lecture 24: the somatosensory system Flashcards
(17 cards)
what is the somatosensory system?
somatosensory receptors and neurons, the afferent axons and neurons that form the somatosensory pathways, neurons of somatosensory cortex
where are sensory receptors distributed throughout the body
- exteroceptive
eg: external skin contact, temperature - proprioceptive
eg: body, limb, joint position - enteroceptive
eg: internal organ status (eg bladder fullness, blood gases
what does the somatosensory do
- it provides the brain with info about the state of the body, and about some aspects of the external environment
- this information is used to help guide behaviour and to maintain homeostatic function
how is the somatosensory cortex set up
- sensory axons from all body locations retain a strict spatial relationship to one another along their projection pathways
- information thereby arrives in somatosensory cortex with a specific “map” of the body
- the sensory map is distorted because regions with high receptor density and smaller receptive field sized occupy a disproportionately large area
whats the pre motor cortex role
sends info out to your muscle from input by your afferent neurons
what are the 4 different sensory neurons (primary afferent neurons)
- multipolar
- multiple processes emanate from the cell body - bipolar
- two (2) processes emanate from the cell body - unipolar
- one process emanates from the cell body
- then branches into dendrite and axon - anaxonic
- no distinct axon
- all processes look alike
role of the dorsal root and ventral horn
- sensory info comes in from the dorsal root
- info leaves the ventral root and links straight to a motor neuron
ganglion def
collection of cell bodies
what are sensory neurons (primary afferent neurons)
- cell body typically located in dorsal root ganglia, or cranial nerve ganglia
- peripheral process typically ends at receptive location (nerve endings in periphery are the cell’s dendrites)
- central process enters CNS
- area over which dendrites ramify defines the receptive field
- nerve endings may be free or encapsulated
- synapses form on secondary sensory neurons
what are the four main features of the stimulus that the somatosensory receptors/neurons can encode
- modality
- receptor specificity, eg: touch and temperature are reported by different receptors and signaled separately to brain (the “labelled line” ) - intensity
- the frequency of action potential firing in a sensory axon, and the number of activated axons encodes the intensity (strength) of the stimulus - location
- the somatotopic mapping of receptors in specific areas allows the location (site) of the stimulus to be known (see somatosensory cortex) - duration
- the beginning/end and pattern of action potential firing can encode the start and end of a stimulus
receptive field
- the region/area whose stimulation affects the activity of the neuron
- location and shape of RF determined by location of the neurons sensory apparatus
- size of field relates to two-point-discrimination
- size of receptive field may vary eg skin on shoulder vs fingertip
- type of receptor end they have = different feeling/response
what is the ascending somatosensory pathways
- dorsal columns (fine touch and vibration)
- spinothalamic (anterolateral) tract
- spinocerebellar tracts (proprioception)
dorsal columns (fine touch and vibration)
- axons enter dorsal roots and ascend in the spinal cord dorsal columns
- then make synaptic contact on neurons in the medulla
- neurons in the gracile and cuneate nuclei project across the midline
- form the medial lemniscus to ascend to the thalamus where they synapse with thalamic neurons
- thalamic neurons project to somatosensory cortex
- projections from periphery to cortex retain somatotopic organization
- due to midline crossing in medulla, body sensations are represented in cortex on opposite side
spinothalamic (anterolateral) tract (pain, temperature, crude/non-discriminative) touch
- axons from pain and temp fibres synapse on neurons in dorsal spinal cord
- SC neurons project across the midline and ascend (spinothalamic tract) to neurons in thalamus (aka the anterolateral pathway)
- thalamic neurons project to somatosensory cortex and other nuclei involved in pain response
spinocerebellar tracts (proprioception)
- info from position-sense receptors goes to cortex and cerebellum via spinocerebellar tracts
- axons from sensory neurons in legs and lower body synapse in spinal cord
- axons from arms and upper body synapse on neurons in medulla
what are the 3 tracts that secondary neurons send axons to the cerebellum via
- dorsal spinocerebellar tract
- ventral spinocerebellar pathway
- cuneocerebellar tract
what are chemoreceptors
–> sensory cells with receptors that respond to presence of a specific chemical
- taste and smell eg: salty taste- NaCl, sour taste - acid
- peripheral chemoreceptors: aortic and carotid bodies detect pCO2, and (H+), and (and O2) in blood, crucial in control of breathing
- central chemoreceptors, on surface of medulla, detect pH of cerebrospinal fluid
