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Sensory Physiology

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Title: Sensory Physiology


1
Sensory Physiology
  • Chapter 10

2
Sensory Organs (Receptors)
  • Monitor the internal and external environment
  • Transmit peripheral signals to CNS for processing
  • Critical for homeostasis

3
Types of SensorsStructural Design
  • Primary Sensors
  • Dendritic endings of sensory neurons
  • Stimulation directly evokes APs in neuron
  • Secondary Sensors
  • Specialized sensory cell
  • Stimulation of sensor induces release of
    neurotransmitter to sensory neuron.

Figs 10.4, 10.7
4
Types of Sensory ReceptorsFunctional Types
  • Chemoreceptors
  • respond to changes in chemical concentration
  • Mechanoreceptors
  • Respond to mechanical energy (touch, pressure
    vibration)
  • Photoreceptors
  • Respond to light
  • Thermoreceptors
  • respond to temperature changes
  • Nociceptors
  • respond to tissue damage (pain)

5
Sensory Adaptation
  • Response of sensors to constant stimulation
  • Phasic receptors
  • exhibit sensory adaptation
  • firing rate of receptor ( APs) decreases with
    constant stimulus
  • Tonic receptors
  • exhibit little adaptation
  • maintain constant firing rate as long as stimulus
    is applied

Fig 10.1
6
Four Steps to Sensation
  • Stimulation
  • application of stimulus
  • Must be strong enough to induce AP in sensory
    neuron
  • Sensors most sensitive to one particular stimulus
    modality (adequate stimulus)
  • Transduction
  • induction of an action potential
  • Stimulation of sensor induces graded potentials
    in sensors
  • generator potentials, or receptor potentials
  • If strong enough depolarization, AP results
  • ? stimulus strength above threshold ? ? AP
    firing rate

Figs 10.2, 10.3
7
Four Steps to Sensation
  • Conduction
  • relay of information through a sensory pathway to
    specific region of CNS
  • Usually three neurons in sensory pathway
  • 1st order neuron
  • from stimulation point to CNS
  • 2nd order neuron
  • e.g., from entry into CNS to thalamus
  • 3rd order neuron
  • e.g., from thalamus to perception site
  • Perception
  • Detection of environmental change by CNS
  • Evaluation of nature of change and magnitude

Fig 8.20
8
Acuity
  • Acuity ability to discriminate size, shape of
    an object in the environment
  • Determined by size of receptive field
  • area of the body that, if stimulated, will cause
    a response from a sensory neuron
  • ? receptor density, ? receptive field size,?
    acuity
  • easier to define borders of an object

Fig 10.5
9
Classification of Sensory Input
  • Somatesthetic senses
  • sensors located over wide areas of the body
  • Information usually conducted to the spinal cord
    first (then possibly the brain)
  • Special Senses
  • Changes detected only by specialized sense organs
    in the head
  • Information conducted directly to the brain

10
Somatesthetic Senses
  • Touch and Pressure
  • Heat and Cold
  • Limb movements
  • Pain

11
Somatesthetic SensesSensor Structure
  • Free nerve endings
  • heat, cold, pain
  • Expanded dendritic endings
  • Ruffini endings and Merkel's disks (touch)
  • Encapsulated endings
  • Meissner's corpuscles, Krause's corpuscles,
    Pacinian corpusles (touch and pressure)
  • Bundled receptors
  • Spindle fibers, Golgi tendon organs

Figs 10.4, 12.26
12
Somatosensory Information Conduction
  • Two possible destinations for sensory information
    upon entering the spinal cord
  • Part of spinal reflex arc
  • Relayed up ascending to somatosensory cortex

Fig 12.26
13
Special Senses
  • Taste
  • Smell
  • Hearing
  • Equilibrium
  • Vision

14
Taste (Gustation)
  • Detection of chemical concentrations in the oral
    cavity
  • Taste buds - chemoreceptors
  • contain microvilli that project to the external
    surface
  • When chemicals come into contact with these
    hairs, buds release NT to sensory neurons ? APs
  • Travel to the parietal lobe (inferior
    postcentral gyrus)

Fig 10.7
15
Taste (Gustation)
  • Different tastes derived from activation of
    different signaling pathways within the cells
  • Salty (high Na)
  • Sour (high H)
  • Sweet (organic molecules)
  • Bitter (toxins)
  • Umami (glutamate)

