Chapter 9Function of the Sense Organs
Human life would be very different without the ability to sense and perceive external stimuli
Imagine your world without the ability to see, hear, smell, touch, and feelIntroduction
Environmental sensation is limited to those forms of energy that sensory receptors are designed to detect.Sensory receptors may convey information to the cortex with awareness or perception and may lead to cerebrally controlled responses.Sensory receptors also serve as afferent pathways for reflex action with or without conscious sensation.
Section 1 Physiology of the Receptor and Sense Organs
I Concept and Classification of the Receptor and Sense Organs
Sensory ReceptorsReceptors are specialized nerve cells that transduce energy into neural signalsReceptors lack axons, form synapses with dendrites of other sensory neuronsReceptors are mode specificLaw of Specific Nerve Energies: sensory messages are carried on separate channels to different areas of the brain Receptors detect a small range of energy levelsEye: 400-700 nMEar: 20-20,000 HzTaste buds: specific chemicals
Spectrum of the Electromagnetic Wave
General sensory receptor structureFree nerve endings: dendrites interspersed among other cells/tissues (pain, temperature, touch)
General sensory receptor structureEncapsulated nerve endings: dendrites with special supporting structures (mechanoreceptors and proprioceptors)
Classification of Receptors:LocationExternoceptors Located on the body surface or specialized to detect external stimuli Pressure, pain, temp, touch, etc.2) Visceroceptors Located within internal organs, detect internal stimuli Blood pressure, pain, fullness.3) Proprioceptors Found in the joints and muscles Also in the vestibular structures and the semicircular canals of the inner ear. Limb and body position and movment.
2 ModalitiesMechanoceptive Detects stimuli which mechanically deform the receptor; Pressure, vibration, touch, sound.2) Thermoceptive Detects changes in temperature; hot/cold3) Nociceptive (pain) Detects damage to the structures4) Photoreceptors Detect light; vision, retinal of the eye5) Chemoceptive Detect chemical stimuli; CO2 and O2 in the blood, glucose, small, taste.
3. ComplexitySimple receptors Usually a single modified dendrite General sense Touch, pressure, pain, vibration, temperature2) Complexity High modified dendrites, organized into complex structures; ear, eye. Special senses: Vision, hearing, smell, taste
Sensation: Receiving messagesStimuli: What messages can be received?Anything capable of exciting a sensory receptor cell can be defined as a stimulusExamples include: sound, light, heat, cold, odor, color, touch, and pressure
Sensation: Receiving messages about the worldSense organs operate through sensory receptor cells that receive external forms of energy and translate these external forms into neural impulses that can be transmitted to the brainThere are two types of sense organs which we will examine in this chapter
Sequence of Events in a ReceptorStimulusBasic FunctionAmplification
II Properties of the Receptors
1. Adequate Stimulus of Sensory Receptors
Each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to normal intensities of other type of stimuli. The stimulus to which a given receptor has the lowest threshold is termed the adequate stimulus of the sensory receptor.For instance, the roes and cones are highly responsive to light but almost completely nonresponsive to heat and cold.
2. Transduction of Sensory ReceptorsTransduction: The process by which an environmental stimulus becomes encoded as a sequence of nerve impulses in an afferent nerve fiber is called sensory transductionSense orgrans transduce sensory energy into neural (bioelectrical) energyConverting one type of energy into another type is the process of transductionYour brain only deals with bioelectrical impulses so transduction must occur; what cannot be transduced cannot be a stimulus
Principles of Transduction
Different kinds of receptor are activated in different ways but the first stage in sensory transduction is the generation of a graded receptor potential.The magnitude of the stimulus is related to that of the receptor potential which in turn is related to either a) the sequence or frequency of all-or-none action potentials generated in the afferent nerve fiber; b) modulated release of transmitter from the receptor cell generating a sequence of action potentials in a second order neurone.
