Neurophysiology and Sensory Systems: Exam 2 Study Guide

Neurophysiology

Axonal Transport

Axonal transport is the process by which materials are moved along the axon of a neuron. This is essential for maintaining neuronal function and structure.

• Fast Axonal Transport: Uses ATP and Ca2+; moves organelles and synaptic vesicles rapidly.

• Slow Axonal Transport: Moves cytoskeletal and soluble proteins; slower rate.

Example: Neurotransmitter vesicles are transported via fast axonal transport.

Synapses

A synapse is the junction between a neuron and another cell (muscle, gland, or neuron) where communication occurs.

• Electrical Synapse: Direct cytoplasmic connections via gap junctions; rapid transmission.

• Chemical Synapse: Uses neurotransmitters; slower, but allows for modulation.

• Presynaptic Neuron: Releases neurotransmitter.

• Postsynaptic Neuron: Receives signal.

Example: Neuromuscular junction is a chemical synapse between a motor neuron and muscle fiber.

Functions of Glial Cells

Glial cells support and protect neurons in the nervous system.

• Astrocytes: Maintain blood-brain barrier, regulate nutrients.

• Oligodendrocytes: Create myelin sheath in CNS.

• Schwann Cells: Myelinate axons in PNS.

• Microglia: Immune defense in CNS.

• Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

Membrane Potentials

Neurons maintain a voltage difference across their plasma membrane, known as the membrane potential.

• Resting Membrane Potential (RMP): Typically -70 mV; maintained by Na+/K+ ATPase and ion channels.

• Equilibrium Potential: Voltage at which there is no net movement of a particular ion.

• Graded Potentials: Local changes in membrane potential; can be depolarizing or hyperpolarizing.

• Action Potentials: Rapid, all-or-none electrical signals; propagate along axons.

Key Equations:

• Nernst Equation:

$E_{ion} = \frac{RT}{zF} \ln \frac{[ion]_{out}}{[ion]_{in}}$

• Goldman-Hodgkin-Katz Equation: Predicts membrane potential considering multiple ions.

$V_m = \frac{RT}{F} \ln \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}}$

Ion Channels and Conductance

Ion channels regulate the flow of ions across the neuronal membrane, affecting excitability and signal transmission.

• Leak Channels: Always open; set RMP.

• Voltage-Gated Channels: Open/close in response to changes in membrane potential.

• Chemically-Gated Channels: Open/close in response to neurotransmitters.

Factors Affecting Conductance:

• Myelination increases conduction velocity.

• Axon diameter: Larger diameter = faster conduction.

Graded vs. Action Potentials

Graded potentials are variable in size and decay with distance, while action potentials are uniform and propagate without decrement.

• Graded Potentials: Can summate; amplitude depends on stimulus strength.

• Action Potentials: All-or-none; threshold must be reached.

Table: Comparison of Graded and Action Potentials

Feature	Graded Potential	Action Potential
Amplitude	Variable	All-or-none
Propagation	Decremental	Non-decremental
Summation	Possible	Not possible
Threshold	No threshold	Threshold required

Excitatory vs. Inhibitory Signals

Neurons receive both excitatory and inhibitory inputs, which determine whether an action potential will be generated.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, increases likelihood of AP.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreases likelihood of AP.

Threshold and Refractory Periods

The threshold is the minimum voltage required to trigger an action potential. The refractory period is the time after an AP during which a neuron cannot fire another AP.

• Absolute Refractory Period: No AP can be generated.

• Relative Refractory Period: AP can be generated with a stronger stimulus.

Saltatory Conduction

Saltatory conduction is the rapid transmission of action potentials along myelinated axons, where the impulse jumps from one node of Ranvier to the next.

• Myelination: Increases speed and efficiency of conduction.

• Nodes of Ranvier: Gaps in myelin sheath where ion channels are concentrated.

Sensory Systems

General Senses

General senses include touch, pressure, temperature, and pain. Sensory receptors detect stimuli and transmit signals to the CNS.

• Mechanoreceptors: Detect touch, pressure, vibration.

• Thermoreceptors: Detect temperature changes.

• Nociceptors: Detect pain.

• Photoreceptors: Detect light (vision).

