chemical signals ear cornea

Chemical Signaling in Animals

Class Goals

  • Understanding of chemical signaling mechanisms in animals.

Lecture Goals to Understand

  • Sensory receptor cells transduce stimuli to changes in membrane potential.

    • Action potentials (APs) are sent to the brain for processing and integration.

    • Hearing:

    • Based on sensory receptor cells that move in response to sound waves of a particular frequency.

    • Vision:

    • Based on sensory receptor cells containing a light-absorbing pigment that changes conformation upon light absorption.

    • Taste and Smell:

    • Involve membrane proteins acting as ion channels or receptors for specific molecules.

  • Animals respond to sensory stimuli through movement.

    • Movement involves antagonistic muscle groups acting on a skeleton.

What Happens When an Action Potential (AP) Arrives?

  • Neurotransmitters are molecules transmitting information:

    • From one neuron to another neuron.

    • From a neuron to a target cell in a muscle or gland.

  • Otto Loewi's Experiments:

    • Investigated chemical transmission of signals from nerve to muscle using frog heart experiments.

The Synapse

  • Definition: The interface between two neurons.

  • Structure:

    • Presynaptic Neuron: Sending cell.

    • Postsynaptic Neuron: Receiving cell.

    • Contains synaptic vesicles within the axon, storing neurotransmitters.

Synaptic Transmission Process

  1. Action Potential Arrival:

    • Triggers release of neurotransmitter.

    • Ions Involved: Na⁺, K⁺, and Ca²⁺.

  2. Voltage-Gated Ca²⁺ Channels Open:

    • Increase in intracellular calcium concentration.

  3. Release of Neurotransmitter:

    • Synaptic vesicles fuse with the presynaptic membrane.

  4. Effect on Postsynaptic Cell:

    • Neurotransmitter triggers change in postsynaptic cell potential.

  5. Termination of Action:

    • Neurotransmitter is either broken down or released.

Qualification of a Molecule as a Neurotransmitter

  • Must:

    • Be present at the synapse and released in response to an AP.

    • Bind to a receptor on a postsynaptic cell.

    • Be taken up or degraded afterward.

Functions of Neurotransmitters

  • Act as ligands: molecules that bind to specific sites on a receptor.

  • Binding to ligand-gated ion channels allows ions to flow along an electrochemical gradient:

    • Converts chemical signals to electrical signals (changes in postsynaptic cell potential).

  • Other neurotransmitters activate enzymes to produce second messengers impacting gene expression, enzyme activity, or membrane potential.

Postsynaptic Potentials (PSPs)

  • Two types:

    • Excitatory Postsynaptic Potentials (EPSPs): Depolarize membrane; increase likelihood of an AP.

    • Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize membrane; decrease likelihood of an AP.

  • Synapses can also be modulatory, modifying a neuron's response to EPSPs or IPSPs.

Graded PSPs

  • EPSPs and IPSPs are graded in size, not all-or-nothing events.

  • Neurons make numerous synapses; combined EPSPs and IPSPs result in short-lived surges of charge in postsynaptic cells.

Summation and Threshold

  • Summation: The additive nature of EPSPs and IPSPs determines whether an AP occurs in the postsynaptic cell.

  • Important structures:

    • Axon Hillock: Contains sodium channels triggering APs.

    • If depolarization reaches threshold, AP is initiated and propagated down the axon.

Sensory Organs and Information Processing

  • Ability to sense environmental changes involves:

    1. Transduction: Converting external stimulus to internal signal (AP).

    2. Amplification: Increasing the intensity of the signal.

    3. Transmission: Sending the signal to the central nervous system (CNS).

Sensory Receptor Cells

  • Sensory neurons or specialized receptor cells detect specific stimuli and make synapses with sensory neurons.

  • Types:

    • Nociceptors: Sense harmful stimuli.

    • Thermoreceptors: Detect temperature changes.

    • Mechanoreceptors: Respond to pressure distortion.

    • Chemoreceptors: Detect specific molecules.

    • Photoreceptors: Respond to light wavelengths.

    • Electroreceptors: Sense electrical fields.

  • Receptor specificity ensures each sensory neuron signals a particular brain region.

Sensory Transduction Explained

  • Sensory receptors change stimuli into electrical signals via changes in membrane potential.

    • Degree of depolarization/hyperpolarization correlates to stimulus intensity (e.g., loudness).

    • Increased intensity leads to changes in AP firing rates directed to the brain.

Anatomy of the Eye

  • Outermost Layer: Sclera (tough white tissue).

    • Cornea: Clear tissue at the front of the sclera.

    • Iris: Muscle controlling light entering the eye through the pupil.

  • Light path: Through cornea and pupil to lens, focusing it on the retina.

