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Detailed Notes on Sensory and Motor Mechanisms I. Introduction II. Types of Sensory Receptors Sensory and Motor Mechanisms All stimuli represent forms of energy. Sensation involves converting energy into a change in the membrane potential of sensory receptors. Sensory Pathways...

Detailed Notes on Sensory and Motor Mechanisms I. Introduction II. Types of Sensory Receptors Sensory and Motor Mechanisms All stimuli represent forms of energy. Sensation involves converting energy into a change in the membrane potential of sensory receptors. Sensory Pathways Sensory pathways have five basic functions: ​ Sensory reception ​ Transduction ​ Transmission ​ Integration ​ Motor output Sensory Reception and Transduction Sensory reception is the detection of stimuli by sensory receptors. Plants and animals have sensory receptors that respond to environmental factors like light, gravity, temperature, and touch. These receptors transduce energy and send information to systems that trigger a response. - In plants, the message is often chemical messengers. - In animals, it results in an action potential and whole-body integration. Sensory receptors interact directly with stimuli, both inside and outside the body by absorbing energy directly from a stimulus. Sensory receptors are transducers: In animals, sensory transduction is the conversion of stimulus energy by the sensory receptor into a change in the membrane potential of a sensory receptor. This change in membrane potential is called a receptor potential. - It involves transducing absorbed energy into electrochemical energy. - Receptor potentials are graded potentials; their magnitude varies with the strength of the stimulus. Sensory Transduction Transmission After energy has been transduced into a receptor potential, some sensory cells generate the transmission of action potentials. In animals, sensory information travels along afferent nerve fibers of a sensory nerve to the brain via the spinal cord. In plants, information travels to the apoplast or phloem. Sensory Transmission Perception Perception (interpretation): Decoding sensation into useful information occurs in the brain of animals. - The brain’s construction of stimuli. - The brain distinguishes stimuli from different receptors based on the area in the brain where the action potentials arrive. Perception occurs in individual cells in plants. Classification of Sensory Receptors Based on energy transduced, sensory receptors fall into five categories: ​ Thermoreceptors ​ Pain receptors ​ Mechanoreceptors ​ Chemoreceptors ​ Electromagnetic receptors Animal Reception Classification of Receptor Type Receptors can be classified based on: Sensory Modality: Thing sensed. Receptor Mechanism: Two kinds of sensory transduction mechanisms: - **Metabotropic** - **Ionotropic** Location: - **Exteroceptors**: Outside (sound, light). - **Interoceptors**: Inside (pH, osmotic concentration). Classification by Location Interoceptors: Inside Visceroreceptors: Stimuli from viscera and blood vessels. - Detect changes in pH, osmotic pressure, body temperature, chemical composition of blood, and tissue stretch. Exteroceptors: Receive stimuli from the outside.  Touch, pressure, pain, and temperature. Most receptors of the sense organs (vision, hearing, equilibrium, taste, and smell). Classification by Mechanism Ionotropic: Stimulus opens ion channels. Very fast. Metabotropic: Stimulus causes a change in shape in the receptor that activates G protein, which activates an enzyme system. Slow. Auditory Receptors: Detection of Sound Vertebrate acoustico-lateralis system: In most terrestrial vertebrates, sensory organs for hearing and equilibrium are closely associated in the ear (the acoustico-lateralis system). The Process of Hearing Ionotropic reception: Outer ear: - Vibrating objects create percussion waves in the air of the auditory canal that cause the tympanic membrane to vibrate. Middle ear: - Vibrations in the tympanic membrane vibrate the ossicles. - Three auditory ossicles conduct and amplify vibrations to the internal ear: 1. **Malleus**: Attaches to the tympanic membrane. 2. **Incus**: Attaches malleus to stapes. 