Chapter 10 - Sensory

 

General Principles
     The afferent neurons of the peripheral nervous system (PNS) pick up information received by sensory receptors.
     Stimuli detected inside the body travel to the CNS by means of visceral afferents and are detected by visceral receptors. We are not aware of these receptors. 
     Visceral receptors include:
Chemoreceptors that monitor O2 and H+ levels in major blood vessels.
Baroreceptors that monitor blood pressure.
Mechanoreceptors that monitor the degree of stretch or distension in hollow organs.
    We are aware of stimuli from the external environment. These receptors fall into two broad systems:
1. Somatosensory system that include various receptors in the skin (somesthetic sensations), and proprioceptors, or receptors that provide information about the position of the limbs and the body.
2. Special sensory system that include for vision, hearing, equilibrium and balance, taste and smell.
Receptor Physiology
     Receptors respond to stimuli which are different forms of energy including chemical gradients, pressure, temperature, sound waves and photons. The specific form of energy of a stimulus is its modality. According to the law of specific nerve energies sensory receptors are designed to respond best to specific modalities. The modality to which the receptor responds to best is the adequate stimulus. 
 
Sensory Transduction
      Transduction (trans - across; duct - lead) is the conversion of one form of energy into another. Sensory transduction converts the energy of the stimulus into a receptor or generator potential. The sensory receptor may be a specialized structure at the end of a peripheral neuron or a separate cell that communicates with an afferent neuron by means of a chemical synapse.
    
Receptor Adaptation
     Adaptation is a decrease in the size of the receptor potential with a constant stimulus. Slowly adapting or tonic receptors show little adaptation in response to a prolonged stimulus. Rapidly adapting or phasic receptors adapt quickly. These receptors detect changes in stimuli intensity.
     Slowly adapting receptors are better at coding the intensity of a stimulus for its entire duration. Rapidly adapting  receptors code changes in stimulus intensity better but not the duration.
Sensory Pathways 
     The specific pathway that transmits information about a specific modality is a labeled line. Stimulation of a labeled line only produces a sensation of its modality no matter what type of energy produces the action potentials. Pathways for different modalities terminate on different places of the cerebral cortex. A pathway begins with a sensory unit. 
     A sensory unit is a single afferent neuron and all receptors associated with it. All the receptors of an afferent neuron can respond to an adequate stimulus in an area called a receptive field. 
 
        In a generalized pathway, the afferent neuron of the sensory unit is the first in a chain of neurons that relays information to the CNS (spinal cord and brain). The afferent neuron is the first order neuron. It synapses with a second order neuron that synapses in turn with a third order neuron in the thalamus. The third order neuron guides the impulse to the sensory cortex where it is perceived.
     
Sensory Coding
  Stimulus type 
     Stimulus type is coded by the receptor type and the pathway activated when the stimulus is applied. The perception of a stimulation often results from the simultaneous activation of more than one sensory pathway. The final perception results from integration in the brain of information from various sensory systems. Pinocchio illusion
  Intensity and Duration
     Intensity is coded by the frequency of action potentials (frequency coding) and the number of receptors activated (population coding). Stronger stimuli produce a higher frequency of actions potentials. 
     Intensity is interpreted by population coding because the more intense the stimulus, the greater the number of receptors stimulated. This results from either a single sensory unit (sensory afferent neuron) sending more action potentials to the CNS or more sensory units sending action potentials to the CNS.
    
  Location
     The receptive fields of specific afferent neurons code for stimulus location. Tactile receptors in the skin illustrate this. The precision of stimulus location is acuity. Acuity depends on:
1. Size and number of receptive fields.
     The smaller the receptive field the greater the acuity. For example, tactile acuity can be measured by two-point discrimination or the ability to perceive two fine points pressed against the skin as two points and not as one. The smaller the receptive fields the smaller the distance between two points of stimulation can be and still be discriminated. Areas with smaller receptive fields (lips, fingertips) have better two-point discrimination than those with larger fields (back, shoulder).
  2. Lateral inhibition  
     Lateral inhibition occurs when a strong stimulus applied to the receptive field of one neuron causes that neuron to inhibit transmission of signals by neurons with neighboring receptive fields. Lateral inhibition increases acuity because it increases the contrast of signals in the nervous system. In other words, the difference between the strength of the signals coming from the central neuron in the affected field and the neurons on the periphery is increased as the information is processed in the central nervous system. 
    