Receptor potentials: Changes in the transmembrane potential of a receptor caused by the stimulus.Generator Potential: A receptor potential that is strong enough (reaches threshold) to generate an action potential.Remember that APs are all-or-none. The stronger the sitmulus (above threshold) the more APs are fired over a given time period; this is translated by the CNS as a strong sensation.
3. Adaptation of Sensory Receptor
Sensory Adaptation is one form of IntegrationPhasic receptors quickly adapt. The frequency of action potentials diminishes or stops if the stimulus is unchanging.
Tonic receptors adaptslowly or not at all.
Most exteroreceptors (receptors that monitor the external environment) are phasic receptors.
Phasic receptors alert us to changes in sensory stimuli and are in part responsible for the fact that we can cease paying attention to constant stimuli.
The slowly adapting receptors (tonic receptors), such as the pain receptors and the baroreceptors of the arterial tree, are useful in situations requiring maintained information about a stimulus.
4. Encoding of Sensory Receptor
The quality of the stimulus is encoded in the frequency of the action potentials transmitted down the afferent fibre and the number of sensory receptors activated.
Stretch Receptors:Weak stretch causes low impulse frequency on neuron leaving receptor.Strong stretch causes high impulse frequency on neuron leaving receptor. Time Membrane potentialFrequency Code
SummaryThe external & internal environments are monitored by sensory receptors. Each type of receptor is excited most effectively by only one modality of stimulus known as the adequate stimulus. The stimulus is converted into an electrical potential.Stimuli are detected as either static or dynamic events. The intensity & duration of the stimulus is frequency coded as bursts of action potentials in the primary afferent nerve.
Section 2 Visual Sense Organ
Functions of the Complete EyeEye functions like a cameraIris allows light into eyeCornea, Lens & humors focus light onto retinaLight striking retina is converted into action potentials relayed to brain
Structure of the EyeballA slightly irregular hollow sphere with anterior and posterior polesThe wall is composed of three tunics fibrous, vascular, and sensoryThe internal cavity is filled with fluids called humorsThe lens separates the internal cavity into anterior and posterior segments
Anatomy of the EyeThree coats or tunicsFibrous: Consists of sclera and corneaVascular: Consists of choroid, ciliary body, irisNervous: Consists of retina
1. Fibrous TunicForms the outermost coat of the eye and is composed of: Opaque scleraClear corneaThe sclera protects the eye and anchors extrinsic musclesThe cornea lets light enter the eye
2. Vascular Tunic (uvea):Has three regions: choroid, ciliary body, and irisChoroid regionA dark brown membrane that forms the posterior portion of the uveaSupplies blood to all eye tunics
Vascular Tunic: Ciliary BodyA thickened ring of tissue surrounding the lensComposed of smooth muscle bundles (ciliary muscles)Anchors the suspensory ligament that holds the lens in place
Vascular Tunic: IrisThe colored part of the eyePupil central opening of the irisRegulates the amount of light entering the eye during: Close vision and bright light pupils constrictDistant vision and dim light pupils dilate
Pupil Dilation and ConstrictionFigure 15.9
Optic Nerve, Blind Spot, Fovea3. The Retina and its Parts
The Retina and its PartsRetina: inner layer on back of eye that contains light-sensitive rods and conesOptic Nerve: bundle of axons running from retina to visual (occipital) cortexBlind Spot: spot on the retina where optic nerve exits eye, there are no receptors (rods or cones) thereFovea: center of the retina where acuity (ability to see fine detail) is greatest
II The Image-Forming Mechanism
The images of objects in the environment are focused on the retina.
1. Principle of OpticsLight rays are bent (refracted) when they pass from one medium into a medium of a different density.Parallel light rays striking a biconvex lens are refracted to a point (principal focus) behind the lens.The principle focus is on a line passing through the centers of a curvature of the lens, at the principal focal distance.
For practical purpose, light rays from an object that strike a lens more than 20 ft (6 m) away are considered to be parallel.The rays from an object closer than 20 ft are diverging and are therefore brought to a focus farther back on the principal axis than the principal focus.
Biconcave lenses cause light rays to diverge.