Senses 1 Flashcards
what are sense organs?
structures containing receptors and interneurons that are specialised for detecting and processing particular types of stimuli
mechanical modalities
touch
pain
hearing
vestibular
joint
muscle
visual modalities
seeing
thermal modalities
cold
warmth
chemical modalities
smell
taste
common chemical
vomeronasal
electrical modalities
electroreception
touch
contact with/deformation of body surface
pain
tissue damage
hearing
sound vibrations in air/water
vestibular
head movement and orientation
joint
position and movement
muscle
tension
seeing
visible radiant energy
cold
decrease in skin temp
warmth
increase in skin temp
smell
odorous substances dissolved in air/water in nasal cavity
taste
substances in contact with tongue
common chemical
changes in CO2, pH, osmotic pressure
vomeronasal
pheromones in air/water
electroreception
diffs in density of electrical currents
what are receptor neurons?
are specialised to detect internal and external stimuli of a particular sensory modality, especially when stimuli change
act as filter for stimulus information, transforming stimulus energy in neural signals that are transmitted to sensory interneurons
coding in diff sensory modalities
all neurons share the same vocabulary of neural signals.
first stage in processing sensory info
receptor neuron
all in PNS – send info to CNS
taste receptors and sensory interneurons in the tongue
photoreceptor and sensory interneurons in the eye
hair cells and sensory interneurons in the ear
Pacinian corpuscle in the skin
don’t have axons that enter CNS
Olfactory receptor in the nose
sensory receptors and interneurons from each sensory modality project to segregated areas or layers within an area of the brain
what other senses do animals rely on?
magneto-reception (e.g. birds, reptiles)
polarisation vision (e.g. insects)
electro-sensation (e.g. eels, teleost fish)
echolocation hearing (e.g. bats, whales)
such additional senses require adaptations at the level of receptor neurons and/or in the processing sensory pathways and brain areas
what are sensory systems?
good examples to demonstrate some of the known fundamental principles of brain organisation and neural network functions.
what are mechanoreceptors?
touch and pain: Diverse receptors in the skin and body
posture control:
Propioreceptors in the body (muscles, joints)
hearing: Hair cells in the inner ear
balance control: Vestibular receptors in the vestibular apparatus
receptive field of a receptor neuron
receptive field is the region in space in which stimuli affect that neuron’s firing rate
small receptive fields
in touch-sensitive receptors: Free nerve endings, Merkel’s disc and Meissner’s corpuscle sense innervate the surface of the skin and are sensitive to stimuli in small areas of the skin
large/wide receptive fields
Pacinian corpuscles and Ruffini’s endings innervate deeper layers of the skin and are sensitive to stimuli over a larger areas of the skin
receptors that transmit signals from skin to spinal cord
Pacinian corpuscle (in skin, muscles – detecting vibration and pressure) is a unipolar cell that extends one branch of its axon to skin and other to spinal cord
afferent projections form the dorsal root (spinal) nerve and the cell bodies are part of the dorsal root (spinal) ganglion.
mechanically-gated ion channels
vibration or pressure on skin deforms the corpuscle and stretches the tip of the axon opening mechanically-gated ion channels.
note the concentric layers of tissue (like in an onion) around the axon tip that amplify the signal
receptors with a graded potential
similar to the dendrites of a postsynaptic neuron, receptors respond to stimulation with a graded potential, the receptor potential
spiking receptor neurons convert the receptor potential into spikes
response thresholds in receptor neurons
receptors respond to a stimulus within a range of stimulus intensities.
different types of receptor neurons vary in their response thresholds for the same type of stimulus.
response rates for three different hypothetical types of receptor neurons with different response thresholds.
low threshold fires no matter what
others need a stronger stimulus
filtering and coding sensory info
sensory systems can combine receptor neurons with different sensitivities (=different thresholds).
this is useful for extending the range of intensities for which they jointly can respond and for distinguishing between different stimuli, if using a combinatorial code.
receptor adaptation: adjustment of thresholds over time
receptors can adapt (to certain extent) their threshold, if the prevailing range of stimulus intensities changes, to optimise coding of the stimulus.
move threshold to experience lower freq if continuously experiencing lower thresholds and vice versa for high
adjustment of receptor response over duration of stimulus
a single stimulus is usually not enough to shift the sensitivity of the receptor neuron (response threshold)
after a short break a second stimulus of the same intensity will elicit the same response.
slow adapting and small receptive field receptors
Merkel’s disc - sensing texture
slow adapting and large receptive field receptors
Ruffini’s ending - sustained contact
fast adapting and small receptive field receptors
Messiner’s corpuscle - vibration
fast adapting and large receptive field receptors
Pacinian corpuscle - initial contact
sensory pathways
segregated projections to different areas of brainstem, thalamus and cortex (labeled-line principle).
where possible, information about the spatial location of stimuli is preserved by separating projections that come from receptors in different locations.
it is important to know where stimulation occurred in the body (e.g. whether the hand or back was touched) or relative to the body (e.g. whether wind blows into the face or neck).
receptive fields of neurons
receptive fields can be mapped for neurons in the sensory pathway
cortical encoding in form of somatotopic maps
primary somatosensory cortex is located in the postcentral gyrus in the parietal lobe of the human brain.
Brodman areas 1,2 and 3a,b
adjacent regions on body are generally encoded in adjacent regions in cortex
(with some curious exceptions)
sensory segregation in cortical somatotopic maps
fast adapting signals (from Meissner’s and Pacinian corpuscles) and slow adapting signals (from Merkel’s disks and Ruffini’s endings) remain segregated in cortex
information from different skin receptor types is projected into the different Brodman areas
organisation of neurons in columns and layers in the primary somatosensory cortex.
experience dependent plastic reorganisation of cortical maps
can be very plastic
cortical representations can change with use
Owl monkey trained for several months at task using fingers 2-4
suppressing sensory input
just as nerve cells need to be inhibited, receptors and the signals they convey also need to be ‘switched off’ at times
often suppression involves accessory organs. Such structures reduce the intensity or alter the stimulus before it reaches the receptor e.g. Eyelids, muscles in ear
or it can be via top-down processes e.g. brain stem sends messages to receptor cells in the ear to selectively dampen sounds
phasic receptors
receptors that show fast loss of response shortly after onset of stimulation
tonic receptors
receptors show a slow loss of response