Somatosensory System
     Body sensations such as pressure, temperature, pain and body position. Somatosensory receptors include:
    Body-sense sensations
Proprioceptors are receptors that give information about body position. These receptors are located in muscles, tendons, ligaments, joints and skin. 
    Somesthetic sensations (senses associated with the surface of the body).
        Mechanoreceptors detect pressure, force and vibration. These include:
     
Merkel's disks and Meissner's corpuscles in the superficial layer of the skin and, 
hair follicle receptors, Pacinian corpuscles and Ruffini's endings in deeper layers.
     Thermoreceptors respond to temperature of receptor endings themselves.
Warm receptors respond to temperature between 30o C and 45o C with action potentials increasing as temperature increases.
Cold receptors respond to temperatures between 35o C and 20o C with action potentials increasing as the temperature falls.
           Both warm and cold receptors respond rapidly to temperature changes and show rapid adaptation. The brain uses the relative changes in the responses of hot and cold receptors to interpret the temperature of the environment. 
     Nociceptors transduce harmful stimuli that we perceive as pain. These consist of free nerve endings. There are three types of nociceptors:
Mechanical - respond to intense mechanical stimuli.
Thermal - respond to intense heat.
Polymodal - respond to a variety of stimuli including mechanical, intense heat and chemicals released from damaged tissue.
Somatosensory Cortex
       Somatosensory sensations from all parts of the body go to the somatosensory cortex. All sensory information (except olfaction) goes to that thalamus and is relayed from there to the cortex. Also, all sensations from one side of the body goes to the thalamus and cerebral cortex on the opposite side. There are two major somatosensory pathways:
              
1. Dorsal Column-Medial Lemniscal Pathway 
   This pathway transmits information from mechanoreceptors and proprioceptors. First order neurons from the periphery enter the spinal cord through the dorsal root. The main axons ascend the spinal cord in the ipsilateral dorsal column and end in the dorsal column nuclei in the medulla oblongata where they synapse with second-order neurons. Second order neurons cross over to the contralateral side of the medulla in the medial lemniscus and ascend to the thalamus. Third order neurons transmit information from the thalamus to the somatosensory cortex.
2. Spinothalamic Tract 
   This pathway transmits information from thermoreceptors and nociceptors. First order neurons from the periphery enter the spinal cord through the dorsal root and may ascend or descend (a few spinal segments) along Lissauer's tract before synapsing with second order neurons in the dorsal horn. Second order neurons cross to the contralateral side of spinal cord and ascend in the anterolateral quadrant of the spinal cord through the brainstem to the thalamus. Third order neurons in the thalamus ascend to the somatosensory cortex.  
Pain Perception Article about "Feeling No Pain"
     Pain is important because it helps us to avoid damaging stimuli and, when tissue damage occurs, helps us prevent further damage. 
Pain Response
     Stimulation of nociceptors causes pain perception but also causes
1. Autonomic responses
2. Fear and anxiety
3. Reflexive withdrawal
     There are two types of pain:
1. Fast pain is perceived as an easily localized, sharp, pricking sensation and is transmitted by Ad  fibers (12-30 m/sec).
2. Slow pain perceived as a poorly localized, dull, aching sensation and is transmitted by C fibers (0.2  - 1.3 m/sec).
     Communication between first and second order neuron involves a neurotransmitter called substance P.
     The information that travels along the spinothalamic tract provides information about the location and type of pain. Pain afferents also ascend along pathways that influence the behavioral and emotional aspects of pain. 
Visceral Pain
     Generally, stimulation of nociceptors in the viscera produces a pain called referred pain. It is called referred pain because it is referred to a body surface. Referred pain is due to second order neurons receiving input from both somatic and visceral afferents. The impulses coming from the second-order neurons is interpreted by the somatosensory cortex as coming from the somatic afferents. 
    
Modulation of Pain Signals
     Pain information ascending to higher centers can be either facilitated or attenuated.
   Gate-Control Theory 
    Describes how pain is modulated at the spinal level. Interneurons inhibit second order neurons that transmit pain. C fibers transmitting pain signals from the periphery inhibit these interneurons. However, Aß fibers transmitting touch, pressure or vibration stimulate these inhibitory neurons. Hence, non-painful mechanical stimuli decrease pain transmission.
   The brain can also influence the perception of pain through descending pathways that are part of the endogenous analgesia (pain-blocking) systems.
   One example is shown in figure above. Neurons in the periaqueductal gray matter of the midbrain communicate with the nucleus raphe magnus in the medulla and lateral reticular formation. Neurons from these areas descend the spinal cord and synapse with inhibitory interneurons that release the endogenous opiate neurotransmitter enkephalin. These in turn synapse with the axon terminals of afferent neurons to decrease the release of substance P and the cell bodies and dendrites of second order neurons to induce inhibitory post synaptic potentials.

