NEURO: Somatosensation I Flashcards
What does the somatosensory system convey?
division of the nervous system that provides sensations from the body e.g.:
- touch
- proprioception
- heat, cold
- pain, itch
What nerves convey sensation?
peripheral nerves
-they connect the body to the CNS
How is the CNS connected to the rest of the body?
spinal nerves (31 pairs) cranial nerves (12 pairs)
Structure of a nerve
made of fascicles (bundle of axons) and connective tissue elements:
epineurium: outer connective tissue of nerve
perineurium: connective sheath surrounding each fascicle
endoneurium: connective tissue surrounding individual axons within fascicles
What is a spinal nerve
Spinal nerve divisions
the peripheral nerve which emerges from the spinal cord
Dorsal root
Ventral root
Dorsal root
sensory (afferent) branch of each spinal nerve;
enters spinal cord on the dorsal side
Dorsal root ganglion
a nodule on a dorsal root that contains cell bodies of afferent sensory spinal nerve neurones
The fate of afferent sensory neurones at the dorsal root
The central portion of the axon enters the spinal cord and either:
- synapses locally
- sends an axon up the white matter ascending towards the brain
Ventral root
motor (efferent) branch of each spinal nerve;
exits spinal cord on the ventral side
Where are motor neuron cell bodies located?
grey matter of the spinal cord
What are the different classes of sensory afferents?
Large fibres (myelinated, fast conduction)
- convey tactile sensation (touch) and proprioception
- A-β fibres and A-⍺ fibres
Small fibres (thinly/unmyelinated, medium/slow conduction)
- convey temperature, pain, itch, crude touch
- A-ẟ fibres and C-fibres
Cutaneous Receptors
nerve receptors in the skin that respond to pressure, temperature, or pain
What are the different cutaneous receptors?
- Meissner’s corpuscle (dermal papillae)
- Merkel’s disks (basal epidermis)
- Pacinian corpuscle (dermis)
- Ruffini’s corpuscle (dermis)
- Free nerve endings (dermis)
Cutaneous nociceptors
Free nerve endings:
- sensitive to noxious stimuli
- sensitive to low resolution tactile
- a sharp, localised type of pain
Afferents associated with cutaneous receptors
A-β tactile afferents
- large diameter
- myelinated
- 2nd fastest conducting (30-70 m/s)
- Meissner’s corpuscle (dermal papillae)
- Merkel’s disks (basal epidermis)
- Pacinian corpuscle (dermis)
- Ruffini’s corpuscle (dermis)
Afferents associated with cutaneous nociceptors
Free Nerve Endings:
>A-ẟ afferents: small diameter, thinly myelinated, moderate conduction velocity (>30 m/s)
>C-fibres: small diameter, unmyelinated, slow conducting (<1 m/s)
Proprioception
What is the main proprioceptor?
our sense of limb & body position
muscle spindle
-sensitive to muscle length
Function of muscle spindle
Stretch receptors which convey muscle length information to the spinal cord via Group Ia (A-⍺) and II afferent axons:
- when muscle lengthens, receptors are stretched, increasing rate of firing in afferent axons in the spindle, transmitting information to spinal cord
- stretch reflex: muscle contraction to prevent overstretch and damage of muscle fibre
Afferents of proprioception
A-⍺ afferents/Group Ia afferents in muscle spindles:
- large diameter
- myelinated
- fastest conducting (>100 m/s)
Relationship of receptive field and tactile discrimination
- Fine/High tactile resolution= lots of small receptive fields close together
- Less tactile resolution= larger receptive fields.
Central Pathways of the Somatosensory System
Spinothalamic Tract (STT)/Anterolateral System Dorsal Column-Medial Lemniscal System (DCML)
What does the Spinothalamic tract (anterolateral system) mediate?
What does the Dorsal Column-Medial Lemniscal System (DCML) mediate?
coarse touch, temperature, pain
discriminative touch, vibration, proprioception
Inputs to Spinothalamic Tract (anterolateral system)
Inputs to Dorsal Column-Medial Lemniscal system (DCML)
Small Fibres:
A-ẟ and C-fibre afferents
Large Fibres:
A-⍺ fibres (proprioception) and A-β fibres (tactile)
Spinothalamic Tract (anterolateral system) pathway
1) C-fibre and A-ẟ fibres enter the dorsal column of the spinal cord and synapses with a second order neurone in the dorsal horn (gray matter)
2) Second order neurone crosses to the other side of the spinal cord
3) Ascends in the anterolateral tract (white matter) and synapses with the ventral posterior nuclear complex of the thalamus.
4) The thalamic neurones are the third order neurones in this pathway, and their axons project to the cerebral cortex, specifically to the primary somatic sensory cortex.
Dorsal Column-Medial Lemniscal (DCML) Pathway
1) A-β or A-⍺ afferents may send out some local branches within the grey matter of the spinal cord, but the main axon branch ascends in the dorsal columns (white matter) of the spinal cord to the dorsal column nuclei (gracile and cuneate nuclei) in the medulla.
2) Axons synapse at these nuclei, and second order neurones originate from the dorsal column nuclei. Second order neurones immediately cross the midline and ascend in a fibre tract called the medial lemniscus on the contralateral side.
3) They then go through the midbrain and synapse in the ventral posterior nuclear complex of the thalamus.
4) The thalamic neurones are the third order neurones in this pathway, and their axons project to the cerebral cortex, specifically to the primary somatic sensory cortex. This would be the first reception of tactile information in the cerebral cortex.
Effect of lesion on right side of spinal cord on tactile sensation and pain and temperature sensation
Loss of tactile sensation on right side (A-β afferents)
Loss of pain and temperature sensation on left side (C-fibres)
Where is the primary somatic sensory cortex located?
anterior end of parietal lobe, partly buried in the sulcus
Somatotopic map
Organisation of the primary somatosensory cortex maintaining a representation of the arrangement of the body.
Distortion of somatotopic mapping
the somatotopic map is distorted due to the uneven representation of body parts in the somatosensory cortex
-areas of the body with the greatest tactile resolution and lots of small densely packed receptive fields have the greatest amount of cortical tissue for processing
Brodmann’s areas
How many Brodmann areas does the primary somatosensory cortex have?
regions of cortex defined by the relative distribution of cell types across cortical layers (cytoarchitecture)
4 Brodmann areas
Is the primary somatosensory cortex sufficienct enough to interpret sensory input?
Not enough to reach primary somatosensory cortex and then have a conscious interpreted sensation.
Many further onward cortical connections are needed before we can actually interpret our sensory inputs. Meaning, the primary somatosensory cortex is well defined, but we have onward connections to what we call secondary somatosensory cortex and also to motor and premotor cortical areas.
List the receptors of the somatosensory system.
PROPRIOCEPTION:
- Muscle spindles
TACTILE AFFERENTS (discriminative touch):
Cutaneous:
- Meissner’s corpuscles (rapidly adapting)
- Merkel’s discs (slowly adapting)
Deep:
- Ruffinni corpuscles (slowly adapting)
- Pacinian corpuscles (rapidly adapting)
FREE NERVE ENDINGS (low-resolution tactile, temperature, pain):
- A delta fibres
- C fibres
