AFFERENT PNS: HLSC exam 2

what do photoreceptors detect and what is the modality of the detection

photoreceptors detect sensation information about vision and this is done through photons of light

what do chemoreceptors detect and by what modality? (five different things)

• taste: via chemicals dissolved in saliva

• smell: via chemicals dissolved in mucus

• pain: via chemicals in extracellular fluid

• blood oxygen: via oxygen dissolved in plasma

• blood pH: via free hydrogen ions in plasma

what do thermoreceptors detect and by what modality

• warm receptors: detect warmth, specifically when there is an increase in temperature between 30 and 43 degrees

• cold receptors: detect cold, specifically when there is a decrease in temperature between 35 and 20 degrees

what do mechanoreceptors detect and how? (three)

• baroreceptors: detect blood pressure by the stretch of specific blood vessel walls

• osmoreceptors: detect osmolarity of extracellular fluid

• hair cells: detect sound via sound waves, and balance and equilibrium via acceleration

what is transduction and how do receptors work?

transduction is the conversion of stimulus energy (modality) into electrical energy by the receptor. receptors are specific to a modality, but also respond in lesser degrees to other modalities.

• when a stimulus is detected by a receptor, a graded potential called a receptor potential generates.

• if axon hillock reaches threshold, an action potential in the afferent neuron will be generated. (afferent neuron may be attached to or apart of sensory receptor or not)

what happens with continuous stimulus on sensory receptors?

not all receptors send the same number of action potentials to the CNS.

• adaptation: refers to a decrease in the amplitude of the receptor potential and corresponding decrease in the frequency of action potentials over time. adaptation decreases the perception of the stimulus by the receptor

• phasic receptors such as for smell and touch, adapt quickly

• tonic receptors such as for proprioceptors and muscle stretch receptors, do not adapt or adapt very slowly.

what is the pathway for sensory information to the brain

• travels along a discrete somatosensory pathway (labelled lines) each pathway is specific for one modality

1. from periphery, action potentials travel along a first-order sensory neuron.

2. this then synapses with a second-order sensory neuron that may cross over or desecrate the midline either in spinal cord or medulla. this signal goes to the thalamus for processing.

3. a third-order neuron projects to a particular part of the somatosensory cortex.

what is a sensory unit and its receptive field?

• the sensory unit refers to a single afferent neuron and all of its associated receptors. within one unit, all receptors are specific for the same modality.

• the receptive field of a unit refers to the physical region over which stimuli can be detected by that particular sensory unit.

what effects sensory coding

sensory coding occurs according to stimulus type, intensity, and location.

• each receptor specific for one modality, but combination of info from various receptors and receptor pathways that are stimulated at any given time that is integrated by CNS and perceived

• stimulus intensity is coded in terms of frequency of action potentials (frequency coding) and the number of receptors activated or recruited (either on same afferent neuron or on multiple afferents aka population coding)

• stimulus location is coded in terms of size and degree of overlap of the receptive field. the smaller the receptive field, the greater the acuity (sharpness). lateral inhibition can increase acuity if there are overlapping receptive fields

what are four kinds of somatosensory receptors

1. mechanoreceptors: some are rapidly adapting such as the Pacinian corpuscle, Meissner’s corpuscle, and hair follicle receptors. some are slowly adapting such as free nerve endings, Merkel’s disks, and Ruffini’s endings.

2. thermoreceptors

3. nociceptors (of skin): all free nerve endings and respond to mechanical, thermal, or chemical stimulu or combos of all three (polymodal)

4. proprioceptors (of muscles and joints): detect positions of the body in space. they are continuously active and DO NOT ADAPT. examples are muscle spindles, Golgi tendon organs, and free nerve endings

what is the dorsal column-medial lemniscal pathway

pathway that transmits information about both proprioception and the somesthetic sensation of touch and pressure to the CNS. the pathway decussates (crosses over) to the contralateral side at the medulla oblongata

what is the spinothalamic tract

transmits somethetic info about pain and temp from thermoreceptors and nociceptors and decussates at level of spinal cord

which receptors detect pain and what is the pathway

pain is complex due to autonomic and emotional components, and detection is situationally dependent and influenced by previous experiences

• nociceptors detect stimuli that are potentially damaging to tissue

• pain stimuli follows the spinothalamic tract in to the cerebral cortex

• non specific pathways also synapse on the reticular formation, hypothalamus, and limbic system

• there are also behavioural and emotional responses to pain when stimuli reaches cerebral cortex

what is referred pain

referred pain is he sensation of pain that is felt at the surface of the body that originates from internal organs (visceral pain)

• it is perceived to arise from a different location than the actual origin this is because of afferent neurons sharing the same second-order neuron

• body parts that experience this are the heart, the liver and gall bladder, appendix, colon, uterers, kidneys, stomach, and esophagus

what is the gate control theory of pain

describes the inhibitory pathways involved in the transmission of pain

modulated pain transmission ( presynaptic inhibition ) involves a modification of the signal by inhibitory interneurons that are excited by collaterals of sensory afferents responding to mechanoreceptors in the skin resulting in a decrease in pain signal

• described by a “gate” at the dorsal horn of spinal cord that acts as a gatekeeper for pain signals

• examples that support this theory include 1. the effectiveness of massage therapy and 2. the soothing nature of rubbing a bumped knee or a stubbed toe

how does presynaptic inhibition suppress pain

• the brain has a built in pain suppressing analgesic system:

• when nuclei in the periaquaductal grey matter or reticular foramen are stimulated, endogenous opiate neurotransmitters are released and attach to opiate receptors and prevent the release of the neurotransmitter substance.