Fig10.8
16
Smell (Olfaction)
  • Detection of chemicals in air
  • Modified bipolar neurons (chemoreceptors)
  • Ciliated receptors located in nasal epithelium
  • respond to chemicals in air
  • APs travel to olfactory bulb
  • Synapse with mitral cells (2nd order) in
    glomeruli
  • Each glomerulus receives signals from one type of
    receptor
  • Info Relayed to olfactory cortex (temporal lobe)
    and medial limbic system

Fig10.9
17
Smell (Olfaction)
  • Defines much of food flavor
  • 1000 different genes for olfactor receptor
    proteins
  • Humans can distinguish among a great variety of
    odors (10,000)
  • Combinatory effect of odorants binding to
    different receptors

18
Hearing
  • Neural perception of vibrations in the air
  • Hair cells - mechanoreceptors
  • vibrations bend stereocilia
  • Opens/closes physically gated ion channels
  • alters release of NT to sensory neurons

Fig 10.13
19
Anatomy of the Ear
Fig 10.17
  • Outer Ear - air-filled
  • Middle Ear - air-filled
  • Inner Ear - fluid-filled

20
Outer (External) Ear
  • Pinna (Auricle)
  • collects and channels sound waves
  • External Auditory Meatus
  • entrance into the skull
  • Tympanic Membrane
  • vibrates when struck by sound waves

Fig 10.17
21
Middle Ear
  • Air-filled chamber
  • Eustachian tube
  • connects middle ear to pharyx
  • Auditory ossicles act as sound amplifiers
  • malleus - against tympanic membrane
  • incus
  • stapes - linked to oval window

Figs 10.17, 10.18
22
Inner Ear
  • Fluid-Filled
  • Two regions
  • Vestibular apparatus
  • equilibrium
  • Cochlea
  • hearing

Figs 10.17, 10.19
23
Cochlea
  • Three snail-shaped tubes filled with fluid
  • Outer canals (continuous)
  • scala vestibuli superior
  • Links to oval window
  • scala tympani inferior
  • Links to round window
  • inner canal Cochlear Duct
  • floor - organ of Corti

Figs 10.19, 10.20, 10.22
24
Organ of Corti
  • Hair cells
  • embedded in supporting cells
  • Basilar membrane
  • Flexible, vibratory
  • Tectorial membrane
  • covers hair cells
  • stereocilia imbedded in membrane

Fig 10.22
25
Conduction of Sound
  • Fluid pressure waves cause basilar membrane to
    vibrate
  • Hair cells move against tectorial membrane
  • Stimulates neurotransmitter release to sensory
    neurons
  • Auditory nerve
  • Signals conducted to auditory cortex (temporal
    lobe)

Figs 10.22, 10.23
26
Equilibrium
  • Changes in position and motion of the head
  • balance and coordination of body movement
  • Hair cells - mechanoreceptors

Fig 10.13
27
Vestibular Apparatus
  • Fluid-filled compartments in the inner ear
  • Semi-circular canals
  • Rotation of the head
  • Otolith organs
  • linear movement of head and orientation relative
    to gravity
  • Sensory information relayed via the vestibular
    nerve to the cerebellum and medulla

Fig 10.12
28
Semicircular Canals
  • Fluid-filled circular tubes oriented in three
    planes
  • Bell-shaped ampulla at one end of each canal
  • contains hair cells covered with gel-like cupula
  • Rotation of head in one direction generates
    inertial pressure in fluid
  • bends cupula
  • stimulates hair cells
  • stimulates vestibular neurons

Figs 10.12, 10.15
29
Otolith Organs
  • Two fluid-filled chambers (utricle and saccule)
  • Macula mound of hair cells covered with
    otolithic membrane
  • jelly like membrane
  • otoliths (CaCO3 crystals)
  • linear movement or tilting of head causes
    otolithic membrane to sag
  • bends hair cells
  • stimulates vestibular neurons

Figs 10.12, 10.14
30
Vision
  • Perception of electromagnetic radiation
  • narrow portion of the EM spectrum
  • Photoreceptors
  • stimulated by photons of light
  • contain photopigments
  • undergo chemical changes in response to light
  • induces metabolic changes in photoreceptors
    leading to receptor potentials

