A3 Perception of Stimuli Flashcards
Sensitivity
the ability of an organism to detect external and internal changes and respond accordingly
Sensory perception
Receptors detect these changes as stimuli, and generate nerve impulses which are relayed to the brain and effector organs
There are different types of receptors that each recognise a different type of stimulus (temperature, light, etc.)
Sensory receptors
Mechanoreceptors
Chemoreceptors
Thermoreceptors
Photoreceptors
Structure of the eye
It consists of two fluid-filled cavities separated by a lens (anterior = aqueous humour, posterior = vitreous humour)
The lens is attached to ciliary muscles, which can contract or relax to change the focus of the lens
The amount of light that enters the eye via the pupil is controlled by the constriction and dilation of the iris
The exposed portion of the eye is coated by a transparent layer called the cornea, which is lubricated by conjunctiva
The internal surface of the eye is composed of three layers – the sclera (outer), choroid (middle) and retina (inner)
The region of the retina responsible for sharpest vision (i.e. focal point) is the fovea centralis (or fovea for short)
Nerve signals from the retina are sent via an optic nerve to the brain (no retina in this region creates a visual blind spot)
Structure of the retina
is the light-sensitive layer of tissue that forms the innermost coat of the internal surface of the eye
Two types of photoreceptors (rods and cones) convert light stimuli into electrical nerve impulses
These nerve impulses are transmitted via bipolar cells to ganglion cells, whose fibres from the optic nerve tract
The photoreceptors line the rear of the retina (adjacent to the choroid), meaning light passes through the other cell layers
Structure of the ear
The external part of the ear is called the pinna, whereas the internal part of the ear is divided into three sections
The outer ear contains the auditory canal, which channel sound waves to the tympanic membrane (or eardrum)
The middle ear contains three small bones called the ossicles, which transfer vibrations to the oval window
The inner ear consists of the cochlea and semicircular canals, as well as a round window which dissipates vibrations
The cochlear converts sound stimuli into electrical nerve impulses, which are transmitted via the auditory nerve to the brain
Photoreception
is the mechanism of light detection (by the eyes) that leads to vision when interpreted by the brain
Light is absorbed by specialised photoreceptor cells in the retina, which convert the light stimulus into nerve impulses
There are two different types of photoreceptors located within the retina – rod cells and cones cells
These cells differ in both their morphology (shape) and function
Rod cells
Rod cells function better in low light conditions (twilight vision) – they become quickly bleached in bright light
Rod cells all contain the same pigment (rhodopsin) which absorbs a wide range of wavelengths
Rod cells cannot differentiate between different colours (monochromatic)
Rod cells are abundant at the periphery of the retina and hence are responsible for peripheral vision
Rod cells produce poorly resolved images as many rod cells synapse with a single bipolar neuron
Cone cells
Cone cells function better in bright light conditions (daylight vision) – they require more photons of light to become activated
There are three different types of cone cells, each with a different pigment that absorbs a narrow range of wavelengths
Cone cells can therefore differentiate between different colours (red, blue and green)
Cone cells are abundant at the centre of the retina (within the fovea) and hence are involved in visual focusing
Cone cells produce well defined images as each cone cell synapses with a single bipolar neuron
How do photoreceptors convert stimuli to impulses
Photoreceptors (rods and cones) convert light stimuli into an electrical nerve impulse (action potential)
This neural information is relayed to the brain via bipolar cells and ganglion cells
Bipolar cells transmit the nerve impulses produced by the photoreceptors to ganglion cells
Many rod cells may synapse with a single bipolar cell, resulting in low resolution of sensory information (poor acuity)
Most cone cells only synapse with a single bipolar cell, resulting in high resolution of sensory information (high acuity)
Ganglion cells transmit nerve impulses to the brain via long axonal fibres that compose the optic nerve
Signals from ganglion cells may be sent to the visual cortex to form a composite representation of surroundings (i.e. sight)
Alternatively, signals may be sent to other brain regions to coordinate eye movements or maintain circadian rhythms
There are no photoreceptors present in the region of the retina where ganglion axon fibres feed into the optic nerve
This region is called the ‘blind spot’ as visual information cannot be processed at this location
The brain interpolates details from the surrounding regions, such that individuals do not perceive a visual blind spot
Contralateral processing - vision
is when a stimulus is processed on the opposite side to where it was detected
Information from the right half of the visual field is detected by the left half of the retina in both eyes and is processed by the left hemisphere (and vice versa for the left half of the visual field)
Information from each eye may swap at the optic chiasma, so that the right or left visual field is processed together
The optic nerves that swap sides are moving contralaterally, while those that stay on the same side remain ipsilateral
Impulses are conducted by the optic nerve to the thalamus, before being transmitted to the visual cortex (occipital lobe)
Thalamic structures (e.g. lateral geniculate nuclei) are involved in coordinating eye movements and circadian rhythms
Sound perception
Sound travels as pressure waves in the air, which travel down the auditory canal and cause the eardrum to vibrate
The degree of vibration of the eardrum (tympanic membrane) will depend on the frequency and amplitude of the sound wave
The eardrum transfers the vibrations via the bones of the middle ear (the ossicles) to the oval window of the cochlea
The function of these bones is to amplify the vibrations from the eardrum (can increase magnification by ~ 20 times)
The vibration of the oval window causes fluid within the cochlea to be displaced – this displacement is detected by hair cells
Activation of these hair cells generates nerve impulses which are transmitted via the auditory nerve to the brain
Function of the middle ear
The middle ear is separated from the outer ear by the eardrum and the inner ear by the oval window
It is an air-filled chamber that houses three small bones (collectively called the ossicles)
The bones of the middle ear are individually called the malleus (hammer), incus (anvil) and stapes (stirrup)
The malleus is in contact with the eardrum and the stapes contacts the oval window (while the incus connects the two)
The function of the ossicles is to amplify the sound vibrations by acting like levers to reduce the force distribution
Sound travelling through air is mostly reflected when contacted by a liquid medium (due to the incompressibility of fluids)
The amplification of sound by the ossicles allows the vibrational pressure to pass to the cochlear fluid with very little loss
The oval window is smaller than the ear drum, which also assists in amplifying the sound energy
Function of the cochlea
The cochlea is a fluid-filled spiral tube within the inner ear that converts sound vibrations into nerve impulses
Displacement of fluid by sound vibrations activates sensory hair cells within the spiral part of the cochlea (organ of Corti)
Hair cells are mechanoreceptors that possess tiny hair-like extensions called stereocilia
The cilia on hair cells vary in length and will each resonate to a different frequency of sound (i.e. specific wavelengths)
When the stereocilia are moved by the cochlear fluid, the hair cell will depolarise to generate a nerve impulse
The nerve impulse will be transmitted via the auditory nerve to the auditory centres of the brain
The kinetic movement of the cochlear fluid (and stereocilia motion) is dissipated by the vibration of the round window
Vestibular system
is a sensory system in the inner ear that is involved in balance and spatial orientation (proprioception)
Within the semicircular canals are gelatinous caps called cupula, which are embedded with numerous hair cells
When the head moves, the fluid in the semicircular canals (endolymph) follows the direction of movement (due to inertia)
This fluid movement exerts pressure on the hair cells embedded in the cupula, triggering nerve impulses
There are three semicircular canals at 90º angles to one another, allowing head movement to be detected in all three planes
The brain integrates information from the semicircular canals in each ear in order to identify head position and movement
