Sensory Systems
Chapter 46
1

Sensory Systems
•
All nerve impulses arrive to CNS as Action
Potential
•
They reach different brain regions so different senses sensed
•
Intensity depends upon number of action potentials received.
2

Categories of Sensory Receptors
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Sensory information is conveyed to the CNS and perceived in a four-step process.
3

3 Main Classes of Sensory Receptors
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Mechanoreceptors – Pressure,
Gravity, Inertia, Sound, Touch,
Vibration
•
Chemoreceptors – Taste,
Smell, Humidity
•
Photoreceptors – Light, Heat,
Electricity, Magnetism
4

Categories of Sensory Receptors
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Sensory receptors transduce stimuli into graded depolarizations which stimulates the production of action potentials.
–
Exteroceptors sense stimuli that arise in the external environment.
–
Interoceptors sense stimuli that arise from within the body.
5

Sensory Transduction
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Sensory cells respond to stimuli because they possess stimulus-gated ion channels in their membranes.
–
Sensory stimulus produces a change in the membrane potential.
 receptor potential
– greater the sensory stimulus, the greater the depolarization of the receptor potential and higher frequency of action potentials
6

Events in Sensory Transduction
7

•
Cutaneous receptors
–
–
Thermoreceptors – naked dendritic nerve endings

Cold – stimulated by fall – inhibited by increase

Heat –
Nociceptors – transmit pain signal to brain
–
–

Some sense tissue damage others more sensitive
Mechanoreceptors – Fine touch – face and fingertips


 hair follicle receptors - with hair
Meissner’s corpulse – no hair – fingers, palms, nipples
Ruffini endings - duration and extent of touch



Merkel cells - duration and extent
Pacinian Corpuscle – deep subcutaneous - pressure
Mechnoreceptors – measure force applied to membrane

Proprioreceptors – measure stretch – reflex Knee-jerk
Baroreceptors – measure stretch in arteries adjust B.P.
8

Muscle Spindle
9

Sensing Taste, Smell, and Body Position
•
•
Chemoreceptors contain membrane proteins that can bind to particular chemicals in the extracellular fluid.
Taste
–
Taste buds mediate taste in vertebrates.
 located in epithelium of tongue and oral cavity within raised papillae
10

Taste
11

Sensing Taste, Smell, and Body Position
•
•
Smell
–
Olfaction involves chemoreceptors located in the upper portion of the nasal passage.

New research suggests there may be as many as a thousand different genes coding for different receptor proteins for smell.
Internal chemoreceptors
– detect variety of chemical characteristics of blood or fluid derived from blood
12

Smell
13

Sensing Taste, Smell, and Body Position
•
Lateral line system
– made up of sensory structures within a longitudinal canal in the fish’s skin
 hairlike processes at their surface project into gelatinous membrane
(cupula)
 vibrations in the environment produce movements of the cupula, which cause hairs to bend
 stimulates sensory neurons
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15

Sensing Taste, Smell, and Body Position
•
Gravity and angular acceleration
– statocyst – invertebrates generally consists of ciliated hair cells with the cilia embedded in a gelatinous membrane containing crystals of calcium carbonate
–
Cilia bend with change in position
–
Tilt to the right cilia on right side bend activate sensory neurons
16

Sensing Taste, Smell, and Body Position
•
•
In vertebrates – fluid filled membranous chamber – labyrinth = organ of equilibrium and hearing
Gravity receptors = two chambers utricle and saccule – possess hair cells similar to the lateral line system
17

Sensing Taste, Smell, and Body Position
•
Inner ear
–
Receptors consist of utricle and saccule .
 hairlike processes embedded within a gelatinous membrane containing calcium carbonate crystal (otolith membrane)
 utricle more sensitive to horizontal acceleration
 saccule more sensitive to vertical acceleration
18

Sensing Taste, Smell, and Body Position
•
Utricle and saccule are continuous, with three semicircular canals oriented in different planes. -
Detect angular acceleration at any angle
– ampullae - swollen chambers at end of canals
 group of cilia protrude into ampullae
 tips of cilia embedded within wedge of cupula that protrudes into the endolymph fluid of each semicircular canal
19

Utricle and Saccule
20

Structure of Semicircular Canals
21

The Ears and Hearing
•
Structure of the ear
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In terrestrial vertebrates, vibrations in air may be channeled through the ear canal to the eardrum (tympanic membrane).

