special Senses

Hormones of the Adenohypophysis (Anterior Pituitary)

The adenohypophysis releases several key hormones into the bloodstream.

Hormone	Function
Prolactin	Stimulates milk production
Luteinizing hormone (LH)	Regulates reproductive processes
Growth hormone (GH)	Stimulates growth and metabolism
Antidiuretic hormone (ADH)	Not released by adenohypophysis; released by posterior pituitary

Eye Anatomy and Physiology

• Choroid: The vascular layer of the eye that absorbs excess light to prevent reflection and scattering.

• Sclera: The tough, outermost layer of the eyeball, providing protection and serving as an attachment for ocular muscles.

• Myopia (Nearsightedness): In myopia, light rays are focused in front of the retina, causing distant objects to appear blurry.

• Fovea: The area of the retina that provides the sharpest vision due to a high density of cone photoreceptors.

Photoreceptors

• Rods: Specialized for scotopic (low-light) vision.

• Cones: Specialized for photopic (color and high acuity) vision.

Gustatory (Taste) Cells

• Location: Found in taste buds on the tongue, soft palate, and epiglottis.

• Function: Chemoreceptors that detect tastants (chemicals in food).

• Replacement: Gustatory cells are continuously replaced.

• Goblet Cells: Not present in taste buds; taste buds have supporting and basal cells, not goblet cells.

Somatic and Sensory Pathways

Proprioceptors

Proprioceptors provide information about body position and movement.

Proprioceptor	Function
Golgi tendon organs	Detect tension in tendons
Pacinian corpuscles	Detect deep pressure and vibration
Ruffini endings	Detect the stretch of skin and joint position
Baroreceptors	Detect blood pressure, not proprioceptors

Reflex Arcs and Integration Centers

• Spinal Cord: The primary integration center for reflex arcs, where afferent neurons synapse with interneurons.

• Example: The knee-jerk (patellar) reflex is integrated in the spinal cord.

Somatic Reflex Testing

• Purpose: Somatic reflex tests can help diagnose conditions such as spinal cord injury by assessing the integrity of neural pathways.

Sensory Pathways

• Spinothalamic Pathway: Transmits pain and temperature sensations from the body to the brain.

• Dorsal Column-Medial Lemniscus Pathway: Transmits fine touch, vibration, and proprioception.

Peripheral Nervous System (PNS) Nerves

• Peripheral Nerves: Include the peroneal, femoral, and sciatic nerves.

• Optic Nerve: Technically part of the central nervous system (CNS), not the PNS.