• since less pain inducing neurotransmitter is released, less pain is perceived

what are the three tissue layers of the eye

1. sclera: outer layer of connective tissue that forms the visible white part of the eye and the anterior transparent surface (cornea)

2. choroid: contains the blood vessels. anterior portion forms ciliary body and iris

3. retina: inner pigmented layer that contains photoreceptors

what are the two fluid filled cavities of the eye and what fluid do they contain

• posterior cavity: contains vitreous humour

• anterior cavity: contains aqueous humour

stimulation of which iris muscles controls the amount of light that can enter the eye through the pupil? and which division of the nervous system controls the muscles?

stimulation of the circular or constrictor muscles and the radial muscles control the amount of light that enters the eye.

• the autonomic nervous system controls controls these muscles

• in bright light or for near vision, the parasympathetic nervous system stimulates the circular or constrictor muscles of the iris to reduce the size of the pupil so that less light can enter.

• in dim light or far vision, the sympathetic nervous system stimulates the radial muscles of the iris to increase the size of the pupil so that more light can enter.

what is accommodation in terms of the eye and it’s association with near vision

accommodation refers to the eyes ability to increase the curvature of the lens to focus light on the retina. this is important in near vision, which also includes contraction of the ciliary muscle by the parasympathetic nervous system

• far vision does not normally required accommodation, thus the ciliary muscle remains relaxed

what does the term emmetropia mean

emmetropia refers to the ability to focus light on the retina for both near and far vision

• length of eyeball or strength of lens can prevent the focusing of light on the retina for near or far vision resulting in common vision disorders known as hyperopia (farsightedness) which requires convex lenses, or

• myopia (nearsightedness) which requires concave lenses

what cell layers does light travel through when it is focused on the retina

1. ganglion cells make up inner layer

2. bipolar neurons make up middle layer

3. photoreceptors (rods and cones) make up outer layer

• in lateral layers (between the three different cell types listed), amacrine cells are between the inner and middle layers (ganglion and bipolar), and horizontal cells are between the outer and middle layers (bipolar and photoreceptors).

• the axons of ganglion cells form the optic nerve (CNII)

explain phototransduction

• photoreceptors in retina do phototransduction

• starts by light stimulating photopigments (conformational change) which inhibits the release of neurotransmitters from the synaptic terminal of receptor

• the decreased release of inhibitory neurotransmitters results in a change in the graded potential of the bipolar cells

• if this change is of significant magnitude, an action potential in the corresponding ganglions cell is generated

differentiate between the “macula lutea” and the “fovea”

• macula lutea: the larger, central area of retina responsible for sharp , detailed, central vision

• fovea: smaller, pit/depression in the centre of the macula lutea where visual acuity is highest has an even greater concentration of (only) cones

differentiate between cones and rods photoreceptors

• cones: are excited b y bright daylight and are responsible for colour vision. cones have high acuity and synapse with ganglion cells in a ratio close to 1:1 (low degree of convergence)

• rods: are highly sensitive and excited by small amounts of light (night vision). rods have lower acuity than cones due to a higher degree of convergence

what is the photopigment of rods and how does it change according to light vs dark adaptation

• photopigment of rods is called rhodopsin

• moving into a dark room from light, eyes switch from cone vision to rod vision. rhodopsin goes from maximally activated to being enzymatically returned to an inactive conformation before it is able to absorb light again

• in light adaptation, the rhodopsin that was in its light sensitive inactive conformation in the dark will initially maximally absorb the light, resulting in a bleaching of rods (perception of being blinded by light). when all rods become saturated with light, rhodopsin is in active conformation can’t absorb any more light, and visual system begins to rely on cone vision for light info, and pupils constrict

where do nerve impulses travel along visual pathway

• nerve impulses that originate from the medial (nasal) half of each eye cross over at the optic chiasm

• nerve impulses that originate from the lateral (temporal) half of each eye do not cross over, but proceed ipsilaterally to the cerebral cortex

what are the three fundamental characteristics of sound waves

1. pitch or tone: determined by the frequency of the sound waves and is measured in cycles/second (hertz).

2. intensity or loudness: determined by the amplitude of the sound waves and is measured in decibels.