 

Vision Web site devoted to vision
   Anatomy of the Eye
     Review the anatomy of the eye.
Nature and Behavior of Light Waves
     Visible light is a part of the electromagnetic spectrum with wavelengths between 350 nm and 750 nm 
     Properties of light: Reflection and Refraction; Refraction through a Prism
Reflection
   Light waves strike and bounce off surfaces that we see. Although we receive some light directly (sun & light bulb) most light is reflected off the objects with non-perceived wavelengths absorbed.
Refraction
   Light waves bend as they pass through transparent materials of different densities.
     When parallel light waves strike a concave lens the waves striking the lens surface at a right angle goes straight through but light waves striking the surface at other angles diverge. In contrast, light waves striking a convex lens converge at a single point called a focal point. The distance from the long axis of the lens to the focal point is the focal length.
     Both the cornea and the lens of the eye have convex surfaces and help to focus light rays onto the retina. The cornea provides for most of the refraction but the curvature of the lens can be adjusted to adjust for near and far vision.
Accommodation Accommodation applet
   Light rays that enter the eye from distant objects are nearly parallel and require little bending or refraction to focus on the retina. Light rays from close objects are diverging as they enter the eye and require more refraction. The eye adjusts its refractive power by controlling the shape of the lens. The lens is suspended by zonular fibers that exert a tension on the lens that causes it to flatten and have a lesser curvature for distant objects. When viewing objects that are near the ciliary muscles contract releasing the tension on the lens. Because of the elasticity of the fibers in the lens, the lens rounds up and the curvature and refractive power increases. 
Clinical Defects in Vision The Human Eye
Emmetropia
   This is normal vision. The eye can see distant objects clearly without accommodation and nearby objects clearly with accommodation. In myopia and hyperopia this is not the case because there is a mismatch between  the refractive power of the lens and cornea and eyeball length.
Myopia
   Near objects are seen clearly but not distant objects because the refractive power of the lens or cornea is too strong for the length of the eyeball. Corrected by the use of concave lens that cause light waves to diverge. Eye accommodates for close vision. 
Hyperopia
   The lens or cornea is too weak for the length of the eyeball. Distant objects can be focused only with accommodation and lens cannot accommodate enough for close objects. Corrected by the use of a convex lens.
Astigmatism
   Irregularities on the surface of the cornea or lens cause uneven refraction and results in distortion.
Presbyopia
   Loss in elasticity of lens with age results in loss of accommodation (reading glasses are needed!).
Cataract
   Age related loss  in transparency of lens.
Glaucoma
   Increase in the volume of aqueous humor raises pressure in anterior cavity of eyeball. Can lead to blindness if unchecked.
Regulating Light Entry (extreme close up slow motion human eye with pupil dilating + expanding)
     The eyes can regulate the amount of light entering the eye by adjusting the size of the pupil. The pupil is constricted in bright light and dilated in dim light. The size of the pupil is controlled by the iris. The iris has a circular muscle layer that constricts the pupil and a radial muscle layer that dilates the pupil. Parasympathetic neurons control the circular muscle layer while sympathetic neurons control the radial muscle layer.
Retina
     Retina is the location of photoreceptors:
Rods give black and white vision under low light conditions.
Cones give color vision in bright light.
     Retina has three distinct layers 
Inner layer (closest to middle of eyeball) contains ganglion cells.
Middle layer contains bipolar cells.
Outer layer contains rods and cones.
     Amacrine and horizontal cells are also present and modulate communication by lateral inhibition.
     To increase visual acuity the retina has an area where cells of the inner and middle layers are laterally displaced. This area is called the fovea (depression). The fovea is the center of a spot that contains a high concentration of cones called the macula lutea (yellow spot). The concentration of the cones is highest here and decreases towards the periphery where there are more rods.
Phototransduction
     Light energy is converted to electrical signals by rods and cones. These photoreceptors can be divided into outer and inner segments. The outer segment contains invaginations of the cell membrane or membranous discs. The membranes contain molecules that absorb light. The inner segment contains the cell nucleus, organelles and the synaptic terminal.
