Exteroception Flashcards
Somatosensory system
collection of receptors whose endings transduce information about the body surface, the skin, although deeper tissues reported as well
2 major subdivisions of somatosensory system
dorsal column (posterior column) or lemniscal system (fine, discriminative, touch perception, limb prioprioception) 2) antero-lateral system (pain/temperature/crude touch)
type s of skin sensory receptors
- cell bodies in DRG
- Merkel’s disk, Pacinian corpuscle, Meisner’s corpuscle, free nerve endings (Ruffini’s endings, hair follicles)
Rapidly adapting receptors
when stimulated with steady touch fire a few APs then stop firing even though sin still pressed
slowly adapting receptors
when stimulated with steady touch, will keep firing throughout time the skin is pressed
Receptive field for skin receptor
area of skin in which a mechanical stimulus elicits a response from the cell
how are somatosensory receptors classified
according to slow vs rapidly adapting, size of receptive field (small fields/sharp borders vs large, poorly defined fields)
- Large - covers entire finger/greater part of palm
Meissner’s corpuscles
correspond to rapidly-adapting, afferents with small receptive fields
Merkel’s discs
afferents innervating these have small fields and slow adapting
Pacinian corpuscles
afferents with large reeptive fields lie deep in dermis and subcutaneous tissues; rapidly adapting
Ruffini endings (free nerve endings)
afferents with slowly adapting, large receptive fields
Small receptive field afferents
- support fine, tactile sense of fingertips
- higher density of Corpuscles/Merkel’s discs
Large receptive field size afferents
- hgih sensitivity to skin deformation over wide area
- Pacinian corpuscles, (vibration)
- Ruffini’s ending (stretch)
Rapid adapting sensory receptors
Meissner’s Corpuscle, Pacinian corpuscle
Slow-adapting sensory receptors
Merkel’s disk, Ruffini’s endings
Hair follicle receptors
dermal tissue around each follicle penetrated by several myelinated axons that produce several unmyelinated branches; some run up/down follicle and others spiral around
- bending of hair shaft activates terminals –> rapidly adapting
Types of axons for mechanoreceptors
large, myelinated axons (Aβ) but smaller/slower than proprioceptors in skeletal muscle
Types of axons for proprioceptors of skeletal muscle
Aα (larger/faster than mechanoreceptors in skin)
Locaiton of cell bodies of mechanoreceptors from skin/proprioceptors
DRG; centrally projecting processes in dorsal root enter spinal cord– bifurcate–local branches prject to dorsal horn for spinal reflexes adn ascending branches enter dorsal column toascend spinal cord
- those from lower limbs– ftravel in fasciculus gracilus
- those from upper body/limbs run in fasciculus cuneatus
where is the synapse onto second order cells in lemniscal system
medulla; axons then cross medulla to form medial lemniscus –> ascend to synapse in ventro-basal complex of the thalamus in the ventra-posterior-lateral nucleus
where does the second synapse ocur in the lemniscal system
Thalamus!
- trunk and limbs represented by cells in ventral-posterior-lateral nucleus
- head represented by cells in ventral-posterior -medial nucleus
- together VPM and VPL form ventrobasal complex
Where do from ventrobasal complex asons project
areas 3, 1, and 2 on posterior bank of central sulcus
Where do axons go from ventrobasal complex
(this is in thalamus) and the axons go to the primary motor cortex
areas richly innervated by primary afferents…
have relatively larger areas of cortical surface devoted to them (homonculus has larger hands and face, esp lips, and relatively small trunk and proximal limbs)
How are functionally related cells organized
columnar organization of cortex in virtually all areas; interconnections– imply cortical columns serve as computational modules that transform information received from the thalamus and redistribute to other regions of the brain
where do different layers of cortex project/receive from
VI: to thalamus
V: projects to other subcortical structures
IV: from thalamus
III: project to other areas of somatosensory cortex
II: to other areas of somatosensory cortex
(II/III- to ipsilateral SII, contralateral SI, posterior parietal cortex, and motor cortex)
Brodmann’s areas 1/2
respond to more complex stimuli (direction-, orientation-0, shape-sensitive
Brodmann’s areas 3a 3b
simple punctate stimulation
- 3a= proprioception
- 3b = touch
Brodmann areas corresponding to somatosensory cortex
1, 2, 3
Somatosensory area II
posterior/inferior to Somatosensory Area I – tend to have more complex stimulus requirements than those in SI
Trigeminal ganglion
“DRG for the head”
afferent fibers from trigeminal ganglion run..
peripherally through 3 branches of trigeminal nerve (opthalmic, maxillary, mandibular)
What senses carreid by trigeminal nerve
discriminative touch (lemniscal) proprioceptive and pain and temperature, some motor and autonomic neurons
soma of mechanoreceptors located where?
trigeminal ganglion; these terminate in different nuclei corresponding to connections of analogous spinal cord brethren
Where are cell bodies of proprioceptive afferents (incl muscle stretch receptors) that run in trigeminal
Centrally located cell bodies (only ones in adult CNS) that make up trigeminal mesencephalic nucleus
(cell bodies for muscles of mastication in motor trigeminal nucleus)
Barrels
group of vertical cells in cortex with similar function
