Chapter 10

sensory transduction

conversion of stimulus energy into information that can be processed by the nervous system, ion channels or second messengers initiate membrane potential change, threshold: minimum stimulus required to trigger an action potential

taste

-closely linked to olfaction -combination of five senses: sweet, sour, salty, bitter, umami -taste buds: composed of taste cells joined near the apical surface with tight junctions -taste transduction: each taste cell senses only one type of ligand 1. Ligands activate the taste cell 2. Various intracellular pathways are activated 3. Ca signal in the cytoplasm triggers exocytosis or ATP formation 4. Neurotransmitter or ATP is released 5. Primary sensory neuron fires and action potentials are sent to the brain

the pupil

-light enters the eye -amount of light that reaches photoreceptors is modulated by changes in the size of the pupil -light is focused by changes in the shape of the lens -pupillary reflexes is activated by light hitting the retina, standard part of neurological examination -create what is known as depth of field

olfactory system

-olfactory bulb: extension of the forebrain that receives input from the primary olfactory neurons -the human olfactory system consists of primary sensory neurons (olfactory sensory neurons) whose axons form the olfactory nerve (cranial nerve I) -secondary and higher order neurons project from the olfactory bulb through the olfactory tract to the olfactory cortex; bypasses the thalamus -olfactory pathways lead to the amygdala and hippocampus -olfactory sensory neurons in humans are concentrated in patch of olfactory epithelium high in the nasal cavity -cilia are embedded in a layer of mucus and odorant molecules must first dissolve in and penetrate the mucus before they can bind to an odorant receptor protein In summary: -The olfactory epithelium lies high within the nasal cavity and its olfactory neurons project to the olfactory bulb. Sensory input at the receptors is carried through the olfactory cortex to the cerebral cortex and the limbic system, involved with emotion and memory. 1. Olfactory neurons in olfactory epithelium (live about 2 months) 2. Cranial nerve I 3. Olfactory neurons synpase with secondary sensory neurons in the olfactory bulb 4. Olfactory bulb 5. Olfactory tract 6. Olfactory cortex 7. Cerebral cortex and limbic system

the lens

-optics describes light behavior and properties -light entering they eye is refracted twice: first when it passes through the cornea, and again when it passes through the lens -the lens is capable of changing its shape to focus light -when light passes from one media to another it is influenced by two factors: 1. the difference in density of the two media 2. the angle at which the light rays meet the surface of the medium into which it is passing -single point where the rays converge is the focal point -the distance from the center of a lens to its focal point is known as the focal length -the process by which the eye adjusts the shape of the lens to keep objects in focus is known as accommodation -closest point at which it can focus an object is known as the near point of accommodation -ciliary muscle ring: ring of smooth muscle that surrounds the lens and is attached to it by the inelastic ligaments called zonules -myopia: near-sightedness -hyperopia: far-sightedness

the retina

-photoreceptors of the retina convert light energy into electrical signals (phototransduction) -five types of neurons in the retinal layers: photoreceptors, bipolar cells, ganglion cells, amacrine cells, horizontal cells -photoreceptors are the neurons that convert light energy into electrical signals; two main types: rods and cones -fovea: small region of the retina free of neurons and blood vessels that would block light reception so the fovea recessive light directly with minimal scattering, area of most acute/sharpest vision -sensory information about light passes from the photoreceptors to bipolar neurons to ganglion cells; axons of ganglion cells form optic nerve which leaves they eye at the optic disk -optic disk has no photoreceptors=blind spot

vision

-the process through which light reflected from objects in our environment is translated into a mental image -can be divided into three steps: 1. Light enters the eye and the lens focuses the light on the retina. 2. Photoreceptors of the retina transduce light energy into an electrical signal. 3. Neural pathways from retina to brain process electrical signals into visual images.

sensory pathways in the brain

1. Olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex. 2. Most sensory pathways project to the thalamus. The thalamus modifies and relays information to cortical centers. 3. Equilibrium pathways project primarily to the cerebellum. -preceptual threshold: the level of stimulus intensity necessary for you to be aware of a particular sensation.

types of sensory receptors

Chemoreceptors -respond to chemical ligands that bind to the receptor (taste and smell) -stimuli: oxygen, pH, various organic molecules such as glucose Mechanoreceptors -respond to various forms of mechanical energy including pressure, vibration, gravity, acceleration, and sound -stimuli: pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, sound Photoreceptors -respond to light -stimuli: photons of light Thermoreceptors -respond to temperature -stimuli: varying degrees of heat

coding and processing stimulus properties

Four properties to distinguish: 1. modality 2. location 3. intensity 4. duration 1. Sensory modality: indicated by which sensory neurons are activated and by where the pathways of the activated neurons terminate in the brain, each modality can be subdivided into qualities 2. Location of the stimulus: coded according to which receptive fields are activated, ex: touch receptors in the hand project to a specific area of the cerebral cortex 3. Intensity of the stimulus: coded in two types of information- the number of receptors activated and the frequency of action potentials coming from those receptors (frequency coding) 4. Duration of the stimulus: coded by the duration of action potentials in the sensory neuron, if a stimulus persists some receptors adapt or cease to respond -tonic receptors: slowly adapting receptors that fire rapidly when first activated then slow and maintain their firing as long as the stimulus is present -phasic receptors: rapidly adapting receptors that fire when they first receive a stimulus but cease firing if the strength of the stimulus remains constant

the ear and hearing

Sound transmission through the ear 1. Sound waves strike the tympanic membrane and become vibrations. 2. The sound wave energy is transferred to the three bones of the middle ear which vibrate. 3. The stapes is attached to the membrane of the oval window. Vibrations of the oval window create fluid waves within the cochlea. 4. The fluid waves push on the flexible membranes of the cochlear duct. Hair cells bend and ion channels open, creating an electrical signal that alters neurotransmitter release. 5. Neurotransmitter release onto sensory neurons creates action potentials that travel through the cochlear nerve to the brain. 6. Energy from the waves transfers across the cochlear duct into the tympanic duct and is dissipated back into the middle ear at the round window.

two types of photoreceptors

Rods: function well in low light and are used in night vision when objects are seen in black and white rather than in color -outnumber cones by a 20:1 ratio except in the fovea Cones: responsible for high-acuity vision and color vision during the day time when light levels are higher -fovea has a very high density of cones Rods and cones have the same basic structure: 1. an outer segment whose tip touches the pigment epithelium of retina 2. an inner segment that contains the cell nucleus and organelles for ATP and protein synthesis 3. a basal segment with a synaptic terminal that releases glutamate onto bipolar cells

proprioception

the awareness of body movement and position in space, mediated by muscle and joint sensory receptors called proprioceptors and may be either conscious or unconscious