Fig 10.25
31
Anatomy of the Eye
Fig 10.26
  • Three distinctive layers of tissue
  • Sclera - outer layer
  • Choroid - middle layer
  • Retina - inner layer

32
Sclera
  • White of the eye
  • Tough connective tissue
  • Protects inner structures
  • Maintains eye shape
  • Cornea (anterior portion)
  • transparent lets light pass into the eye
  • fixed lens (bends light)
  • covers the anterior cavity
  • filled with aqueous humor

Fig 10.26
33
Choroid
  • Contains blood vessels for the eye
  • Specialized structures anteriorly
  • Iris
  • Ciliary Muscle
  • Lens

Fig 10.26
34
Iris
  • Thin ring of pigmented muscle in front of lens
  • pupil - opening in muscle
  • Muscles alter pupil size, thus amount of light
    passing
  • Radial muscles - open pupil in dim light
    (sympathetic)
  • Circular muscles - close pupil in bright light
    (parasympathetic)

Fig 10.26, 10.27
35
Ciliary Muscles and Lens
  • Lens
  • solid but pliable transparent body
  • used to focus light on the retina
  • Ciliary Muscle
  • ring-shaped smooth muscle
  • linked to lens by suspensory ligaments
  • adjusts shape of lens to focus light

Fig 10.32
36
Accommodation
  • Changing lens shape to focus light from objects
    at different distances on the retina
  • Far objects
  • light from narrow range of angles
  • ciliary muscles relax, lens stretched
  • less convex, less bending of light
  • Near objects
  • light from wide range of angles
  • ciliary muscles contract, lens recoils
  • more convex, more bending of light

Fig 10.33
37
Refraction of Light
  • Light bends when passing between mediums with
    different densities
  • Four different refractive mediums in the eye
  • cornea
  • aqueous humor
  • lens
  • vitreous humor (btw lens and retina)
  • bending of light leads to projection on the
    retina
  • lens is responsible for focusing the image

Fig 10.30
38
Retina
  • Inner layer of the eye
  • Contains photoreceptors
  • rods and cones
  • Fovea centralis
  • point where light is focused
  • high density of cones
  • Optic disk
  • where optic nerve joins the eye
  • no photoreceptors - blind spot

Figs 10.29, 10.30
39
Retina Cells
  • Photoreceptors
  • deepest layer
  • rods and cones
  • Bipolar cells
  • modified neurons
  • receive signals from cells
  • transfer signals to ganglion cells
  • Ganglion cells
  • sensory neurons
  • conduct signals to CNS via the optic nerve

Fig 10.35
40
Photoreceptors
  • rods - light intensity
  • more numerous than cones
  • highly sensitive to light
  • low light levels detected
  • low visual acuity
  • cones - color
  • less sensitive to light
  • need high light levels to respond
  • high visual acuity

Fig 10.36
41
Photoreceptors
  • Each photoreceptor has two segments
  • Inner segment
  • metabolic machinery
  • synaptic endings
  • Outer segment
  • contains layers of internal membranes containing
    photopigments
  • rhodopsin - rod cells
  • photopsins - cone cells

Figs 10.36, 10.37, 10.40
42
Phototransduction
  • photoreceptors synapse with bipolar cells
  • bipolar cells synapse with ganglion cells
  • in absence of light, photoreceptors release
    inhibitory NT
  • hyperpolarize bipolar cells
  • inhibit bipolar cells from releasing excitatory
    NT to ganglion cells

Fig 10.39
43
Phototransduction
  • when stimulated with light, photoreceptors STOP
    releasing inhibitory NT
  • bipolar cells depolarize
  • release excitatory NT to ganglion cells
  • ganglion cells undergo APs

Fig 10.39
44
Conduction of Light
  • Cornea and aqueous body
  • Pupil - adjust light level
  • Lens - focus light
  • Vitreous body
  • Retina (fovea centralis)

Fig 10.30
45
Transduction of Light
  • Rods and Cones cease release of inhibitory NT
  • bipolar cells depolarize
  • release excitatory NT
  • Ganglion cells depolarize
  • AP in optic nerve
  • Signal conducted to visual cortex in occipital
    lobe

Figs 10.35. 10.43
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