Retina Structure

  • Comprises three layers:

    • Photoreceptors: Light-sensitive cells at the back.

    • Bipolar Cells: Connect photoreceptors and ganglion cells.

    • Ganglion Cells: Front layer, axons form the optic nerve.

Photoreceptors: Rods and Cones

  • Rods: Sensitive to dim light, not color.

  • Cones: Less sensitive, respond to different light wavelengths (colors).

    • Most rods are located across the retina with a small fovea area having only cones.

Rhodopsin and Vision

  • Rods and Cones Structure:

    • Contain opsin protein and retinal pigment.

    • Absorption of light changes shape of retinal, activating opsin, sending action potentials to the brain.

Light Detection Mechanism

  • Inverts existing membrane potentials and neurotransmitter release in rod cells:

    • Depolarizes in darkness, hypopolarizes in light exposure.

Signal Transmission in Photoreceptors

  1. Activation of Rhodopsin: Light induces retinal shape change.

  2. Activation Pathway: Rhodopsin → Transducin → Phosphodiesterase (PDE).

  3. Effect of PDE: Breaks down cGMP, closing cGMP-gated Na⁺ channels.

  4. Final Outcome: Decrease in Na⁺ influx hyperpolarizes the cell, reducing neurotransmitter release.

Insights into Color Detection

  • Three cone types exist with distinct opsins for blue, green, or red light.

  • Richer color perception in species with additional opsin types, linked to environmental adaptation.

Hearing Process

  • Hearing detects sound waves (pressure changes in air/water).

    • Frequency: Number of pressure waves per second, perceived as pitch.

  • Mechanisms are consistently based on mechanoreceptor cells responding to pressure.

Classification of Hearing Responses

  • Sound-receptor Cells: Depolarize in response to sound stimulus, and respond more strongly to loud sounds.

    • Known relationship between sound level and action potentials per second.

Structure of the Mammalian Ear

  • Tympanic Membrane: Size ratio with oval window amplifies sound.

  • Cochlea: Contains membranes dividing it into chambers with hair cells detecting frequency.

Cochlea Functionality

  • Basilar membrane stiffness varies, causing it to vibrate maximally at specific spots based on sound frequency, influencing hair cell response and brain interpretation of pitch.

Hair Cells Structure

  • Stereocilia: Stiff outgrowths for pressure detection.

  • Lower or equal variable height, kinocilium presence, extending into fluid-filled chamber.

Signal Transduction Process in Hair Cells

  • Bending Directions:

    • Toward kinocilium: Ion channels open, leading to depolarization.

    • Away from kinocilium: Channels close, causing hyperpolarization.

  • Uniqueness in K⁺ ion behavior: Influx leads to depolarization; closure leads to hyperpolarization, transmitting electrical signals via the auditory nerve to the brain.

Chemical Signals in Animals

  • Chemical Signals: At least six primary types used by animals.

  • Hormones: Present in tiny concentrations but impactful across body,

    • Critical for embryo development, sexual maturation, response to environmental change, and homeostasis.

  • Hormonal signaling tightly regulated by nervous inputs and other hormones.

Cell-to-Cell Signaling

  • Long-lasting effects of chemical signals versus short-term electrical signals.

  • Combination of electrical and chemical signaling aids coordination of bodily activities.

Major Categories of Chemical Signals

  • Six classes of signals, not strictly different classes; a single messenger can function in multiple categories.

The Endocrine System

  • Secretes hormones through specialized glands directly into the bloodstream.

  • Hormones circulate and have long-lasting effects on distant targets, often responding to external/internal conditions.

Insulin and Blood Glucose Regulation

  • Examples of feedback loop regulating blood glucose step-by-step based on nutrient intake and hormonal response (e.g., increased insulin production by pancreas upon elevated glucose levels).

Hormonal Signaling Pathways

  • Hormones operate through three pathways, often controlled by negative feedback.

  • Electrical signals modulate endocrine releases, influencing long-acting responses to conditions.

Hormone Specificity

  • Hormones circulate systemically yet only affect specific target cells due to receptor specificity, preventing non-specific effects.

Hormones and Homeostasis

  • Hormonal messages maintain homeostasis, particularly managing energy reserves for periods of food scarcity, primarily stored as triglycerides in adipose tissue.

Research Study Summary: Mice and Leptin

  • Study of ob and db mutations revealing significant differences in feeding behavior and metabolism linked to hormonal signaling

    • Observation: Obese (ob/ob) and diabetic (db/db) mice show distinct eating patterns when parabiosed with lean mice.

Conclusions from Parabiosis Experiment

  • Satiation Hormone Hypothesis:

    • Leptin, encoded by the ob gene, reduces appetite in response to increased fat stores.

    • db/db mice lack receptors for leptin, while ob/ob mice do not produce it.