Physio 1, Fall ’08, LPC

Chapter 10 – Sensory Physiology

I – General Properties

Stimulus is physical energy that acts on a sensory receptor.
Receptor transduces stimulus into an intracellular signal.
Action potential passes along sensory neuron to CNS.

Fig 1

Simple receptor – free nerve endings
Many nerve endings encased in connective tissue capsules
Special sense receptors release neurotransmitter onto sensory neuron

4 major groups of sensory receptors

chemoreceptors
mechanoreceptors
photoreceptors
thermoreceptors

Transduction

->graded potential

requires minimum stimulus -> threshold and depolarization
“adequate stimulus” is the form of energy to which a receptor is most sensitive
excessive stimulation of another type of stimulation will -> response (ex: poking eye will -> seeing colors)

Receptive field

Primary sensory neuron responds to defined area
Secondary receptive field can include several receptive fields of primary neurons, often through convergence on the secondary sensory neuron.
Fig 2
Some body areas show more convergence than others.

Routing sensory information in CNS – Fig 10-4 – very good

All sensory information, except olfactory & equilibrium, routed through thalamus before being sent appropriate center in cortex
Olfactory pathways go directly to olfactory cortex
Equilibrium information goes to cerebellum
Some information routed through brain stem before thalamus.

Can decrease perception of a stimulus by inhibitory modulation

CNS must distinguish 4 properties of a stimulus

1. Modality – determined by type or sensory neuron activated and activated brain area (ex: touch)
2. Location of stimulus – determined by activated receptive field

   Sensory regions of cerebrum organized as to source of stimulus
   Auditory information reaches each ear at a different time. Difference in time translated to direction or source.
   Lateral inhibition helps discriminate, ie, area of most intense stimulation sends message while area of less intense stimulation receive inhibitory message to secondary neuron. Fig 10-6

3. Intensity- coded by number of receptors activated & frequency of action potential. Fig 10-7

4. Duration – duration of stimulus determines length of series of action potentials

If a stimulus persists, some receptors adapt. Fig 10-7
Tonic receptors slowly adapt by firing more slowly as stimulation persists (loud noise)
Phasic receptor adapts rapidly to a constant stimulus and turn off.  Will fire again when stimulus turns off. (wearing sox)

Summary on page 335 – very good

II-Somatic Senses

Pathways – Fig 10-9, Table 10-3

Primary sensory neuron terminates onto secondary sensory neuron in:

Medulla if fine touch, proprioception, or vibration
Dorsal horn of spinal cord if irritants, temperature, or coarse touch

Secondary sensory neuron terminates in thalamus

Tertiary sensory neuron goes to specific area of somatic sensory cortex

Area in somatosensory cortex is proportional to importance of body area

Many different types of sense receptors  (pressure, vibration, stretch, texture, etc)

Nocioreceptors

Respond to a variety of chemical, mechanical, thermal stimuli
Activation modulated by local chemicals released upon tissue injury
Activate reflexive protective responses, integrated in spinal cord & message to cerebral cortex that becomes conscious
Pain

Pain is a subjective perception
Pain pathways send branches to limbic system & hypothalamus (can -> emotional distress, nausea, vomiting, sweating)

III-Chemoreception: Smell & Taste

Olfaction – Fig 10-14

Primary sensory neurons (olfactory cells) embedded in olfactory epithelium in nasal cavity and go through holes in bone.

Olfactory cells synapse with secondary sensory neurons of olfactory bulb which is an extension of the forebrain.

Secondary and higher-order neurons project to olfactory cortex, go through parts of brain involved with emotion & memory.

Odorant molecules dissolve in mucus before binding to odorant receptor.

Olfactory cell sensitive to single odor. Combination of odors -> perception of many different smells.

Taste – Figs 10-15 & 16

Five sensations: sweet, sour salty, bitter, umami. Specific taste is a combination of these.

Taste buds in surface of tongue contain many taste cells.
Each taste cell sensitive to one taste. Villi increase area.

Taste cell is a polarized epithelial cell – apical contacts environment, basal side forms a synapse with primary sensory neuron.

Substance dissolves in saliva & mucus, then interacts with receptor on taste cell.
Multiple intracellular pathways increase intracellular Ca++.
Increased Ca++ -> exocytosis of neurotransmitter.
Neurotransmitter causes action potential in primary sensory neuron.
Action potential sent to brain, through thalamus to gustatory cortex.