• Chemoreceptors: Detect chemical stimuli (taste, smell).

Receptor Properties

Receptors have specific properties that allow the CNS to distinguish different stimuli.

• Modality: Type of stimulus (e.g., light, sound).

• Location: Where stimulus is detected.

• Intensity: Strength of stimulus.

• Duration: How long stimulus lasts.

Label-Line Coding

Label-line coding refers to the relationship between the sense and the nerve, where specific pathways carry specific types of sensory information.

• Lateral Inhibition: Enhances contrast and accuracy by inhibiting adjacent neurons.

Tonic vs. Phasic Receptors

Tonic receptors respond continuously to stimuli, while phasic receptors adapt rapidly and respond only to changes.

• Tonic: Slow adaptation (e.g., pain receptors).

• Phasic: Fast adaptation (e.g., touch receptors).

Somatic Sensory Pathways

Somatic sensory pathways involve primary, secondary, and tertiary neurons transmitting signals from receptors to the brain.

• Primary Neuron: Receptor to spinal cord/brainstem.

• Secondary Neuron: Spinal cord/brainstem to thalamus.

• Tertiary Neuron: Thalamus to sensory cortex.

Pain and Temperature Sensation

Nociceptors detect pain, while thermoreceptors detect temperature changes. Pain can be acute or chronic, and is transmitted by different types of nerve fibers.

• Acute Pain: Fast, sharp pain (A-delta fibers).

• Chronic Pain: Slow, dull pain (C fibers).

Special Senses: Smell and Taste

Olfactory and gustatory systems detect chemical stimuli.

• Olfactory Pathway: Olfactory receptor → CN I → olfactory bulb → olfactory tract.

• Taste Pathway: Facial nerve (CN VII), glossopharyngeal nerve (CN IX), vagus nerve (CN X).

Special Senses: Hearing and Equilibrium

The auditory system detects sound waves and interprets them as hearing. The vestibular system detects head position and movement for balance.

• Basilar Membrane: Vibrates in response to sound; location of vibration determines pitch.

• Cochlea: Contains hair cells that transduce sound.

Special Senses: Vision

The visual system detects light and processes images.

• Retina: Contains photoreceptors (rods and cones).

• Fovea Centralis: Area of highest visual acuity.

• Accommodation: Lens changes shape to focus light.

• Myopia: Nearsightedness.

• Hyperopia: Farsightedness.

• Astigmatism: Distorted vision due to irregular cornea/lens.

Autonomic Nervous System (ANS)

Divisions of the ANS

The ANS controls involuntary functions and is divided into sympathetic and parasympathetic divisions.

• Sympathetic: Prepares body for 'fight or flight'.

• Parasympathetic: Promotes 'rest and digest'.

ANS Control Centers

Control centers in the brain regulate autonomic functions.

• Hypothalamus: Integrates autonomic responses.

• Medulla Oblongata: Regulates cardiovascular and respiratory functions.

Neurons in the ANS

Preganglionic and postganglionic neurons transmit signals from the CNS to target organs.

Feature	Preganglionic Neuron	Postganglionic Neuron
Cell Body	CNS	Autonomic ganglion
Axon	CNS → ganglion	Ganglion → target organ
Neurotransmitter	Acetylcholine (ACh)	Sympathetic: norepinephrine; Parasympathetic: acetylcholine
Function	First leg of ANS signal	Second leg to effector

Sympathetic Effects

• Pupil: Dilates

• Salivary Glands: Inhibits secretion

• Heart Rate: Increases

• Blood Flow: Diverted from digestive organs, skin, kidneys

• Digestive Functions: Decreased

• Adrenal Medulla: Releases epinephrine and norepinephrine

Parasympathetic Effects

• Pupil: Constricts

• Salivary Glands: Stimulates secretion

• Heart Rate: Decreases

• Digestive Functions: Increased

• Urinary Tract: Promotes urination

Receptors in the ANS

• Adrenergic Receptors: Bind norepinephrine/epinephrine (sympathetic).

• Cholinergic Receptors: Bind acetylcholine (parasympathetic).

Additional info: Some details, such as specific cranial nerve numbers and advanced receptor subtypes, were inferred for completeness.