3. **Stapes**: Attaches incus to oval window, an opening in bone surrounding the internal ear. - Located in the middle ear. - Articulations between are the smallest synovial joints in the body. - Small skeletal muscles that insert on malleus and stapes contract to reduce the amount of vibration to protect the tympanic membrane. Inner ear: - The three bones of the middle ear transmit the vibrations to the oval window on the cochlea (on which the stapes rests). - These vibrations create pressure waves in the fluid in the cochlea that travel through the vestibular and tympanic canals. - Pressure waves travel down the vestibular canal around the apex and down the tympanic canal. - The wave causes the basilar membrane to vibrate, bending its hair cells in the Organ of Corti. The Organ of Corti Hair cells with bundles of hairs that project from them: The ear conveys information about: - **Volume**: The amplitude of the sound wave. - **Pitch**: The frequency of the sound wave. Different volumes create different numbers of action potentials. The cochlea can distinguish pitch because the basilar membrane is not uniform along its length. - Each region of the basilar membrane is tuned to a particular vibration frequency. This bending of hair cells depolarizes the membranes of mechanoreceptors and sends action potentials to the brain via the auditory nerve. Events Involved in Hearing ​ Sound waves arrive at the tympanic membrane. ​ Movement of the tympanic membrane causes displacement of the auditory ossicles. ​ Movement of the stapes at the oval window establishes pressure waves in the endolymph of the vestibular membrane.  ​ The pressure waves distort the basilar membrane on their way to the round window of the tympanic membrane. ​ Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane. ​ Information about the region and the intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII. Detection of Gravity and Body Orientation Ionotropic reception in animals. Gravity Response in Plants Gravitropism: Roots show positive gravitropism; shoots show negative gravitropism. Plants may detect gravity by the settling of statoliths, dense cytoplasmic components. - Statoliths settle to the lowest sides of root cap cells. - The signal from statoliths is not understood but may involve: - Acidification of the apoplast by activation of a H+ ATPase. - Calcium as a second messenger. Detailed Notes on Sensory Systems and Visual Reception Gravitropism in Plants Roots of Maize Bending Gravitropically Statoliths settle to the lowest sides of root cap cells. Auxins (plant hormone) migrate down the root. Calcium accumulates on the upper side. - Calcium inhibits growth. - Auxin stimulates growth. Vertebrate Acoustico-Lateralis System Detection of Gravity and Body Position Sensory organs for hearing and equilibrium are associated in the ear. Vestibular Organ detects equilibrium. - **Three Semicircular Canals** - Detect angular acceleration of the head. - Filled with fluid called **endolymph**. - Oriented at right angles to each other. - **Ampulla** at the base contains hair cells. - **Two Otolith Organs** - **Sacculus** and **Utriculus** detect linear movement and acceleration. Hair Cells in Vertebrates Structure Hair Bundle at the apical end consists of stereocilia of increasing height. - **Stereocilia**: Long microvilli, narrow at the apical end. - **Kinocilium**: A single true cilium. - Held rigid by actin filaments.  Function Change in acceleration and position moves endolymph against hair cells. Ion channels open or close depending on the direction of displacement. - **Toward Tallest Stereocilia**: Opens K+ ion channels, increases ion influx, depolarization, and neurotransmitter release. - **Away from Tallest Stereocilia**: Closes ion channels. Semicircular Canals and Head Movement Rotation of Head causes: Movement of endolymph. Movement of cupula to the side. Distortion of receptor processes. Movement in one direction stimulates hair cells; opposite direction inhibits hair cells. Stopping rotational movement stops endolymph movement and cupula returns to normal. Specific Canals Anterior Semicircular Duct: Responds to "yes" motion. Posterior Semicircular Duct: Responds to tilting head to the side. Three semicircular ducts lie in three rotational planes, each responding to one rotational movement. Equilibrium Detection Utricle and Saccule Contain granules called otoliths.  Allow perception of position relative to gravity or linear movement. Electromagnetic Receptors Detection of Electromagnetic Energy Includes light, electricity, and magnetism. Some animals, like snakes, have infrared receptors for detecting body heat. Many animals use Earth's magnetic field for migration. Visual Receptors in Animals Detection of Electromagnetic Energy Diverse organs for vision. Common evolutionary origin for light capture mechanism. Vertebrate Eye Cornea: Allows light entry, dense matrix of collagen fibers, no blood vessels. Lens: Held by suspensory ligaments connected to the ciliary body. Retina: Contains photoreceptors, supporting cells, neurons. Choroid: Contains nutrient-carrying blood vessels. Sclera: Dense connective tissue, stabilizes eye shape, insertion point for eye muscles. Fovea: Center of visual field, high density of cones, no rods. Optic Disc: Blind spot where ganglion cell axons pass through. Visual Reception and Transduction Retina Contains rods and cone photoreceptors and a network of neurons.  