     There are four different kinds of photoreceptors each containing a different photopigment that absorbs light. One type of photopigment is found in rods and three different photopigments are found in the three kinds of cones.
     Each photopigment contains a light absorbing portion retinal and a protein called opsin. The kind of opsin determines which wavelengths of light are optimally absorbed by the retinal.
     The photopigment of the rods is rhodopsin. It is associated with a G protein called transducin and the enzyme phosphodiesterase which degrades cyclic GMP (cGMP).
The Process of Phototransduction Photoisomerization of rhodopsin
A. Photoreceptor in the Dark Transduction Animation
   In the dark levels of cGMP are high.
1. cGMP in cytosol opens sodium channels in the membrane of the outer segment.
2. Na+ moves in and depolarizes the membrane.
3. Depolarization spreads to the inner segment.
4. Depolarization opens Ca++ channels.
5. Ca++ enters the cell and triggers release of transmitter by exocytosis.
6. Transmitter causes graded potential in the bipolar cell.
B. Photoreceptor in the Light 
1. Light is absorbed by rhodopsin. Retinal changes shape and is released by opsin.
2. This creates bleached opsin that activates transducin.
3. Activated transducin activates phosphodiesterase.
4. Phosphodiesterase breaks down cGMP.
5. Decreased cGMP causes Na+ channels to close.
6. K+ continues to leak out hyperpolarizing the membrane.
7. Hyperpolarization spreads to the inner segment and closes Ca++ channels.
8. Less Ca++ enters the cell.
9. Less transmitter is released.
10. Graded potential in bipolar cells decreases.
     Rods are sensitive to light in a wide spectrum but is most sensitive to the blue-green range. Rods are highly sensitive and can respond to a single photon. 
     Cones are not as sensitive as rods and respond best to narrower ranges of wavelengths:
S (blue) cones - most sensitive at 430 nm
M (green) cones - most sensitive at 530 nm
L (red) cones - most sensitive at 560 nm (see Table 10.4)
Bleaching of Photoreceptors in Light Illusions Blind Spot
     When exposed to bright light the rhodopsin becomes bleached and opsin is in its active form. No more light can be absorbed. When a dark environment (movie theater) is entered bleached rods are not sensitive to light. Retinal and opsin reassociate in dim light and rhodopsin becomes sensitive to light again.
Neural Processing in the Retina
     There are varying degrees of photoreceptors converging onto bipolar cells. The lesser the convergence the greater the visual acuity. For example, at the macula lutea where there is maximum visual acuity very few photoreceptors converge on a bipolar cell. In the periphery where there is less visual acuity but more sensitivity many photoreceptors converge on each bipolar cell.
     Both the photoreceptor and bipolar cells are incapable of generating action potentials and the graded potentials caused by stimuli is modulated by the horizontal and amacrine cells.
     The ganglion cells transmit signals to the brain by action potentials. Each ganglion cell has a receptive field that can be complex. For example, there are on-center, off-surround ganglion cells which fire most strongly when there is light in the center of the field but no light in the surround. There are also off-center, on-surround ganglion cells that fire most strongly when the opposite is true. The axons of the ganglion cells form the optic nerve.
Neural Pathways for Vision
     The optic nerve exits each eye and combines in front of the brainstem to form the optic chiasm. The light from the left visual field strikes the nasal retina of the left eye and the temporal retina of the right. The ganglion axons from the left nasal retina cross over to the right brain at the optic chiasm while those from the right temporal retina stay on the same side. The same is true for information from the right visual field.
     The result is that, after information arrives at the optic chiasm, input from the left visual field goes to the right side of the brain and vice versa. The fibers that continue from the optic chiasm to the lateral geniculate body of the thalamus due so in the optic tract. At the lateral geniculate body synapses are formed with neurons that go to the visual cortex as part of the optic radiation.
     The visual field is mapped onto the cortex in a topographic organization.
Parallel Processing
     For each point in the visual field information about different qualities of the stimuli such as color, shape and movement are transmitted by parallel pathways to the primary visual cortex where they are interpreted.
 
