Senses Flashcards
What is the overview of the pathway of senses?
Physical/chemical environment Reception Transduction Encoding/transmission Perception
What are labelled lines?
One pathway per modality
Meaning each sense has only one pathway
How are senses recieved?
Properties (e.g. location) of stimuli are mapped onto brain structures e.g. retinotopic, chemotopic and tonotopic maps
How can the intensity of coding stimulus be activated?
It can be encoded by either spike rate and the number of axon receptors being activated
What are the different pathways of the senses?
Visual Pathway
Auditory pathway
Olfactory (smell) pathway
Gustatory (taste) pathway
Describe some features of the eye?
Optic nerve - blind spot, that carries nerve imulses to the brain
Ciliary muscle - changes the thickness of the eye lens
Cornea - transparent layer that protects the eye and helps focus light on the retina - refraction
Iris - ring of muscle controlling the amount of light entering
Lens - focuses light on to the retina
Pupil - hole that allows light through
Retina - light senstive cells recieve the light rays and they are converted to neural signals
What is used in phototransduction?
The retina - which is made up of rod and cone cells
Information about cone cells?
High in the fovea
Iodopsin pigment - 3 types: blue, green and red sensitive
Each one is connected to a separate bipolar neurone
High visual acuity
Low sensitivity in low light
Information about Rod cells?
High in the side eye Rhodopsin pigment Many rod cells are connected to each bipolar neurone Low visual acuity High sensitivity in low light
What is the retina made up of? How do they generate a nerve impulse?
Choroid
Rod and cone cells
Biopolar cells
Ganglion cells
Light changes the protein conformation of the receptor
G-protein is activated - binding GTP
This decreases second messenger and there is a decrease in Na+ conductance from the ion channels = action potential
What is the visual pathway in mammals?
Retina
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus (in the thalamus)
Projection fibres (optic radiation)
Primary visual cortex of cerebral hemispheres
What is the stimulus in the audition pathway?
Stimulus - pressure, waves and air
Sound is actually a series of changes in are pressure which form a wave
What is the ear made up of?
External ear
Middle ear - tympanic cavity, auditory ossicles
Internal ear - cochlea
What does the middle ear do?
This amplifies the waves
Transmit pressure waves in air into pressure waves in liquid
This results in ~22x greater force at the oval window
What does the Cochlea do?
Vibration causes the organ of corti to move up and down
It is the inner hair cells (sterocillia) within the organ of corti that when perturbed = mechano-electrical transduction
Movement of the hair K+ channel opens = depolarisation
Which then opens Ca2+ channels = neurotransmitter release onto spiral ganglion cells
These axons form the auditory nerve
What is tonotopy?
The spatial arrangement of where sounds of different frequency are processed in different areas of the brain
What is the auditory pathway in mammals?
Cochlea - stimulation of hair cells leads to AP
Cochlear neucleus (brainstem)
Inferior colliculus
Medial geniculate nucleus (in the thalamus)
Primary auditory cortex (temporal lobe)
Give an overview of the olfactory (smell) pathway?
Olfactory sensory neurons are located in the olfactory epithelium which lines the nasal cavity below cribriform plate
Odourants enter nasal cavity & dissolve in mucus secretion
Dissolved odourants bind to receptors on the cilia of olfactory sensory neurons
How does transduction happen in the olfactory pathway (smell)?
All receptors are metabotropic and initiated a second messenger model:
Odourants bind to G-protein coupled receptors on cilia
Activation = production of cAMP
This leads to depolarisation as:
cAMP binds to and opens cation (Ca2+ & Na+) channels
Ca2+ opens Cl- channels -> Cl- efflux
What happens in encoding and transmission in the olfactory pathway (smell)?
Olfactory sensory neurons send their axons through the cribriform plate to synapse in the olfactory bulb (the 1st stage of olfactory processing)
Signals are then sent directly to many brain areas
What are the signal targets within the brain in the olfactory pathway (smell)?
It targets cortical and subcritical structures directly
Projects directly to emotion (amygdala) and memory (hippocampal) centres
Give an overview of the gustatory system (food)?
Majority of ‘taste’ comes from smell, texture, visual appearance
First step is break down of food by enzymes in saliva to solubilise
What are the different sections of the tongue?
Circumvallate papillae – largest, contain many thousands of taste buds, located at posterior
Foliate papillae – elongated structure, contain hundreds of taste buds, lie along posterior lateral edge
Fungiform papillae – smallest, contain one or two taste buds, widespread across anterior portion and tip of tongue
Taste buds localised to these three types of papillae
Throughout the tongue it has sweet, sour, bitter and salty taste buds (all spread out)
How does fast transduction take place?
Dissolved molecules interact with receptors
Triggers membrane depolarisation & action potential firing
Accompanied by increase in intracellular calcium which initiates transmitter release
Transmitters excite afferent nerve fibres
What do different neurones of the gustatory pathway project to in the brain?
1st order neurons project to medulla
2nd order neurons project to thalamus
3rd order neurones project to gustatory cortex
