vphy week 5 part 1

Differentiate between metabotropic (GPCR) and ionotropic receptors regarding their mechanism of action.

Ionotropic receptors are integral ion channels that open directly upon ligand binding, causing immediate changes in membrane potential. In contrast, metabotropic GPCRs are not ion channels themselves; they activate intracellular G proteins which then indirectly modulate ion channels or cellular excitability via second messenger cascades. \ Thus, GPCRs have a slower but often more prolonged and varied effect.

Explain the critical sequence of events that lead to the initiation of an action potential (AP) from a resting membrane potential.

From a resting membrane potential, excitatory postsynaptic potentials (EPSPs) caused by Na+ or Ca2+ influx depolarize the membrane. If this depolarization reaches a specific threshold voltage, voltage-gated Na+ channels open rapidly, leading to a massive influx of Na+ ions. This rapid positive feedback loop further depolarizes the membrane, driving the fast upstroke of the action potential.

Describe the 'all-or-none' principle of action potentials and how stimulus intensity is encoded.

The 'all-or-none' principle states that if the membrane depolarization reaches the threshold, an action potential will be generated with a consistent, maximal amplitude, regardless of the strength of the suprathreshold stimulus. Stimulus intensity is not encoded by the size of the action potential, but rather by the frequency (firing rate) of action potentials generated and the number of neurons activated (population activity).

What are temporal and spatial summation, and how do they contribute to synaptic integration?

Temporal summation occurs when a single presynaptic neuron fires multiple times in quick succession, causing successive postsynaptic potentials (PSPs) to overlap and summate at the postsynaptic membrane. Spatial summation happens when multiple different presynaptic neurons fire simultaneously, causing their PSPs to summate at the postsynaptic neuron. Both mechanisms allow a neuron to integrate multiple subthreshold inputs to reach the action potential threshold.

Distinguish between afferent and efferent neurons and ascending vs. descending pathways.

Afferent neurons carry sensory information from the periphery toward the Central Nervous System (CNS), often via ascending pathways towards the brain/cortex. Efferent neurons carry motor commands away from the CNS to muscles or glands, primarily via descending pathways from the brain/spinal cord to the periphery.

List the four principal somatosensory modalities in order from fastest to slowest pathway speed and provide a reason for the speed in the fastest modality.

The order from fastest to slowest is: Proprioception > Temperature > Touch > Pain. Proprioception is typically the fastest because it often relies on large-diameter, heavily myelinated axons, which allow for very rapid conduction of action potentials to the CNS, crucial for immediate feedback on body position and movement.

Explain the relationship between receptive field size and two-point discrimination accuracy, using examples.

Receptive field size is inversely related to two-point discrimination accuracy. Smaller receptive fields allow for higher spatial resolution and more accurate two-point discrimination, meaning two closely spaced stimuli can be perceived as distinct. For example, fingertips and the tongue have small receptive fields and excellent two-point discrimination. Conversely, areas with large receptive fields, such as the back, have poor two-point discrimination because multiple stimuli within a single large field are perceived as one.

Outline the basic components of GPCR signaling, detailing how second messengers can modulate ion channels to produce EPSP or IPSP.

GPCR signaling begins with a ligand binding to a receptor, causing G protein activation. The activated G protein subunits then interact with effector enzymes or ion channels, often leading to the production of second messengers (e.g., cAMP, IP3, DAG). These second messengers can then directly or indirectly modulate the opening or closing of ion channels. For instance, if the modulation leads to the opening of Na+ or Ca2+ channels (influx), an EPSP (depolarization) results. If it leads to the opening of K+ channels (efflux) or Cl- channels (influx), an IPSP (hyperpolarization) occurs.

Briefly describe the significance of the retina as an extension of the CNS in studying neural integration and the effects of genetic mutations.

The retina is considered an accessible and well-organized extension of the CNS, making it an excellent model system for studying complex neuronal interactions and integration. Its structured circuitry and distinct cell types allow researchers to investigate how signals are processed and transformed. Furthermore, its direct accessibility and relevance to visual function make it ideal for studying the impact of genetic mutations on neuronal development, function, and eye health, with implications for a broader understanding of CNS disorders.

In an action potential, what happens during the repolarization phase, and what typically causes afterhyperpolarization?

During repolarization, voltage-gated Na+ channels inactivate, and voltage-gated K+ channels open, allowing K+ ions to flow out of the cell. This efflux of positive charge rapidly returns the membrane potential towards its resting state. Afterhyperpolarization, or undershoot, often occurs because the K+ channels remain open for a brief period after the membrane potential has reached rest, causing the membrane to become even more negative than the resting potential before returning to normal.