What is a sensory receptor? They keep us aware of our internal body or external world A specialized cell (not a protein) or a specialized dendritic ending (e.g. Pacinian corpuscle) or free nerve ending

Characteristics of Sensory Receptors Dendrites ‘transduce’ (convert) the ‘adequate’ stimulus into a membrane potential. E.g. chemoreceptors Na+ channels open up, Na+ flows in, depolarization (receptor potential) occurs, and this stimulates voltage-gated channels to open. e.g. photoreceptors adequate stimulus is light.

Classification of Sensory Neurons Somatic sensory neurons – (EXAMPLES NEXT SLIDE) -- information reaches conscious awareness -- two major subgroups: Exteroceptors – provide info about external environment Proprioceptors – provide info about position of head and body in space Visceral sensory neurons – -- information does not reach consciousness or is poorly localized -- Interoceptors – EXAMPLES COMING UP!

Somatic sensory neurons: Exteroceptors Exteroceptors: a) photoreceptors b) chemoreceptors 1. olfactory receptors 2. taste buds 3. nociceptors- pain - stimulated by chemicals released from damaged tissue c) mechanoreceptors 1. touch and pressure receptors 2. vibration, tickle, itch receptors 3. auditory (cochlear) receptors d) thermoreceptors

Somatic sensory neurons: Proprioceptors Proprioceptors: a) Muscle stretch receptors (muscle spindle fibers) b) Golgi tendon organs – detect c) Joint receptors d) Vestibular apparatus (balance and equilibrium)

Visceral sensory neurons: Interoceptors 1. chemoreceptors: O2, CO2, H+ or visceral nociceptors 2. baroreceptors – detect pressure 3. Osmoreceptors – detect osmolality 4. Visceral stretch receptors – detect stretch of an organ

NEURAL PATHWAYS FOR SENSORY INFO FIG. 8.24

All somatesthetic information goes to the somatosensory cortex -- information from the same area of the body projects to the same area of the somatosensory cortex --disproportionately large areas for hands and feet FIGURE 8.7

The sense of “hurt” resulting from pain -- Why do we feel sad from pain? -- probably a result of impulses passing from the thalamus to the cingulate gyrus, which is part of the limbic system (emotional center). FIGURE 8.18

Referred Pain -- Referred pain is pain that is felt in a somatic location, but instead is the result of damage to an internal organ. -- referred pain is due to the synapsing of visceral and somatic sensory neurons at the same interneuron as they enter the spinal cord. Angina pectoris – pain resulting from damage to the heart. Angina pectoris is often felt as stimulation of a nociceptor in the left arm.

TONIC AND PHASIC RECEPTORS: Sensory Adaptation PHASIC -After awhile you no longer sense the effect. Neurons send many action potentials at first, but then they send fewer or no action potentials. e.g. spray perfume, or e.g. putting your glasses on your head TONIC RECEPTORS – E.G. Nociceptors (pain) do not adapt.

Law of Specific Nerve Energies The law says that no matter what ending is stimulated, you will always feel it by its original modality (adequate stimulus). E.g. stimulation of photoreceptors results in the person seeing light. Why? Because that neuron goes to a specific location in the brain. Example: a person with an amputated leg often has a “phantom” limb because the dendrite ending remains in the spinal cord after amputation.

How Does the Receptor Work? It creates a “generator” potential (or receptor potential), which is similar to an EPSP. It is a partial depolarization (graded); nearby are voltage-regulated gates.

Sensory Dendrites contain the voltage-regulated gates -- interneurons and motor neurons do not have voltage regulated gates on their dendrites.

Two-Point touch Threshold and Receptive Field Receptive field – an area of the body that, when stimulated by a sensory stimulus, activates a particular sensory receptor. - depends on the density of receptors in that area Two point threshold – minimum distance at which 2 points of touch can be perceived as separate.

Taste buds Taste buds have 50 – 100 different taste cells in them Each cell responds to a different taste 5 types: salt (sodium), sour (H+), Sweet (sugar), bitter (quinine), umami (savory; responds to amino acids, such as glutamate).

Taste

SMELL (Olfaction) Olfactory receptors have cilia, which project into the nasal cavity. Detect 380 different smells; originally we had 1000 genes. Many are now pseudogenes (mutated). Nasal stem cells divide every 1-2 months G proteins (up to 50) may be associated with each receptor, which may account for olfactory sensitivity. Olfactory neuron axons synapse with 2nd order neurons in the olfactory bulb. They do not go to the thalamus, but go directly to the cerebral cortex.