Vibrations of the tympanic membrane cause movement of three small bones
(ossicles) in the middle ear.
 malleus
 incus
 stapes
22

Outer ear Middle ear
Inner ear
Pinna
Auditory canal
Eustachian tube
23

Malleus Incus
Skull
Stapes Semicircular canals
Auditory nerve to brain
Tympanic membrane
Oval window
Round window
Eustachian tube
Cochlea
24

The Ears and Hearing
•
•
The middle ear is connected to the throat by the Eustachian tube which equalizes the air pressure between the middle ear and the external environment.
Inner ear is composed of the cochlea.
–
The cochlear duct is located in the center of the cochlea.

The area above is the vestibular canal and the area below is the tympanic canal.
25

Cochlear duct
Bone
Vestibular canal
Organ of Corti
Tympanic canal
Auditory nerve
26

The Ears and Hearing
•
Transduction in the cochlea
– bottom of the cochlear duct, basilar membrane, quite flexible and vibrates in response to pressure waves
 cilia of sensory hair cells project into tectorial membrane
 organ of Corti
 cilia of hair cells bend in response to the movement of the basiliar membrane relative to the tectorial membrane
27

Hair cells
Tectorial membrane
Basilar membrane
Sensory neurons
To auditory nerve
28

The Ears and Hearing
•
Frequency location in cochlea
–
When a sound wave enters the cochlea from the oval window, it initiates a traveling motion of the basilar membrane.

Flexibility of the basilar membrane limits the frequency range of human hearing to between approximately 20 and
20,000 cycles per second (in children).
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30

Incus
Malleus
Stapes Oval window
Basilar membrane
Tympanic membrane
Round window
Base
Vestibular canal
Cochlear duct
Tympanic canal
Apex
High frequency (20,000Hz)
Medium frequency (2000Hz)
Low frequency (500Hz)
31

Sonar
•
Some mammals such as bats emit sounds and then determine the time it takes for the sound to return.
– locate themselves in relation to other objects such as prey
32

Evolution of the Eye
•
Structure of the vertebrate eye
– sclera - white portion of the eye, formed of tough connective tissue
– iris - colored portion of the eye

Contraction of the iris muscles in bright light decreases pupil size.
33

Evolution of the Eye
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Light enters the eye through a transparent cornea which begins to focus the light.
–
Light then passes through the pupil to the lens , a transparent structure that completes the focusing of light onto the retina.
 lens attached by suspensory ligament to the ciliary muscles
34

Human Eye
35

Vertebrate Photoreceptors
•
Vertebrate retina contains two photoreceptors.
– rods - black and white vision
 photopigment - rhodopsin
– cones - sharpness and color vision

Both have an inner segment rich in mitochondria, with numerous vesicles filled with neurotransmitter molecules.
 photopigment - photopsins
 red, blue, and green cones
36

Color Vision
37

Vertebrate Photoreceptors
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Sensory transduction in photoreceptors
–
Inverse to the usual way stimuli are detected
–
In the dark, photoreceptors release inhibitory neurotransmitter the hyperpolarizes the neurons
–
Light inhibits the photoreceptors from releasing their inhibitory neurotransmitter, and thus stimulates the bipolar cells and the ganglion cells, which transmit action potentials to the brain.
38

Structure of the Retina
39

Visual Processing in the Vertebrate Retina
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Action potentials propagated along the axons of ganglion cells are relayed through the lateral geniculate nuclei of the thalamus and projected to the occipital lobe of the cerebral cortex.
–
The brain interprets this information as light in a specific region of the eye’s receptive field.
40

Visual Processing in the Vertebrate Retina
•
•
Color blindness
– inherited lack of one or more types of cones
– more common in men due to sex-linkage
Binocular vision
– ability to perceive three-dimensional images and sense depth
 each eye sees object at a slightly different angle
41

Pathway of Visual Information
42

Diversity of Sensory Experiences
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Heat
– pit vipers
–
Pits have membrane that is warmed by infrared radiation stimulating thermal receptors
•
•
Electricity
–
Elasmobranchs – ampullae of Lorenzini
Magnetism
– eels, sharks, bees, and birds – navigate in magnetic field of earth
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