Darkthorn’s Blog

These are my revision notes on the senses from my university lectures. Once again, Copyright belongs to Monash University, and has been paraphrased by me for my revision purposes.

Auditory System

Sensation – the initial message from the senses

Perception – process by which messages from the senses are given meaning

Accessory structure (eye lens or outer ear) modifies energy à sensory receptors transduces energy into a neural response à sensory nerves transfer the coded activity to the central nervous system à thalamus processes and relays the neural response à cerebral cortex receives input and processes the sensation and perception.

Adaptation – process by which responsiveness to an unchanging stimulus decreases over time

Coding – translating the physical properties of a stimulus into a pattern of neural activity

Doctrine of specific nerve energies – stimulation of a particular sensory system provides codes for that sense, no matter what the form of the stimulation

Temporal codes – coding attributes of a stimulus in terms of changes in the timing of neural firing

Spatial codes – coding attributes of a stimulus in terms of the location of firing neurons relative to their neighbours

Liberal response bias – able to see stimulus when one hasn’t been presented

Conservative response bias – refuse to report a stimulus until you are certain it is there

Amplitude of sound – the intensity, the loudness of the sound

Pitch – how high or low the sound is, depending on the frequency of the sound waves.

Timbre – quality of sound

Auditory system – sound waves are collected in the pinna à pinna funnels sound down into the ear canal à sound waves reach middle ear where they strike the eardrum/tympanic membrane à vibrations on the membrane are passed on by the three bones of the ear (malleus/hammer, incus/anvil and stapes/stirrup) à amplify the changes in pressure by focussing vibrations on the oval window membrane à sounds enter the inner ear (cochlea) where transduction occurs à basilar membrane runs the base of the cochlea à when a sound wave passes through the fluid, bending the hair cells of the organ of the Corti and membraneà the hair cells connect to the fibres of the auditory nerves which transfers the sound to the brain for processing.

Aspect of Sensory System	Elements	Key Characteristics
Energy	Sound – pressure fluctuations of air produced by vibrations	The amplitude, frequency and complexity of sound waves determine the loudness, pitch and timbre of sounds
Accessory Structures	Pinna, tympanic membrane, malleus, incus, stapes, oval window, basilar membrane	Changes in pressure produced by the original wave are amplified
Transduction Mechanisms	Hair cells of the organ of Corti	Frequencies are coded by the location of the hair cells recieveing the greatest stimulation and by the firing frequency of the neurons
Pathways and Representations	Auditory nerve to thalamus to primary auditory cortex	Neighbouring cells in the auditory cortex have similar preferred frequencies, thus providing a map of sound frequencies

Visual System

Cornea – the curved, transparent layer through which light waves enter the eye.

Pupil – an opening in the eye just behind the cornea through which light passes

Iris – the colourful part of the eye, which constricts and relaxes to allow light to enter the eye

Lens – the part of the eye behind the pupil that bends light rays, focussing them on the retina (accommodation)

Retina – the surface at the back of the eye which the lens focuses light rays

Photoreceptors – nerve cells on the retina that code light energy into neural activity

Photopigments – chemicals in photoreceptors that respond to light and assist in converting light into neural activity

Rods – highly light-sensitive, but colour insensitive, photoreceptors that allow vision in low light

Cones – photoreceptors that distinguish colours

Fovea – region of the centre of the retina where cones are highly concentrated

Acuity – visual clarity which is greatest in the fovea

Lateral inhibition – creates more difference between dark and light areas because of interneurons

Ganglion cells – cells in the retina that generate action potentials, locked onto bipolar cells and interneurons

Receptive fields – the portion and the visual world that affects a given ganglion cell

Optic nerve – the bundle of fibres that is composed of a bundle of ganglion cell axons

Blind spot – the area at which the ganglion cells converge and exit the eyeball

Optic chasm – part of the bottom of the brain where half of the optic fibres cross sides

Lateral geniculate nucleus – a region of the thalamus in which axons from most of the ganglion cells in the retina end and form synapses

Primary visual cortex – an area at the back of the brain to which neurons in the lateral geniculate relay visual input

Feature detectors – cells in the cortex that respond to a specific feature of an object

Aspect of Sensory System	Elements	Key Characteristics
Energy	Light – electromagnetic radiation from about 4000mn to about 750mn	The intensity and wavelength of lightwaves determine the brightness and colour of visual sensations
Accessory Structures	Eye – cornea, pupil, iris, lens	Light rays are bent to focus on the retina
Transduction Mechanisms	Photoreceptors (rods and cones) in the retina	Rods are more sensitive to light than cones, but cones discriminate between colours. Interactions between the cells of the retina exaggerate differences in the light stimulus
Pathways and Representations	Optic nerve to optic chasm to LGN of thalamus to primary visual cortex	Neighbouring points in the visual world are represented at neighbouring points at the LGN and primary visual cortex. Neurons then respond to the stimulus.

Seeing colours

Hue – the essential colour determined by the dominant wavelength of light

Saturation – the purity of the colour

Brightness – the sensation of the overall intensity of all the wavelengths that make up light

Trichromatic theory – a theory of colour vision identifying three types of visual elements (blue, green and red), each of which is most sensitive to different wavelengths of light

Opponent-process theory – a theory of colour vision stating that colour-sensitive visual elements are grouped into green-red, blue-yellow and black-white elements

Chemical Senses

Synthesia – a blending of sensory experiences that causes some people to ‘see’ sounds or ‘taste’ colours

Anosmia – people who are unable to distinguish between different smells

Olfaction – the sense of smell

Olfactory system – employs thousands of receptors, and sensory information goes directly into the brain

Olfactory bulb – a brain structure that receives messages regarding olfaction

Pheromones – chemicals released by one animal and detected by another, that affects the second animal’s behaviour or physiology

Vomeronasal organ – a portion of the mammal olfactory system that detects pheromones

Gustation – the sense of taste

Papillae – structures on the tongue containing groups of taste receptors or taste buds

Sensations – salty, sweet, sour, bitter, astringent, umami

Aspect of Sensory System	Elements	Key Characteristics
Energy	Smell – volatile chemicals Taste – chemicals in solution	The amount, intensity and location of the chemicals determine taste and smell sensations
Structures of Taste and Smell	Smell – chemical receptors on the mucous membranes of the nose Taste – taste buds grouped in papillae in the mouth	Odour and taste molecules stimulate chemical receptors
Pathways to the brain	Olfactory bulb and taste buds	Axons from the nose bypass the thalamus and extend directly to the olfactory bulb

Somatic Senses

Somatic senses – senses of touch, temperature, pain and kinaesthesia

Touch – dependant on free nerve endings, but is subject to habituation, coded for by the neurons (baseline firing rate)

Temperature – warm fibred (sensory neurons) produce effects, but often are combined with touch

Pain – very similar to touch and temperature in receptiveness, uses A-delta and C fibres to carry pain sensations

Gate control theory – a theory that suggests a functional gate in the spinal cord can either let pain impulses travel up to the brain or block their progress

Analgesia – the absence of pain sensations in the presence of a normally painful stimulus

Natural analgesics – serotonin and endorphins

Kinaesthesia – the sense that tells you where the parts of your body are in relation

Proprioceptive senses – the sensory systems that allow us to know about where we are and what each part of our body is doing

Vestibular senses – the proprioceptive sense that provides information about the position of the body

Vestibular sacs – organs in the inner ear that connect to the semicircular canals and the cochlea

Otoliths – small crystals in the vestibular sacs that stimulate nerves cells

Semicircular canals – tubes in the inner ear whose fluid, when shifted by head movements, stimulating nerves cells