IV-The Ear - Hearing

Transduction steps:

Sound waves enter ear canal and move tympanic membrane.
Tympanic membrane moves 3 little bones (malleus, incus, stapes) which vibrate and move membrane of oval window.
Vibrations of oval window create fluid waves within cochlea.
Fluid waves move flexible (tectorial) membrane which bends hair cells.
Hair cells release neurotransmitter onto primary sensory neurons, -> action potential in nerve fibers of cochlear nerve.
Coded information about sound sent to auditory cortex.

Signal Transduction in Hair Cells. Fig 10-21

Stereocilia in hair cell transduce vibrations to action potential in primary sensory neuron
Tallest stereocilium (kinocilium) embedded in overlying tectorial membrane.
Movement of tectorial membrane moves tallest stereocilium which moves progressively shorter stereocilium.
Stereocilia movement allows entry of cations into hair cell, -> depolarizes cell, -> neurotransmitter release to primary sensory neuron.
Movement towards tallest stereocilium -> more + ion entry, -> more action potentials in primary sensory neuron.
Movement away from tallest stereocilium -> less + ion entry, -> fewer action potentials in primary sensory neuron.

Cochlear processing of sound

Pitch

Basilar membrane (below support cells which are below tectorial membrane) sensitive to different frequencies along its length
Area closest to round & oval windows sensitive to high frequencies (pitch)
Area distal to round & oval windows sensitive to low frequencies

Loudness

Louder sound -> more rapid action potentials in sensory neuron

Sound waves -> primary sensory neurons -> medulla oblongata-> information from both ears goes to both sides of brain

V-The Ear – Equilibrium

Fig 10-23

Inner ear detects changes in acceleration and direction, -> important information about location of body parts and their relation to each other and the environment.

Three semicircular canals arranged at right angles to each other to detect left & right movement, up & down movement and forward & back movement.
Semicircular canals filled with fluid that comes to area with gelatinous cupula that move hair cells or macula with otoliths that move in response to gravity and stimulate gelatinous material that stimulates hair cells.

Hair cells in inner ear stimulated by gravity & acceleration
Hair cells moved by either gelatinous cupola or otoliths (little stones) moving membrane in which they are embedded

Different orientation of hair cells detect different directions of movement.

Hair cells stimulate primary sensory neurons that synapse in medulla or cerebellum.
Cerebellum coordinates most information and reactions to equilibrium information. Cerebral cortex gets some information.

VI-Vision

Vision – the process through which light reflected from objects in our environment is translated into a mental image.  Three steps:

1. Light enters the eye and is focused on the retina by the lens.
2. Photoreceptors of the retina transduce light energy into an electrical signal.
3. The electrical signals are processed through neural pathways.

Anatomy

Cornea, tissue on the anterior eye surface
Pupil changes amount of light entering eye
Lens bends light to focus it on retina
Muscle alters curvature of lens to accommodate different depths of focus
Retina contains photoreceptors (rods and cones)
Photoreceptors transduce light energy into electrical signals
Photoreceptors stimulate bipolar neurons which synapse with ganglion cells which form optic nerve
Optic nerve from each eye sends information from both eyes to thalamus to visual cortex.
Information from left visual field goes to right side of brain, from right visual field goes to left side of brain. Fig 10-41

Light path

Light goes to pigment epithelium
Fig 10-37
Rods (monochromatic vision) and cones (color vision) embedded in pigment epithelium
Outer segment of rods and cones contain disks with visual pigments that are excited by specific wavelengths of light

Rhodopsin, in rods, is model for phototransduction

Fig 10-39
Darkness - rhodopsin composed of retinal and opsin which are together. Na+ (goes in) and K+ (goes out) channels open. (Membrane potential = -40mV.) Tonic release of neurotransmitter onto bipolar neurons.
Light – Retinal released from rhodopsin. Na+ channel closes. Cell hyperpolarized (to -70 mV). Neurotransmitter release decreases in proportion to amount of light.
Recovery phase – retinal recombines with opsin to form rhodopsin

Great deal of convergence between photoreceptors and bipolar cells.  Even more convergence between bipolar cells and ganglion cells.

Retina uses contrast rather than absolute light intensity for better detection of weak stimuli. One area will be stimulated, surrounding area inhibited, ->  better resolution.

One type ganglion cell more sensitive to information about movement, another to signals that pertain to form and fine detail.