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Sensory Systems

Chapter 46

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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.

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Categories of Sensory Receptors

• Sensory information is conveyed to the CNS and perceived in a four-step process.

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3 Main Classes of Sensory Receptors

• Mechanoreceptors – Pressure, Gravity, Inertia, Sound, Touch, Vibration

• Chemoreceptors – Taste,

Smell, Humidity

• Photoreceptors – Light, Heat, Electricity, Magnetism

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Categories of Sensory Receptors

• 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.

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Sensory Transduction

• 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

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Events in Sensory Transduction

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• 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.

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Muscle Spindle

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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

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Taste

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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

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Smell

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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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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

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• 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

Sensing Taste, Smell, and Body Position

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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

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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

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Utricle and Saccule

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Structure of Semicircular Canals

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The Ears and Hearing

• Structure of the ear– 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 incusstapes

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Middleear

Innerear

Outer ear

Auditory canal

Eustachian tube

Pinna

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Roundwindow

Tympanicmembrane

Malleus Stapes Semicircularcanals

Auditorynerveto brain

Ovalwindow

Skull

Incus

Cochlea

Eustachian tube

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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.

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Vestibularcanal

Auditorynerve

Organ of Corti

Bone

Cochlearduct

Tympaniccanal

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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

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To auditorynerve

Sensoryneurons

Hair cells

Basilarmembrane

Tectorialmembrane

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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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Effect of Sound Waves

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Vestibular canal

Roundwindow

Tympanicmembrane

MalleusIncus

Stapes Oval window

High frequency (20,000Hz)

Medium frequency (2000Hz)

Low frequency (500Hz)

Cochlearduct

Tympanic canalApex

Base

Basilarmembrane

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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

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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.

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Evolution of the Eye

– 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

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Human Eye

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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

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Color Vision

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Vertebrate Photoreceptors

• 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.

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Structure of the Retina

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Visual Processing in the Vertebrate Retina

• 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.

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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

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Pathway of Visual Information

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Diversity of Sensory Experiences

• 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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