Somatosensory Function I: Sensation Flashcards
Sensory receptors
• Term ‘Receptor’ used in different sense
to cell biology
• Not ligand binding to protein
• Specialised cell or part of cell
• Sensory pathways
• Including peripheral and cranial nerves
• Conduct sensory information from
receptors to CNS
• Brain areas dedicated to processing
• Process/decode/make sense of sensory
information
Sensory receptors detect a range of sensory stimuli, largely defined as three different sensations:
- Proprioception
- Touch
- Pain & Temperature
Mechanoreceptors
Modality; Touch, Audition, Vestibular
Receptor; Pacinian corpuscle, Hair cell
Location; Skin, Organ of Coni Macula, semicircular canal
Photoreceptors
Modality; Vision
Receptor; Rods and cones
Location; Retina
Chemoreceptors
Modality; Olfaction, Taste, Anerial Po; pH of CSF
Receptor; Olfactory receptor, Taste buds
Location; Olfactory mucosa, Tongue Carotid and aortic bodies, Ventrolateral medulla
Theroreceptors
Modality; Temperature
Receptor; Cold receptors, Warm receptors
Location; Skin
Nociceptors
Modality; Extremes of pain and temperatUre
Receptor; Thermal nociceptors, Polymodal nociceptors
Location; Skin
Sensory Pathways
• Function
• Including peripheral and cranial nerves
• Conduct sensory information from
receptors to CNS
- Conduction velocity
- Slow vs Fast
Anatomy
- Myelinated vs Unmyelinated
* Axon diameter
Anatomically, sensory pathways are found within
mixed neuronal bundles within spinal nerves.
Describe receptor type, axon diameter and conduction velocity in proprioception sensory pathways
Muscle spindle
13-20um
80-120 m/s
Describe receptor type, axon diameter and conduction velocity in touch sensory pathways
Merkel, meissner, pacinian and ruffini cells
6-12um
35-75 m/s
Describe receptor type, axon diameter and conduction velocity in pain, temperature sensory pathways
Free nerve endings
1-5um
5-30 m/s
Describe receptor type, axon diameter and conduction velocity in pain, temperature sensory pathways (unmyelinated)
Free nerve endings (unmyelinated)
- 2-1.5um
- 5-2 m/s
Functional divisions of spinal nerves:
- Root
- Primary
- Peripheral
• Spinal Nerve
• Mixed sensory and motor
fibre
Dorsal Root
Bifurcation of sensory
fibres from spinal nerve
Ventral Root
• Bifurcation of motor
fibres from spinal nerve
Fibres of the dorsal root carry
sensory information.
Lateral division of dorsal root
• Myelinated and unmyelinated fibres. Temperature & pain sensation.
Medial division of medial root
• Myelinated fibres .
• discriminative touch, pressure,
vibration, and conscious proprioception.
pseudounipolar
Sensory nerve fibres a pseudounipolar, with their cell bodies located within the dorsal root ganglion.
Dorsal Root Ganglion
Sensory nerve fibres from the ANS input to the Dorsal Root Ganglion from the Grey Ramus.
Typically associated with organ pain.
Motor fibres from the ventral root input to ANS fibres via the White ramus.
Link to mechanical analgesia.
White matter:
Fibre pathways
Grey matter:
Neuronal nuclei
Dorsal Horn:
Sensory input
Ventral Horn:
Motor output
Sensory fibres input via the dorsal horn, then:
• Enter the white matter tracts
and project in ascending tracts.
• Synapse with interneurone, then motor neurone, exiting via the Ventral horn.
• Decussate, before either of the above.
Brain areas dedicated to
processing
• Process/decode/make sense of sensory
information
Proprioception & Fine Touch
Fine touch, often referred to as discriminatory touch, enables the animal to localise.
Crude touch, often referred to as non- discriminative touch, enables sensation without localisation.
Difference between modalities a combination of both receptors activated and spinal pathways.
Describe the pathway in the brain of fine touch, proprioceptionn, vibration, nociception, temperature and coarse Touch
Pain, temperature, and coarse touch cross the midline in the spinal cord.
Fine touch, vibration, and proprioception pathways cross the midline in the medulla.
Sensory pathways synapse in the thalamus.
Sensations are perceived in the primary somatic sensory cortex.
Dorsal Column System
- Dorsal input enters via the dorsal root, ascending via the Fasiculus cuneatus (1st Order Neurone).
- Decussates at the medulla, ascending via the Medial leminiscus (2nd Order Neurone).
- Synapses at the Thalamus before projecting to the sensory cortex (3rd Order Neurone).
Spinothalamic Tract
- Dorsal input enters via the dorsal root, synapses within the Dorsal horn (1st Order Neurone).
- Decussates, ascending via the Lateral spinothalamic tract (2nd Order Neurone).
- Synapses at the Thalamus before projecting to the sensory cortex (3rd Order Neurone).
Thalamus
Incoming spinal sensory pathways synapse within the ventral posterior nucleus of the Thalamus.
Inputs stratified by sensation:
• Superior (VPS) Proprioceptive
• Inferior (VPI) Pain & Temperature
• Medial (VPM) and Lateral (VPL) Fine touch
The medial and lateral aspect are further stratified by sensory region:
• Medial (VPM) Face
• Lateral (VPL) Foot
• Middle: Hand