Depth Perception
     Most areas of the right and left visual fields are detected by both eyes. This is necessary for depth perception because the right and left eyes see objects from different angles. The cortex converts these differences into three dimensional images.

 

Ear
Anatomy
     Review the anatomy on your own.
Nature of Sound Waves
     Sound waves are mechanical waves caused by air molecules in motion. Sound has properties of loudness and pitch. Loudness (amplitude) is due to differences in the densities of compressed and rarified areas. The loudness is expressed in decibels (db) on a logarithmic scale. Pitch is determined by the frequency of sound waves. Frequency is measured by the number of waves per second or Hertz (Hz). The average range of hearing is 20-20,000 Hz with the greatest sensitivity in 1000-4000 Hz.
Sound Amplification in the Middle Ear Tutorial 45.1 Sound Transduction in the Human Ear
     It takes greater pressure to produce waves in the fluid of the cochlear than in the air of the outer ear. The ear amplifies the pressure wave in the air by two means:
1. The three ear ossicles are arranged to function as a series of levers. The movement of the maleus causes a lesser movement of greater force of the incus which in turn causes a lesser movement of greater force of the stapes. The oval plate of the stapes then presses on the fluid of the cochlear duct with greater force in producing a pressure wave. 
2. The diameter of the tympanic membrane is much larger than the membrane on the oval window. The pressure on the larger surface translates to a larger pressure on the smaller surface.
Anatomy of the Cochlea
   Sound transduction occurs in the cochlear. Review the anatomy of the cochlea.
Anatomy of the Organ of Corti
   The organ of Corti is on top of the basilar membrane and contains hair cells, supporting cells and an overlying tectorial membrane. The hair cells have stereocilia the tips of which are embedded in the tectorial membrane.
Sound Transduction by Hair Cells Sound Transduction
     Sound waves causes pressure waves in  the endolymph of the cochlear duct. The basilar membrane moves relative to the tectorial membrane in which the stereocilia of hair cells is embedded. This causes the stereocilia to bend. The distortion of stereocilia causes potassium channels to either open or close. Because the concentration of K+ in endolymph is higher than that in the cell, the opening of K+ channels causes the cell to depolarize as K+ rushes in. The closing of these channels cause hyperpolarization.

     The stereocilia are arranged in decreasing sizes and are connected by elastic protein filaments. When the hair cell is at rest potassium channels are partially open and partial depolarization causes Ca++ channels to open. This results in the release of neurotransmitter by the hair cell which causes neurons of the cochlear nerve to fire action potentials. 
     When stereocilia bend in the direction of the tallest stereocilia the increased tension on stereocilia open the potassium channels more increasing depolarization. This increases Ca++ entry and neurotransmitter release and the frequency of action potentials in afferent neurons. When stereocilia bend away from the tallest stereocilia potassium channels close and the results are opposite.
Coding Sound Intensity and Pitch
     The greater the intensity (loudness) of sound the greater the bending of the stereocilia. The larger the variations in transmitter release produces greater variations in action potentials in the afferent neurons.
     The basilar membrane  varies in structure over its length with the membrane being narrow and stiff near the oval and round windows and wider and more flexible near the helicotrema. High frequency sounds cause greater deflection of the basilar membrane where it is narrow and stiff and lower frequency sounds produce greater deflection where the basilar membrane is loose and flexible. The frequency of sound (pitch) is coded by where along the basilar membrane there is the greatest deflection.
Neural Pathways for Sound
     The afferent neurons travel in the cochlear nerve (VIII) with frequency of action potential coding intensity of sound. In the brainstem afferent neurons synapse with second-order neurons in the cochlear nuclei that travel to the medial geniculate body of the thalamus. Third-order neurons travel to the auditory cortex of the temporal lobe. The frequency of sound is mapped out in the auditory cortex in a tonotopic manner.

 

Ear and Equilibrium
     The ear in addition to transducing sound also detects the acceleration of the body and the position of the head in relation to the rest of the body. Please note, that acceleration is a change in the motion of a body. It can be a change of speed (linear) or a change in direction (rotation). 
Anatomy of Vestibular Apparatus
    Review anatomy 
The semicircular canals detect rotational acceleration. There are three:
1. Anterior  
2. Posterior  
3. Horizontal 
Utricle detects linear acceleration forward and backward.
Saccule detects linear acceleration up and down.
Transduction of Rotational (Angular) Acceleration 
     Hair cells are found in the ampulla of each semicircular canal. The stereocilia of these receptor cells are similar to those of the organ of Corti with the largest ones next to a cilium called a kinocilium. The stereocilia and kinocilium are embedded in a gelatinous mass called the cupula.
     When the head is at rest the hair cells are partially depolarized and cause action potentials in the afferent neurons at a low frequency. When the head rotates the bony labyrinth rotates but the endolymph within the membranous labyrinth lags behind because of inertia. Depending on the angle of rotation, the stereocilia within the cupula either bend toward or away from the kinocilium. If the stereocilia bend away from the kinocilium the hair cell is hyperpolarized and action potentials in the afferent neuron decrease. If the stereocilia bend toward the kinocilium the hair cell is depolarized and action potentials in the afferent neuron increase.
Transduction of Linear Acceleration
     Within the utricle and saccule are areas containing hair cells with stereocilia embedded in a gelatinous mass. The surface of the gelatinous mass has calcium carbonate crystals called otoliths. During linear acceleration hair cells within the utricle and saccule are subjected to the same kinds of mechanical distortion as described in the cochlea and semicircular canals. These cells not only detect linear acceleration but provide information about the relative position of the head in space.
Neural Pathways for Equilibrium
     The afferent neurons from the semicircular canals and utricle and saccule travel to the brain in the vestibular nerve. The nerve goes to the vestibular nuclei and the cerebellum. The information is compared to that received from the eyes, proprioceptors and somesthetic receptors to enable balance and equilibrium and to control eye movements.