Photoreceptor Cells - **Rod Cells**: Night vision, contain rhodopsin. - **Cone Cells**: Color vision, high acuity. Rhodopsin: Visual pigment in rod cells, consists of opsin and retinal. Light Absorption and Signal Transduction Light induces conversion of 11 cis-retinal to trans-retinal. Trans-retinal activates rhodopsin, which activates transducin (G protein). Activated transducin activates cGMP phosphodiesterase. Decrease in cGMP closes cGMP gated ion channels. Na+ influx decreases, leading to photoreceptor hyperpolarization. In The Dark: The photoreceptor is depolarized by an influx of Na+ and Ca2+ through open channels gated by cGMP cGMP is made by membrane-bound guanylyl cyclase Transducin and phosphodiesterase are inactive Dark and Light Responses Dark Response: Rhodopsin inactive, Na+ channels open, rod depolarized, glutamate released, bipolar cell hyperpolarized/inhibited. Light Response: Rho dopsin active, Na+ channels closed, rod hyperpolarized, no glutamate released, bipolar cell depolarized. Detailed Notes on Light Detection in Plants and Color Vision in Humans Color Vision in Humans General Mechanism Rods and Cones: In the dark, rods and cones release the neurotransmitter glutamate into synapses with neurons, initiating an action potential that travels along the optic nerve. Optic Nerve Pathway: Optic nerves meet at the optic chiasm near the cerebral cortex. Sensations from the left visual field of both eyes are transmitted to the right side of the brain. Sensations from the right visual field are transmitted to the left side of the brain. Most ganglion cell axons lead eventually to the primary visual cortex in the cerebrum. Cones and Color Vision Types of Cones: Humans have three types of cones, each containing a different type of photopsin: Red Cones: About 74% of cones. Green Cones: About 10% of cones. Blue Cones: About 16% of cones. Photopsin: Each type of photopsin contains different amino acids that determine the wavelengths of light absorbed. These differences allow for the perception of different colors. Color Vision: Based on the ratio of three classes of photoreceptors sensitive to different wavelengths of light. Retinal circuitry integrates color contrasts based on red-green and blue-yellow contrasts. Comparative Color Vision Vertebrates: Most fish, amphibians, and reptiles, including birds, have very good color vision. Mammals: Humans and other primates are among the minority of mammals with the ability to see color well. Nocturnal Mammals: Usually have a high proportion of rods in the retina, which are more sensitive to low light levels. Light Detection in Plants Importance Light cues many key events in plant growth and development. Plants detect not only the presence of light but also its direction, intensity, and wavelength (color). Blue-Light Photoreceptors Types: Various blue-light photoreceptors, such as cryptochromes (CRYs). Functions: Control elongation, stomatal opening, and phototropism. Blue light suppresses extension growth, resulting in shorter and thicker plants. Cryptochromes (CRY): Found in plants, animals, archaea, bacteria, and algae.  Play roles in growth, development, stress tolerance, and establishing the circadian clock. Low blue light turns on CRY genes that promote growth. Phytochromes Nature: Photoreversible pigments that undergo changes in shape/conformation when exposed to light of different wavelengths. Forms: Pr (inactive form): Converted to the active form (Pfr) when exposed to red light. Pfr (active form): Enters the nucleus and regulates gene expression. Converted back to Pr when exposed to far-red light. Functions: Control physiological processes such as seed germination, seedling development, induction of flowering, and shade avoidance. Act as light-driven molecular switches. Photoperiodism and Control of Flowering Photoperiodism: Some processes, including flowering in many species, require a certain photoperiod. Short-Day Plants: Flower when a light period is shorter than a critical length. Long-Day Plants: Flower when a light period is longer than a certain number of hours. Day-Neutral Plants: Flowering is controlled by plant maturity, not photoperiod. Critical Night Length: Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness. Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness. Light Interruption:  Red light can interrupt the nighttime portion of the photoperiod. A flash of red light followed by a flash of far-red light does not disrupt night length. Phytochrome is the pigment that receives red light, as shown by action spectra and photoreversibility experiments. Conversion and Responses Red Light (660 nm): Converts Pr to Pfr. Far-Red Light (730 nm): Converts Pfr to Pr. Responses: Red light increases germination, while far-red light inhibits germination. Slow conversion of Pfr to Pr occurs in darkness in some plants. Enzymatic destruction of Pfr occurs at night via ubiquitin attachment.