Tracts Flashcards
(35 cards)
what are the 5 properties of primary afferent neurones?
- modality
- threshold
- adaptation rate
- conduction velocity (of axons)
- site and extent of peripheral termination (receptive field)
define modality
principle type of adequate stimulus that is transduced into an electrical signal by a primary afferent neuron
define threshold
threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential
low threshold units: respond to low intensity stimuli (non-painful and non-damaging)
high threshold: respond to high, but not normally low intensity stimuli. painful but subject to modulation e.g. thermal, chemical and polymodal nociceptors
define adaptation
slow vs rapid adaptation
rapidly acting dynamic response only responds to changing stimulus whereas slow is continuous information to CNS
define conduction velocity
axons from skin:
- Aa (proprioceptors of skeletal muscle)
- Ab (mechanoreceptors of skin)
- Ao (pain, temperature)
- C (temperature, pain, itch)
define receptive field
the Rf of an afferent neuron is the region that when stimulated an adequate stimulus causes a response in that neuron
where do sensory neurones travel to?
travel to the dorsal root to enter gray matter of the spinal cord
describe segmental organisation of the spinal cord in terms of sensory pathway
- most peripheral nerves communicate with CNS through spinal cord
- area of skin innervated by left and right dorsal roots of a single segment is a dermatome
- arrangement of paired dorsal and ventral roots is repeated 31 times
describe sensory input to the spinal cord?
spinal cord consists of grey (cell bodies and sensory afferent terminals) and white matter (ascending and descending tracts)
- gray matter - dorsal and ventral horns and 10 distinct laminae of rexed
what is the ascending somatosensory pathway?
made up of the:
- dorsal column medial lemiscal pathway
- anterolateral system e.g. spinothalamic tract
what is the DCML pathway?
the major route by which touch and proprioceptive information ascends to the cerebral cortex
sensory pathway of the CNS system
as well as:
- pressure
- vibration
- two-point discrimination
from the skin and joints
describe the 2 parts of the DMCL pathway
consists of 2 parts.
1. The dorsal (post) column which runs from spinal cord to medulla
2. Medial lemniscus which runs as a continuation of the dorsal column, from medulla -> cortex
in the cortex the DCML pathway projects onto the primary somatosensory cortex of the postcentral gyrus.
here sensation location is ‘mapped’ using a somatotopic arrangement, represented as a homunculus.
what is the dorsal column formed by?
2 large fasciculi (bundles of nerve fibres) running through the posterior spinal cord:
- fasciculus gracilis
- fasciculus cuneatus
these fasciculi gather sensory info from body’s periphery via skin and joint receptors, + send this info to superior cerebral structures
Name 3 of the most important conscious cutaneous receptors for the DCML system?
- ruffini endings (bulbous corpuscles): detect tension deep in the skin and connective tissue or fascia
- merkel nerve endings (merkel discs): detect sustained pressure
- lamellar corpuscles (Pacinian corpuscles): detect rapid vibrations (200-300Hz)
describe the DCML pathway from the start? (long!)
- when an AP is generated by a mechanoreceptor in the tissue, the impulse travels along the peripheral axons of the 1st order neuron
- the 1st order neuron is pseudounipolar in shape with its body in the dorsal root ganglion
- the input travels along its axon, through the dorsal root and into the posterior horn, via medial dorsal root entry zone
- once in spinal cord, these axons give off small collaterals in order to facilitate spinal reflexes or send part of the sensory input through spinothalamic tracts
- majority, however, will leave the dorsal horn gray matter without synapsing and enter the dorsal funiculus in order to constitute either the fasciculus gracilis or fasciculus cuneatus
- together these fasciculi ascend the spinal cord to reach lower part of the medulla oblongata.
- in the medulla, axons in the gracile fasciculus synapse with gracile nucleus and cuneate fasciculus synpase w neurones in the cuneate nucleus.
- neurones in the 2 medullary nuclei are second order neurones.
- their axons cross over to the other side of the medulla and are named internal arcuate fibers. crossing is known as = sensory decussation.
- internal arcuate fibers eventually form medial lemniscus.
what information does the gracilis fasciculus carry?
- tactile and proprioceptive information from the lower half of the body - T7 to the first coccygeal nerve.
- lies medial to the fasciulus cuneatus within post half of posterior spinal cord
- lower = medial spatial relationship
does the dorsal column fibers of the fasciculus gracilis ascend and cross over?
no, they do not cross - they only ascend until they reach the gracilis nucleus
- here they synapse w 2nd order neurones (internal arcuate fibers) and run anteromedially and then cross the midline of the MIDBRAIN - to the contralateral side
- after decussation - the inernal arcuate fibers form the medial lemniscus which ascends towards the ventral posterolateral nucleus of the thalamus and then projects onwards to the primary somatosensory cortex.
does the gracilis fasciculus or the fasciclus cuneatus run most medially?
- gracilis fasciculus
what information does the fasciculus cuneatus carry?
- it carries input from the C1 - T6 spinal cord levels
- responsible for transmitting vibration, conscious proprioception and fine (discriminative) touch sensations from the upper body.
- it REPRESENTS the lateral portion of the dorsal column
where does the fasciculus cuneatus decussate?
- the fibers of fasciculus cuneatus reach the ipsilateral cunetaus nucleus in caudal medulla, where they synapse w second-order neurones
- these second order-neurones then cross over as internal arcuate fibers and form the medial lemniscus on the contralateral side
where do axons from the second-order neurons go after decussation at the level of the medulla?
- travel up the brainstem as the medial lemniscus on the contralateral side
- the medial lemniscus terminates and sypnases w third-order neurones - located in the ventral posteronucleus (VPLN) of thalamus within each hemisphere
after the third order neurones have sypases with medial lemniscus where does it go?
- axons of third-order neurones ascend from VPLN, through the posterior 1/3 of the posterior limb of the internal capsule, and then through corona radiata, to reach the primary somatosensory cortex of the post central gyrus in the parietal lobe.
what pathway allows us to have well developed discriminative sensation/touch in the fingers of humans?
- DCML pathway
- it provides us w the ability to detect fine textures and to determine an unknown object by its shape, using our hands w/o visual or auditory input
DCML pathway
- damage below the crossing point will cause symptoms on what side of the body?
it will cause symptoms on the same side of the body as the lesion
- if the damage occurs above the level of decussation, then the symptoms will show up on the OPPOSITE side of the body
