Sensory System Flashcards
The sensory System - what it does
-General principles
-Detects changes in the external and internal environment
General Principles:
-Specialised receptor cell converts physical or chemical signal into electrical signal
-Electrical signal travels via PNS to CNS
-signals processed by CNS
-Efferent signals from CNS elicit appropriate response
Mechanoreceptors - how they work (generally)
-e.g.
-stretching of cell membrane causes opening of ion channels
e. g. Pressure and vibration
- Osmoreceptors
- balance (equilibrium)
- Sound
- muscle length and tension
- joint position and movement
Chemoreceptors
- How they work (general)
- e.g.
- Chemicals bind to specific receptors on cell membrane
- Open channels via secondary messengers
e. g. CO2, pH, various organic and inorganic molecules
*thermoreceptors operate in similar way, but they respond to temperature
Photoreceptors
- How they work
- Default position
- Respond to light
- When stimulated, initiate chain in chemical reactions terminating in breakdown of secondary messenger molecules and closure of ion channels
- Dark is default position
- Go out in sunlight = ion channels close
Sensory Neuron axon features
-Have a peripheral axon (where signal comes in) and a central axon (where signal goes out)
Sensory Transduction
-how works if sensory receptor is specialised nerve ending
- Stimulus opens ion channels, depolarising membrane and producing receptor (generator) potential)
- Receptor (generator) potentials are graded potentials - if afferent nerve sufficiently depolarised, APs generated, propagating to CNS.
Sensory transduction
-How works if receptor cell is separate from afferent nerve
- Stimulus changes membrane potential of receptor, opening or closing Calcium channels, increasing or decreasing calcium conc in cell
- Triggers or inhibits release of chemical transmitter
- signals receptor on afferent neuron
- excitory or inhibitory potentials generated in afferent neurons
- if sufficiently depolarised, APs generated, travelling to CNS
Sensory Systems in Vertebrates (3 broad types)
- Somatosensory system (senses external enviro)
- e.g. mechanoreceptors in skin detect touch, stretch and vibration, muscles, tendons, joints
- thermoreceptors
- nociceptors in skin detect tissue-damaging mechanical, thermal or chemical stimuli
- e.g. mechanoreceptors in skin detect touch, stretch and vibration, muscles, tendons, joints
- Visceral sensory system (sense internal environment)
- e.g. mecahnoreceptors for blood pressure; chemoreceptors and nociceptors
- Special sensory systems (sense external environment)
- involved in structures
- photoreception, mehcanoreception and chemoreception
- involved in structures
CNS processing of sensory information
- Conscious and unconscious signal perception
- Where processed in brain
- Some are perceived at lvl of conscious awareness(goes to cortex);
- some somatic senses (touch, temp, conscious proprioception and noxious stimuli)
- Special senses (taste, smell, vision, hearing) - Others processed at subconscious level (goes to cerebellum);
- Some propioceptive signals (eg. muscle length and tension)
- Signals from visceral sensory system (blood, pressure, body temp)
Sensory Coding
-4 things nervous system is able to identify
- When a stimulus sensory receptor, nervous system able to identify:
- Modality (Receptor type and signal pathway)
- Location (Receptive fields)
- Intensity
- Duration of stimulus (receptor adaptation)
Modality Receptor Type
-how it works
- Each type of sensory receptor responds only to specific form of energy (or modality)
- i.e. eye has photoreceptors
- modality to which receptor responds best is called adequate stimulus
- modalities other than the right stimulus may activate receptor, but only at high energy levels (e.g. getting hit hard in eye)
Modality: Labelled lines
-what it is
- each form of sensory stimulus follows fixed specific neural p/way to CNS
- same pathway activated every time
- p/way for each modality terminates in specific area of brain (if occurs in cerebral cortex, modality perceived)
Stimulus Location: Receptive fields
-2 ways stimulus localisation is enhanced
- In skin, stimulus localisation enhanced by;
- Smaller receptive fields
- Greater overlap of receptive fields of different afferent nerves
- lips more sensitive than back in humans
- difference in acuity differs over body surface and between sides
How is the intensity of a stimulus worked out?
- Action potential Rate and Burst duration
- Also, recruitment of additional neurons - stimuli of increasing intensity activates greater number of receptors
- may be within a single sensory unit of by stimulation of additional units
Stimulus duration: Tonic receptors
- what they are
- what suited for
- e.g.
- Most receptors adapt to stimulus
- w/ constant stimulus intensity, there is a decrease in magnitude of receptor potential and AP rate in afferent neuron
- tonic receptors adapt slowly -> are suited to signaling prolonged stimuli
e.g. tension receptors in tendons and stretch receptors in skin
Stimulus Duration: Phasic Receptors
- What they do
- what suited for
- e.g.
- Phasic receptors adapt rapidly
- Suited to detecting dynamic qualities of mechanical stimuli
- examples are Pacinian corpuscls in skin (detect high-frequency vibrations)
Sensor Receptors on Body surface
- Mechanoreceptors: detect various forms of mechanical energy, including pressure, vibration, touch and stretch
- Thermoreceptors: detect temperature
- Nociceptors: detect tissue-damaging (noxious) mechanical, therma and chemical stimuli.
Temperature Receptors
- how they work
- temperatures detected
- Free nerve endings (mainly in skin, lining or oral cavity and on surface of tongue)
- activation of receptor opens ion channels in cell membrane -> allows ions to enter cell, eliciting generator potential
- are separate receptors for cold, cool, warm and hot
In humans: perceived as thermal gadations from cold to hot (43 are tissue damaging)
- distribution of receptors not uniform
- cold receptors more superficial and in greater numbers
Thermo transient receptor protein
- what it is
- what else they can detect
Thermoreception in snakes
-what’s special about it?
- Series of 6 temp.-activated ion channels called transient receptor potential (TRP)
- some also respond to chemicals
- TRPA1, TRPV1 and TRPV2 are nociceptive
In snakes;
- some snakes have highly sensitive thermoreceptors -> enables them to locate prey in darkness
- are in small pits in skin on eiyher side of head (in pit organs)
Nociceptors on Body surfaces
-what they are perceived as
- Free nerve ending receptors that respond to tissue-damaging stimuli
- in brain, signals perceived as pain - activated by high intensity mechanical and thermal stimuli - most also w/ chemical
Proprioceptors in Muscles, tendons and Joints
- Where occurs
- where signal travels to (3)
- Mechanoreceptors in S.M., tendons and joints
- Detect changes in muscle length, changes in muscle tension and position of joints
- Signals go to cerebral cortex (conscious perception), spinal cord (generation of spinal reflexes) and unconscious area of brain (cerebellum)
Spinal Reflex
- info from somatosensory receptors travels to spinal cord (which acts as integrating centre)
- initiate reflex response w/out input from brain
- but also travel to brain where conscious perception occurs
- initiate reflex response w/out input from brain
Spinal reflexes: Knee jerk reflex
- Hammer tap stretches tendon, which, in turn, stretches sensory receptors in leg extensor muscle
- sensory neuron synapses w/ and excites motor neuron in spinal cord
- sensory neuron also excites spinal interneuron
- interneuron synapse inhibits motor neuron to flexor muscles
- Motor neuron conducts AP to synapses, causing contraction
- Flexor muscle relaxes b.c. activity of its motor neurons has been inhibited
- leg extends
Somatosensory Cortex
-how organised
- Sensory area of brain in which somatic sensations perceived
- info arising from adjacent areas of body register in adjacent areas of cortex
- size of specific area relates to sensitivity of body region
- areas close together on skin are close together in brain
- called somatrophic organisation
