SENSORY PHYSIOLOGY LECTURE 14 CH 10

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. photoreceptors Na+ channels open up, Na+ flows in, depolarization (receptor potential)occurs, and this stimulates voltage-gated channels to open.

Classification of Sensory Neurons Somatic sensory neurons – -- information reaches conscious awareness -- two major subgroups: exteroceptors – provide info about the external environment proprioceptors – provide info about the position of the head and body in space Visceral sensory neurons – -- information does not reach consciousness or is poorly localized -- interoceptors –

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

Student Activity Turn to your neighbor and talk about how you think Ben Gay works. Active ingredients: menthol, methylsalicylate (aspirin), which does not go very far into the body. But Ben Gay feels hot/cold/hot/cold

Somatic sensory neurons: Proprioceptors Proprioceptors: provide info about the position of the head and body in space (kinesthesia, which is the sensation of movement). 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+, visceral nociceptors 2. baroreceptors 3. osmoreceptors 4. visceral stretch receptors 5. irritant receptors

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.

Lateral Inhibition Receptors that are most strongly stimulated inhibit those around them. This allows us to perceive well-defined sensations at a single location instead of a “fuzzy” border This process occurs in the CNS, where interneurons that pass between sensory neurons are inhibitory. Lateral inhibition allows us to distinguish different pitches, closely related odors, or the borders of light and darkness, for example.

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)

SMELL (Olfaction) The 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.

Questions for students: Which receptors are used by both gustatory and olfactory senses? Why do Ramen noodles taste meaty even though there’s no meat in them?

Vestibular Apparatus: Equilibrium Two parts: 1) semicircular canals -- rotational, or angular, acceleration -- oriented in 3 planes -- help us maintain balance when turning our head, spinning, tumbling 2) otolith organs (utricle and saccule) --provide information about linear acceleration -- utricle: horizontal acceleration --saccule: vertical acceleration

Vestibular Apparatus: Mechanism Bending of hair cells results in the production of action potentials Bending opens up ion channels  depolarization Bent towards kinocilium: depolar- ization bent away from kinocilium: hyperpolarization

Vestibular Apparatus: The otolith organ The hair cells project in an endolymph-filled membrane the membrane contains CaCO3 crystals horizontal acceleration bends utricle hair cells; vertical acceleration bends saccule hair cells

Vestibular Apparatus: Semicircular canals Three different semicircular canals detect movement in 3 orientations: front-back; up-down; side-to-side hair cells at the base of each canal detect endolymph movement How does the brain know which direction you’re spinning? Hair cells send action potentials at a constant rate when you’re not moving. But when you rotate your head in one direction it increases the frequency of action potentials.

Neural Pathways

VESTIBULOOCULAR REFLEX Click here for a 30 second you tube video. Vestibular nystagmus – visual tracking to keep a field of visual space within view vertigo – a loss of equilibrium resulting in a sense of spinning

Student Activity Meniere’s disease is thought to be due to pressure changes in the inner ear. Why do patients show up at the doctor complaining of dizziness and exhibiting Nystagmus?

Hearing: The Cochlea Sound waves enter the auditory canal and vibrate the tympanic membrane Middle ear: malleus, incus, stapes oval window

Hearing: The Middle Ear This is a close up of the middle ear The stapes is the last bone in the sequence It vibrates the oval window, which mechanically vibrates the endolymph inside the cochlea The eustachian tube (auditory tube) is connected to the nasopharynx. It equalizess the pressure of the middle ear with the changing atmospheric pressure.

Hearing: The Cochlea

Hearing: Cross Section through Cochlea the membrane that the hair cells are stuck into is called the tectorial membrane Fluid makes the basilar membrane bounce and this bends the hair cells and causes depolarization Action potential is sent to cranial nerve VIII

Hearing: PITCH High pitch wiggles the basilar membrane closer to the oval window medium pitch wiggles the basilar membrane further along the path low frequency wiggles the basilar membrane further away In the figure to the left, this is shown as 20,000 Hertz (the lowest frequency).

Neural Pathways for Hearing Sensory neurons from the vc nerve synapse in the medulla oblongata neurons extend from medulla to midbrain neurons extend from midbrain to auditory cortex of temporal lobe

Hearing Impairment Conduction deafness: Sound waves are not conducted from the outer to the inner ear. May be due to a buildup of earwax, too much fluid in the middle ear, damage to the eardrum, or overgrowth of bone in the middle ear Impairs hearing of all sound frequencies Can be helped by hearing aids

Hearing Impairment, cont Sensorineural/perceptive deafness: Nerve impulses are not conducted from the cochlea to the auditory cortex. May be due to damaged hair cells (from loud noises) May only impair hearing of particular sound frequencies and not others May be helped by cochlear implants Presbycusis – age-related hearing impairment