 

Taste
   Anatomy of Taste Buds
   The taste buds contain 50-150 receptor cells and numerous support cells. Taste receptor cells have microvilli that extend through a pore exposing the microvilli to the saliva on the surface with its dissolved food molecules.
   Signal Transduction in Taste
There are four primary tastes 
A. Sour - Caused by the presence of H+ in food.
1. Hydrogen ions bind to potassium channels.
2. Potassium channels close.
3. Taste receptor depolarizes and calcium channels open up.
4. Calcium triggers release of transmitter by exocytosis.
B. Salty - Caused by the presence of sodium in food.
1. Increased concentration of sodium increases flow of sodium into cell.
2. Voltage-gated calcium channels open as cell depolarizes.
3. Calcium triggers transmitter release.
C. Sweet - Presence of organic molecules with structure similar to sucrose.
1. Organic molecule binds with membrane receptor.
2. G protein called gustducin activated.
3. Stimulates production of cAMP.
4. cAMP activates protein kinase.
5. Kinase catalyzes phosphorylation of K+ channels and they close.
6. The cell depolarizes and voltage gated Ca++ channels open.
7. Ca++ triggers release of transmitter.
D. Bitter - Associated with a variety of nitrogen-containing compounds. Two mechanisms involved:
1. "Bitter" molecules block K+ channels.
2. K+ leakage decreases.
3. Cell depolarizes and calcium channel opens.
4. Calcium enters the cell and transmitter is released.
Other bitter molecules:
1. Ligands bind to receptors on cell membrane.
2. Receptor inactivates gustducin.
3. Decreases activity of adenylate cyclase
4. Decrease levels of cAMP
5. Calcium channels close.
A fifth taste has been discovered that is activated by amino acids such as glutamate (monosodium glutamate MSG) and are flavor enhancers. This taste has been called "umami" (delicious).
     Receptors cells have all four transduction mechanisms. However, receptor cells respond stronger to one primary taste than the other. 
     Several receptor cells converge on a single afferent neuron and afferent neurons respond in a complex fashion to different tastes.
Neural Pathway for Taste
     Taste afferent neurons travel in VII, IX and X. Afferent neurons synapse in gustatory nuclei of the medulla with second order neurons that travel to the contralateral thalamus. Third order neurons go from thalamus to the gustatory cortex.

 

Olfaction
     Odorants (specific molecules) dissolve in mucus and bind to chemoreceptors.
   Anatomy of Olfactory System
     Olfactory epithelium consists of supporting cells, basal cells and olfactory receptor cells. Basal cells replace olfactory receptor cells.
   Olfactory Signal Transduction
1. Odorant molecules bind to membrane receptors and activates a G protein.
2. G protein activates adenylate cyclase.
3. cAMP is formed.
4. cAMP binds and opens Na+ and Ca++ channels causing depolarization.
5. Ca++ also causes chloride channels to open and Cl- to exit from the cell, increasing the depolarization. 
6. If the depolarization is great enough, action potentials are triggered.
   Neural Pathway for Olfaction

 

     The olfactory neurons form the olfactory nerve (cranial nerve I). The axons form synapses with second-order neurons in the olfactory bulb in structures called glomeruli (sing. glomerulus). The second-order neurons (mitral cells) form the olfactory tract which goes to the olfactory tubercle. 
     From the olfactory tubercle olfactory information goes either to the olfactory cortex concerned with the discrimination of smells, or to the limbic system for triggering olfactory-driven behaviors, such as sexual behaviors.