Special senses Flashcards
General senses
Receptors are scattered through the body. E.g touch, pressure, pain, vibration, tem
Special senses
have a special organ/structure with distance receptor cells devoted to it and all localised in the head region
Importance of sensory systems
The ability to sense changed in the environment and in the body allowing survival
Survival depends on:
- Sensation: the awareness of changed in the internal and external environment
- Perception: the conscious interpretation of those stimuli
How we interpret sensation determines how:
- we develop physically, cognitively
- our memories are formed
- we interact socially
- we become unique
Simple organisms e.g Hydra
- Respond to light, pH and nutrient presence
- basic nerve connections and primitive histology
- hydra defy standard sensory receptor modelling
- Show endogenous activity when kept in a homogenous environment
General principles in higher animals
Receptor cells
Purpose
- specialised receptor cells are distributed throughout eukaryotic organisms
- purpose: to convert physical or chemical signal into electrical signal so that is can be used as an action potential
- electrical signals travel PNS to CNS
Afferent
Signals received and processed by CNS
Efferent
Signals from CNS elicit appropriate response in body
Free nerve endings
Non-encapsulated
Pain and thermoreceptors
Light tough and hair follicle e receptors
Encapsulated nerve endings
Pacinian (lamellar) corpuscles for touch and pressure
Tactile corpuscles for discriminative touch
Muscle spindles: proprioceptors respond to muscle stretch
Specialised receptor cells e.g.
Photoreceptor cells in the retina of the eye respond to light stimuli - rods
Tastebuds
Sensory receptors process
Stimuli in the environment activate specialised receptor cells in the PNS –> integration in brain –> send motor output to effectors
Different types of stimuli are sensed by different types of receptor cells
How are receptor cells classified
Structurally by cells type, position and function
General arrangement of neurones in ascending tracts: first order neuron
Has the receptor
Synapses to second order neutron
In grey matter of spinal cord or in medulla oblongata
General arrangement of neurones in ascending tracts: second order neuron
interneuron
crosses over to other side
Decussation
Ascends to thalamus
General arrangement of neurones in ascending tracts: this order neuron
Axons pass to somatosensory cortex
There the sensory information is received and interpreted = perception of peripheral stimulus
Ascending tracts
Afferent signals travel towards brain for processing
Descending tracts
efferent signals from CNS to elicit appropriate response elsewhere in body
Spinal cord
- Ascending and descending tracts located in specific areas of the spinal cord
Allocated pathways for different senses so that the brain knows where the stimuli is coming from
Different nerve tracts lead to different parts of the brain
Tracts are symmetrical
Ascending pathways Dorsal Tracts Conscious Sub-conscious
Somatosensory signals travel along three main pathways on each side of spinal cord:
Dorsal column pathway: Spinothalamic tract and spinocerebellar tract
Conscious information: dorsal column,ns, fasciculus gracilis, fasciculus cuneatus, spinothalamic tracts
Subconscious: spinocerebellar
Descending Pathways
Descend from brain to effector region - motor neurone to create effect
In the pyramidal pathways of the spine
Sensory receptors (2)
Stimuli in the environment activate specialised receptor cells in the PNS –> integration in brain –> send motor output to effects
Different types of stimuli are sensed by different types of receptor cells
Receptor cells classified structurally by cell type, position and function
Exteroceptors
Respond to stimuli arising outside of body
Receptors in skin for touch, pressure, pain and temperature
Most special sense organ
Interceptors (visceroceptors)
Respond to stimuli arising in internal viscera and blood vesseld
Sensitive to chemical changes, tissue stretch, and temp changes
Proprioceptors
Respond to stretch in skeletal muscles, joints, ligaments, and connective tissue covering of bones and muscles
Inform brain of one’s movements
Mechanoreceptors
Stretching of cell membrane opens ion channels
Respond to physical stimuli like touch, pressure, vibration and stretch. Balance, sounds, muscle length and tension, joint position and movement
Thermoreceptors
Allows ions to enter cell, eliciting generator potential
Several proteins that detect temperature
Sensitive to changes above or below normal body temp
Free nerve endings, mainly in skin, lining oral cavity and on tongue
Chemoreceptors
Chemicals bind to specific receptors on cell membrane
Respond to chemicals (smell, taste, blood chemistry) = external (nose) or internal
Photoreceptors
Initiate chain of reactions to cause breakdown of 2nd messenger molecule and closure of ion channels
Always has an influx of ions to get the membrane potential
Respond to light energy
Osmoreceptors
Respond to solute concentrations of body fluids
Nociceptors
Free nerve ending chemoreceptors that respond to tissue-damaging stimuli
In the brain the signals are perceived as pain
Sensitive to pain-causing stimuli (extreme heat or cold, excessive pressure, inflammatory chemicals) - have different properties in terms of where
Neuronal communication after detection
For sensation to occur, the stimulus must excite a receptor and the action potential must be created
This action potential must reach the CNS.
The transmission of a nerve impulse across a synapse is by means of a chemical called a neurotransmitter
Neurons characteristics
Long-lived (>100)
Hugh metabolic rate - depends on continuous supply of oxygen and glucose
Plasma functions in electrical signalling and cell-to-cell interaction during development
Chemical synapses
- Neuron secrete neurotransmitter that binds to specific receptors in cell membrane of second neuron
- most common way in which neurones communicate
- axon terminal contains packets of neurotransmitter e.g. acetylcholine
- electrical signal to chemical signal and then back to an electrical signal when it reached the next neuron
Signal transmission at synapse
- AP arrive at axon terminal
- Voltage-gated Ca2+ channels open
- Ca2+ enters cell – communicates with the synaptic vesicles to begin the transfer
- Ca2+ signals to vesicles
- Vesicles move to membrane
- Vesicles dock and release neurotransmitter by exocytosis
- Neurotransmitter diffuses across synaptic cleft and binds to specific receptors
- Binding of neurotransmitter to receptor activated signal transduction pathway
Sensory transduction
Physical energy –> electrical energy
electrical energy produced by receptor = receptor potential
Stimulus acts by affecting opening/closing of channels or affects calcium levels inside separate cell
Graded potentials and special senses
- a receptor potential is a graded response to a stimulus that may be depolarising or hyperpolarising
- receptor potentials have a threshold in stimulus amplitude that must be reached before a response is generated
- graded means the amplitude of the receptor is proportional to the size of the stimulus
- the transduction process couple’s stimulus detection to the opening or closing of ion channels
Stimulus intensity: frequency or recruitment
- the more intense stimuli produce higher frequency of action potentials
- stimuli of increasing intensity activate, or recruit, greater number of receptors
What happens in the brain
- any given neurone usually received signals from many other neurons
- special sense pathways can form complex neural pathways when they enter the brain
- when groups of neurones synapse there can always be integration
