L16 - Somatic Sensation Flashcards
(18 cards)
what does the somatosensory system do?
- provides the brain with information about the state of the Boyd, and about some aspects of the external environment
- the information is used to help guide behaviour and to maintain homeostatic function
- the brain also receives important sensory information from the special senses (vision, hearing, balance, smell, taste)
what are the somatic sensations?
Somatic = ‘of the body’ sensations
- touch
- temperature
- vibration
- proprioception
- pain
- temperature
- chemicals eg. blood gases, body fluid composition
NOT special senses
what are the somatosensory receptors
Convert a physical stimulus into action potentials in primary sensory neuron (transduction)
Peripheral receptors -> sensory information -> somatosensory cortex
- temperature - thermoreceptors
- touch/position - mechanoreceptors, proprioceptors
- pain - nociceptors
- chemical - chemoreceptors
what are the four features of a stimulus that result in information coding?
- modality - type of receptor which was activated
- intensity - frequency of action potentials in sensory axon encodes intensity of the stimulus
- duration - beginning, end and pattern of action potential firing can encode the beginning and end of stimulus
- location - mapping of receptor location to somatosensory cortex
describe receptor modality
signal transduction is receptor-specific:
- touch, pressure, vibration and proprioception is by mechanoreceptors
- pain is by nociceptors
- temperature is by thermoreceptors
- body fluid composition is by chemoreceptors
describe skin mechanoreceptors
Sensitive to mechanical deformation
Axons are myelinated
Meissner’s corpuscles:
- in glabrous skin (hairless areas) mostly
- fine touch and pressure, and vibration
Merkel’s discs/corpuscles:
- in all skin
- fine touch, small receptive fields - info about shape and texture
Pacinian corpuscles:
- in all skin (deeper), interosseous membranes, viscera
- deep pressure and vibration, large receptive fields
Ruffini endings/corpuscles:
- in all skin (deeper)
- deep pressure, skin stretch
Hair units:
- in non-glaborous skin
- detects hair discplacement
describe skin thermoreceptors and nociceptors
free nerve endings:
- in all skin, viscera
classified by axon type:
- myelinated axons - temperature (detect change)
- unmyelinated or thinly myelinated axons - nociceptors: noxious stimuli (mechanical, thermal, chemical)
describe proprioceptors
Mechanoreceptors:
- in muscles and tendons
- stretch receptors in skin (Ruffini endings)
Provide information relating to change in muscle length and force
Used by CNS to sense limb movement and lib/body position
Crucial for:
- stretch reflex
- tension reflex
- planning and monitoring movement
describe interoceptors/visceroceptors
- sensory receptors and afferent nerves associated with internal organs
- normally associated with autonomic nervous system function
- includes: mechanoreceptors, chemoreceptors, nociceptors - visceral pain
- often not consciously aware of their outputs
- crucial for many important homeostatic reflexes (eg. blood pressure control, breathing)
describe chemoreceptors
sensory cells with receptors that respond to presence of a specific chemical
peripheral chemoreceptors:
- aortic and carotid bodies detect pCO2, and [H+], (and O2) in blood
- crucial in control of breathing
central chemoreceptors:
- on surface of medulla
- detect pH of cerebrospinal fluid
describe what a sensory unit is
unipolar neuron: (one process leaves body cell body, then branches into dendrite and axon)
receptors in peripheral processes (dendrite)
- space of receptive area = receptive field
- nerve ending can be free or encapsulated
cell body in dorsal root ganglia, or cranial nerve ganglia
central process enters CNS
release neurotransmitters into synapses between sensory unit and secondary sensory neuron
- location of synapse depends on type of signal being transduced
how does the intensity of received information change and how do receptors adapt
- stimuli can very bit APs are all-or-nothing
- need to encode both weak and strong signals of the same type of stimulus
- stronger signal leads to more frequent APs
Adaptation is a decrease in receptor sensitivity:
- decrease in AP frequency in affect neuron despite continuous presence of stimulus
Rapidly adapting receptors:
- generate receptor potential and APs at the onset of a stimulus, but very quickly cease responding
- important for monitoring: rapidly moving or changing sensory stimuli (eg. vibration) and persistent stimuli that don’t need to be monitored closely (eg detecting pressure when you first sit down)
Slowly adapting receptors:
- maintain a persistent or slowly decaying receptor potential during a constant stimulus
- important for monitoring stimuli that need to be constantly monitored (eg. muscle and joint receptors for maintaining balance)
describe what a receptive field is
- the region/area whose stimulation affects the activity of the neuron
- receptive field size affects discrimination aquity (small receptive field = high location discrimination and vice versa)
- receptor ending overlap leads to activity being triggered in multiple sensory units
how can you increase the precision of sensation? (look at diagrams on slides)
lateral inhibition can increase the accuracy of sensory information
- mediated by inhibitory interneurons
- localises sensation to a resticted area of skin
- is used when accurate location of stimulus is required
describe how the location of axons and how that translates to somatotopic map and receptive fields)
Sensory axons formt eh body carry information along ascending projection pathways to the somatosensory cortex
Recall the somatotopic map (homunculus) - ordered representation of the body
Regions with high receptor density and smaller receptive fields occupy a disproportionate area of somatosensory cortex
Representations are modifiable (plastic) with use or lack of use
- violin/guitar players
- amuptation/injuy
describe the dorsal column pathway (how info travels through it) - diagram on slides
Carries sensations of fine touch and vibration - 3 neurons in pathway
1. axons enter dorsal roots and ascend in the spinal cord dorsal columns to make synaptic contact on neurons in the medulla (in brainstem)
2. neurons in the medulla project across the midline to ascent to the thalamus
3. thalamic neurons project to somatosensory cortex
- projections from periphery to cortex retain somatotopic organisation
- due to midline crossing in medulla, body sensations are represented in cortex on opposite side
describe the anterolateral pathway (how info travels through it) - diagram on slide
Carries sensations of pain, temperature and some touch - 3 nerons in pathway
1. axons enter via dorsal root and synapse on eurons in spinal cord
2. spinal cord neurons project across the midline and ascend to neurons in the thalamus (aka the spinothalamic tract)
3. thalamic neurons project to somatosensory cortex
- retain somatotopic organisaton
- due to midline crossing in SC, body sensations are represented in opposite cortex
ANTS neumonic
- Anteriolateral patwhay
- nociceptive
- temperature
- some touch
ants crawl along the ground so think about it as the lower crossing of the pathways
how is sensory information from the head carries (since above brainstem)
- somatosensory information from he head and face enters the brains directly vis the cranial nerves
- neurons cross the midline, usually at the level of their synapse and ascend to the thalamus
- thalamic neurons project to somatosensory cortex
