SENSORY RECEPTORS, TASTE AND SMELL

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Define sensation and describe its process

Sense organ - A structure composed of nerve tissue and other tissues that enhance its response to a certain type of stimulus.
Main purpose of sensory receptors is transduction, conversion of one form of energy to another. The transducer is the device that converts the energy ie sense organ.
Receptor potential - the local electrical change from a stimulus on a neuron.
1.Define sensation and describe its process
Sensation:
•Conscious or subconscious awareness of changes in the external or internal environment
•Sensory impulses may only reach the level of the spinal cord (e.g. stretch reflex), the lower brain stem (e.g. reflexes like blood pressure) or the cerebral cortex and we become aware of them (e.g. touch, pain, hearing etc)
•The process of sensation begins in a sensory receptor which can be either a specialised cell or the dendrites of a sensory neuron
The following must take place for sensation to occur:
•A stimulus must occur within the receptive field of the receptor (receptive field is the pickup area of a particular neuron).
•The sensory receptor transduces energy in a stimulus (e.g. mechanical, thermal, chemical) into a graded/receptor potential (electrochemical energy). Note that each type of sensory receptor can only transduce one kind of stimulus; selectivity
•If graded potential reaches threshold, an action potential is triggered which propagates to CNS via 1st order neurons (the 1st order neuron is always associated with the receptor/stimulus)
•Integration of sensory input takes place in a particular region of the CNS via 2nd & 3rd order neurons with conscious sensation perceived in cerebral cortex only. Note that most sensory input is filtered out at brainstem level

The 4 kinds of info that sensory receptors trasmit and describe how CNS encodes each one

1. Modality - type of stimulus, encoded by "labeled line code", if signal comes from retina it must be a visual signal; all action potentials are the same. Labeled "sensory modalities" like taste, hearing, vision, pain i.e. coming from the tongue must be taste.
2. Location:
•each sensory receptor receives input from its "receptive field"
•sensory projection - brain identifies site of stimulation
•receptive fields on the skin for instance are correlated with their sensory homunculus region on the postcentral gyrus. Smaller receptive fields have larger regions dedicated to it in the cortex (see Figure 14.23)
Remember the sensory homunculus
•The area of the cortex dedicated to sensations from specific body parts is proportional to the sensitivity of that body part (i.e. the number of sensory receptors per unit area)
•It's a topographical representation of the sensory receptive fields of the body

The 4 kinds of info that sensory receptors trasmit and describe how CNS encodes each one

3. Intensity- Intensity is encoded in 3 ways:
•frequency of action potentials can increase or decrease
•number of nerve fibres stimulated rises with intensity increase
•which fibres are activated depends on weak or strong stimuli as nerve fibres can differ in their sensitivity and hence the threshold for which an action potential is generated
Intensity; firing rate
This can be achieved by the firing rate of the sensory receptor neuron (eg. olfactory receptors, somatosensory receptors) or the release rate of transmitter from the sensory receptor cell to activate the second order neuron (eg. rods and cones, hair cells, taste sensory receptor cells).
Firing rate - greater response of pain from conscious awareness.
4. Duration (how long a stimulus lasts)
is encoded by changes in the firing frequency of a nerve fibre over time
Receptors exhibit sensory adaptation which means if the stimulus is prolonged then the firing of the neuron will get slower over time (e.g. jumping in to a hot bath). We become less aware of the stimulus.
Receptors classified according to how quickly they adapt
phasic receptor - burst of activity and quickly adapt (smell and hair receptors). May stop signaling even if stimulus continues.i.e. smell gas leak for several seconds, then the sensation fades in intensity even if the stimulus is still there.
tonic receptor - adapt slowly, generates impulses continually (proprioceptors, nociceptors), slow dull ache.

Outline 3 ways in which receptors can be classified

By modality:
chemo-, thermo-, mechano-, photo- receptors and nociceptors
By origin of stimuli
interoceptors - detect internal stimuli (from organs) - produce feelings such as pressure and pain etc
proprioceptors - sense body position and movements - in muscles, tendons and joints etc
exteroceptors - detect external stimuli - vision, hearing, taste etc
By distribution in the body
general senses - widely distributed (somatic)
special senses - limited to head, innervated by CNs