3. timbre (quality of sound that distinguishes it from another sound): depends on overtones that add richness and help us distinguish one voice from another and locate sound.

list the order of the small bones of the middle ear in order from most outer to most inner

• MOST OUTER: malleus (next to ear drum)

• MIDDLE: incus

• MOST INNER: stapes

what route do sound waves travel

• through the external auditory meatus of the external ear to the tympanic membrane

• tympanic membrane then vibrates at the same pitch and amplitude as the incoming sound waves

• this causes the ossicles (three ear bones: malleus, incus, stapes) to move. this amplifies sound so that the oval window of the cochlea vibrates the fluid that is within the cochlea

what are the three fluid filled chambers of the cochlea

1. scala vestibuli: filled with perilymph (resembles extracellular fluid: rich in Na+) to conduct sound and pressure waves ***in bony labyrinth***

2. scala tympani: filled with perilymph

3. cochlear duct: filled with endolymph (resembles intracellular fluid: rich in K+) to generate electrical signals ***in membrane labyrinth***

where and what are the actual auditory receptors

they are hair cells of the organ of Corti in the cochlear duct. these hair cells sit on the basilar membrane and can bend in relation to the overlapping tectorial membrane

what happens when the hair cells of the organ of Corti are bent

• mechanically gated cation channels open resulting in depolarization of the cell and formation of a graded potential

• this can translate into an action potential in the cochlear division of the vestibulocochlear nerve (CN VII)

• the info travels to the medial geniculate nucleus of the thalamus (main relay centre for auditory information) and then to the auditory cortex of the temporal lobe

what does intensity coding for loudness involve

• occurs in terms of the amplitude of the wave

• coded for in terms of the degree of deflection and opening of the ion channels in the sterocilia (hair like non motile projections)

what does frequency coding for pitch involve

• coded for in terms of the location of the deflection on the basilar membrane

where are the receptors for equilibrium and what do they detect

• located in the inner ear in the vestibular apparatus

• detect changes in motion and the position of the head

• three kinds:

• 1. semicircular canals: detect rotational or angular acceleration or deceleration of the head

• 2. utricle: and 3. saccule: are otolith organs that provide info about the position of the head relative to gravity and the rate of linear motion

how does sound transduction by stereocilia in hair cells work in resting stage vs bending towards vs bending away from tallest sterocillium ( kinocilium ) (located in a gelatinous cupola above ampulla)

• resting: partially depolarized state due to some mechanically gated K+ channels are open, allowing K+ to enter hair cells. also allows small amounts of Ca+ to enter cell through voltage gated channels which results in the release of of small amounts of neurotransmitter for communication with afferent neuron. this causes a low frequency of action potentials evident in afferent neuron under resting conditions

• stereocilia bend in direction of tallest sterocillium, and more K+ channels are opened and more K+ enters cell thus greater depolarization. this means more Ca+ also enters cell thus more neurotransmitter and a higher frequency of action potentials

• when stereocilia bend away from tallest sterocillium, more K+ channels close compared to resting state, less K+ enters, causing hyperpolarization. very small amounts of Ca+ enter, very little neurotransmitter is released at very low frequency of action potentials in afferent neuron.

what kind of head movement do the anterior, posterior, and lateral canals detect

1. anteior canal detects movement of the head up or down (ex. nodding yes)

2. posterior canal detects movement of the head from a vertical to a horizontal position (ex. touching ear to shoulder)

3. lateral canal detects movement of the head from side to side (ex. shaking head no)

• axons from these hair cells join to form the vestibular branch of the vestibulocochlear nerve (CNVIII)

where are the olfactory receptors located and what other cells are present in the olfactory epithelium

• located in a small patch of tissue in the roof of the nasal cavity

• the axons project brought the cribiform plate of the ethmoid bone to form the olfactory nerve (CN1)

• also found in the olfactory epithelium are supporting cells: to secrete mucus and basal cells: are the precursor cells for new receptor cells

how can an odourant be detected

• odourant must be volatile and soluble in water

• approx. 1000 diff types of olfactory receptors, therefore various parts of an odour are dissected and stimulate particular receptors

• the more intense an odour is, therefore various parts more action potentials are produced

• these action potentials are generated through a G-protein signalling cascade

what is the pathway to the brain for processing odours

• initial processing of odours at the glomeruli within the olfactory bulb (humans can distinguish ~10,000 diff odours)

• information then sent to either the limbic system ( amygdala and hippocampus ) or to the primary olfactory cortex (temporal lobe and partly frontal lobe) for interpretation

• second order neurons in olfactory pathway are called mitral cells which form the olfactory tract

where are taste buds found

• primarily found on the tongue, but some are found in pharynx

• taste receptor cells are spindle shaped and surround a central taste pore and detect dissolved chemicals

what are the different tastant categories

1. sweet

2. sour

3. bitter

4. salty

5. umami

which cranial nerves are responsible for transmission of taste information to the brain stem, then thalamus, then cortical gustatory area of the cortex (or limbic system and hypothalamus)

• facial (VII):

• glossopharyngeal (IX):

• vagus (X):