Somatic and sensor analyzer.

How to classify sensory receptors?A. Energy transduced

Chemoreceptors Taste buds, olfactory receptors, aortic

and carotid bodies Photoreceptors

Rods and cones in retina Thermoreceptors

Heat and cold Mechanoreceptors

Touch and pressure

B. By the type of stimulus

Sensory neurons transmit specific signals; require “adequate stimulus” to do so

Classification by Location Exteroceptors – sensitive to stimuli arising from outside the body

Located at or near body surfaces Include receptors for touch, pressure, pain, and temperature

Interoceptors – (visceroceptors) receive stimuli from internal viscera Monitor a variety of stimuli

Proprioceptors – monitor degree of stretch Located in musculoskeletal organs

C. Type of sensory information delivered

Proprioreceptors Within muscles; at joints (position and

movement) Cutaneous receptors

In skin:touch and pressure; heat and cold; pain Special sensory organs

Eyes, ears, olfactory, taste buds Extero- and interoreceptors

External and internal stimuli Lots of overlap!

Sensory adaptation

Phasic (fast-adapting) and tonic (slow-adapting) receptors

Note different firing patterns

Stimulation pattern resembles EPSPs

Cutaneous receptors

What is a somatesthetic sensation? From cutaneous and

proprioreceptors Myelinated sensory

neurons extend all the way to medulla oblongata; crossing over

Always extends to postcentral gyrus

Referred pains along same pathway

Modulating cutaneous sensations Receptive field:

density of receptors Some body areas

Lateral inhibition helps “pinpoint” sensation or increase acuity

Applies to other senses in addition to touch

General somatic – include touch, pain, vibration, pressure, temperature

Proprioceptive – detect stretch in tendons and muscle provide information on body position, orientation and movement of body in space

Somatic Senses

Location of pathway terminations

video

Smell and Taste

• Smell aka Olfaction (Latin olfacere)

• Taste aka Gustation (Latin gustare)

• Minor Senses - Really?

• Chemical Senses

• Smell and Taste very closely related to each other

Where does smell figure?

In humans

• Strong link to sense of taste

• Strong link to emotional states

• Exploited by commercials/marketing

• Used for identification of gender

• Pheromones - signal sexual arousal or a readiness for mating

Basics: What do you need for smell?

• Odorous Substance

• Airborne

• Perceptual system that can parse or segregate the information in airborne odorant molecules

Olfactory epithelium with olfactory receptors, supporting cells, basal cells

Olfactory receptors are modified neurons Surfaces are coated with secretions from olfactory glands Olfactory reception involves detecting dissolved chemicals

as they interact with odorant binding proteins

Smell (Olfaction)

Olfactory Receptors Bipolar sensory neurons located within olfactory epithelium

Dendrite projects into nasal cavity, terminates in cilia Axon projects directly up into olfactory bulb of cerebrum Olfactory bulb projects to olfactory cortex, hippocampus, and

amygdaloid nuclei

Categorizing Smell

• Aristotle: Pungent, Succulent, Acid and Astringent

• Hennings Smell Prism• Based on verbal descriptors of smell

• Multidimensional Scaling (MDS)• Subjects rate only the similarity of odor-pairs

• Consistent arrangement in an N-dimensional space

MDS

Eugenol (cloves)

Methyl Salicylate (cloves)

Vanillin

Aldehyde C14 (flowers)

Nitrobenzene (almonds)

Citronelli (lemons)

Turpentine

Propionic acid (sweaty socks)

Acetic acid (vinegar)

Scatole (feces)

Hydrogen Sulfide (rotten eggs)

Physiology

• Nasal Cycle

• Olfactory Epithelium

• Olfactory Receptor Cells (Complete Neurons)

• Free Nerve Endings

• Olfactory Nerve (Axons of the ORCs)

Physiology contd.

Olfactory Receptor Cells have cilia

Cilia are bathed in mucous

Mucous contains Odorant binding proteins (OBP)

OBP transmits the odorant molecules from the air to receptor sites on the cilia

Olfactory Pathways

Neural Representations

Very little known

• No odorotopic map found.

• All cells fire for all odors but form different patterns different odors => Population code.

• Other complications: Change in intensity leads to increase in the firing rate of neurons but triggers changes in the perceived quality of the odorant.

Odor PerceptionOdor (as well as taste) are much more phenomenological

than vision or audition.

• Odor Detection• Very good: Mercaptan 1 part per 50 billion parts of air.• Varies greatly from odor to odor (molecular properties)• Varies with a variety of other factors

• Time of day, Age and Gender

Smoking in injurious to odor detection!

• Odor Identification• Much worse• Shows context dependence• Varies once again with age and gender

Odor Identification

• Determined by Gender•Women vastly superior

• Can be improved with practise - benefits both genders

• Stimulus Saliency• Infants and mothers can recognize each other by smell alone• Infants hedonic reactions to smell develops with age

• Odor Familiarity - Seen in elderly people

Smoking is injurious to odor identification!

Some more amazing facts• Odor Constancy

• Natural sniffs lead to constant odor perception but artificial blowing of air does not

• Common Chemical Sense • Odors judged pleasant at moderate concentrations are judged unpleasant at high concentrations• Related to the action of free nerve endings - stimulated in an indiscriminate manner• Warning system for the organism

• Adaptation - Adapt to a particular odor• Cross Adaptation: Adapt to a different odor• Could have led to odor categorization but turns out that cross-adaptation is not symmetric!

Disorders of Smell

Anosmia or “Odor Blindness”

• Often caused by a blow to the head

• Temporary since ORCs can regenerate

• Sometimes can be total but in that case is specific to certain odors, e.g. sweat, urine

• Anosmia can have serious consequences such as loss of apetite and weight

Taste – Gustation

Taste receptors Occur in taste buds

Most are found on the surface of the tongue

Located within tongue papillae

Two types of papillae (with taste buds) Fungiform papillae Circumvallate papillae

Taste Buds

Collection of 50 –100 epithelial cells

Contain three major cell types (similar in all special senses) Supporting cells Gustatory cells Basal cells

Contain long microvilli – extend through a taste pore

Taste Sensation and the Gustatory Pathway Four basic qualities of taste

Sweet, sour, salty, and bitter A fifth taste – umami, “deliciousness”

No structural difference among taste buds

Gustatory Pathway from Taste Buds

Figure 16.2

Taste information reaches the cerebral cortex Primarily through the

facial (VII) and glossopharyngeal (IX) nerves

Some taste information through the vagus nerve (X)

Sensory neurons synapse in the medulla Located in the solitary

nucleus