List and identify the different types of somatosensory receptors

Receptor types can be classified according to whether the dendrites are wrapped in CT or glial cells (encapsulated nerve endings) or (unencapsulated nerve endings)
Unencapsulated receptor types include:
Free nerve endings - for pain (nociceptors) and temperature (thermoreceptors such as cold receptors that respond to falling temps and warm receptors that respond to rising temps)
Tactile (Merkel) discs (flattened)- associated with cells at base of epidermis.
Hair receptors- monitor movement of hair
Encapsulated receptor types include:
tactile corpuscles (meisners corpuscles) (Phasic) - light touch and texture
Tall/ovoid to pear shaped and move upward through a mass of schwann cells
krause end bulb (phasic) - tactile; in mucous membranes
Ovoid with a CT layer around a sensory nerve fibre
lamellated corpuscles (pacinian corpuscles) (phasic) - large, onion like in cross section
deep pressure, stretch, tickle and vibration
concentric layers of connective tissue
ruffinicorpuscles (tonic) - heavy touch, pressure, joint movements and skin stretching
Flattened elongated capsules

Differentiate between somatic pain, visceral pain and referred pain

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
•Nociceptors - or pain receptors, allow awareness of tissue injuries and are found in all tissues except the brain
•Somatic pain is pain from the skin, muscles and joints.
•Visceral pain is pain from the viscera, and can result from stretch, chemical irritants or ischemia (lack of blood flow) of viscera (poorly localized)
•Injured tissues release chemicals that stimulate pain fibres (bradykinin, histamine, prostaglandin)

Misinterpreted pain (referred pain):
•brain "assumes" visceral pain is coming from skin
•heart pain felt in shoulder or arm because both send pain input to spinal cord segments T1 to T5
•Results in part from convergence of neural pathways from sensory input on inter-neurons at the spinal cord segments T1-T5 and then this input following similar pathways to thalamus and cerebral cortex
•Brain gets it wrong basically

Describe the receptors for taste and explain how they are stimulated

Taste receptors:
Gustation - sensation of taste
Results from action of chemicals (tastants) on taste buds
Taste buds are banana shaped epithelial cells that:
•have tufts of microvilli called taste hairs which project into a pit called a taste pore
•Lie between supporting cells
•Have synaptic vesicles at base of cell which release neurotransmitter when stimulated by a tastant
•Liberated neurotransmitter molecules trigger nerve impulses in the 1st order sensory neurons that synapse with the base of the gustatory receptor cells
•Taste buds are located mainly on tongue in protrusions called papillae (foliate, fungiform, vallate&filiform).
Basal cells are stem cells that multiply and replace dead taste cells.
Supporting cells have no sensory role on synaptic vesicles.

List the 5 primary taste sensation and state their location on the tongue

Molecules must dissolve in saliva and flood the taste pore.
5 primary sensations - throughout tongue
Sweet - concentrated on tip
Salty - lateral margins
Sour - lateral margins
Bitter - posterior (back of tongue) (lowest threshold)
Umami (meaty or savoury) - taste of amino acids (MSG)

Describe the projection pathways for taste

Taste buds are innervated by the:
facial nerve (VII) - anterior 2/3's of tongue
glossopharyngeal nerve (IX) - posterior 1/3
vagus nerve (X) - palate, pharynx, epiglottis
•Taste fibres project to medulla (solitary nucleus)
•Second order neurons project signals to either the hypothalamus and amygdala (reflex responses such as salivating or gagging) or the thalamus then cerebrum (consciousness of taste)

Describe the receptors for smell and explain how they are stimulated

Olfactory receptors:
•Are found in the olfactory mucosa in superior part of nose just below cribriform plate on 5cm2 of superior nasal concha
•The olfaction receptors are 1st order bipolar neurons
Olfactory cells (are neurons)
•Have olfactory hairs that project from the dendrites
•bind odor molecules (odorants) in thin layer of mucus
•Have axons that pass through the cribriform plate
•survive 60 days
•Have supporting cells
Physiology of smell:
•Most odours contain a mixture of many different chemicals. These activate a range of different receptors (up to 1000). Thus we can distinguish up to 10,000 different odours.
•Odorants bind to receptors on olfactory hairs
•Binding of odorant opens ion channels in membrane for Na+ or Ca2+, depolarizes it and creates a receptor potential, this triggers an action potential that travels to the brain.
•Receptors adapt quickly (sensory adaptation), due to synaptic inhibition in olfactory bulbs. i.e. odours can change in quality and significance depending on whether you are hungry, just eaten or ill.

Describe the projection pathways for smell

•Sensory neurons project via the olfactory nerves to synapse with mitral cells in olfactory glomeruli (spherical structures that receive input from one type of olfactory receptor) in the olfactory bulb.
•These mitral cells (second order neurons) in the olfactory bulb project via the olfactory tract to the olfactory cortex (enterhinal cortex) in the medial temporal lobe directly, or via another pathway through the thalamus to orbitofrontal cortex.
•From olfactory cortex signals travel to insula, frontal cortex, hippocampus, amygdala and hypothalamus.
•Input to these areas can evoke memories, emotions and visceral reactions.

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