NROC64H3 Study Guide - Thalamus, Cortical Map, White Matter
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Sensory and Motor Systems
Mechanoreceptors also vary in the persistence of their resPonses to long lasting stimuli
Meissners and Pacinian corpuscles respond quickly at first but then stop firing even though the stimulus
continues ---these receptors are called rapidly adapting ---Local / government MP'S are rapidly adapting!
Merkel's and Ruffinis are slow adapting and generate a sustained response
hairs are part of a sensory receptor system
for most animals, hair is a major sensory system
i.e a rat , navigates by using its facial vibrissae (whiskers)
hairs grow from follicles embedded in the skin
each follicle is richly innervated by free nerve endings that either wrap around it or run parallel to it.
there are several diff types of hair follicles including some with erectile muscles (i.e facilitate goosbumps)
and the details of their innervation differ
in all cases, the bending of hair cause a deformation of the follicle and surrounding skin tissues...in turn
stretches, bends or flattens the nearby nerve endings which then respond appropriately by increasing or
decreasing their firing frequency. mechanorecpetors of hair follicles may be either slowly adaptign or
diff mechanical sensitvities of mechanoreceptors mediate diff sensations
Pacinian corpuscles are most sensitive to vibrations of baout 200-300 Hz
Meissners corpuscles around 50Hz
VIBRATION AND PACINIAN CORPUSCLE
selectivity of a mechanoreceptor axon depends on primarily the structure of its special ending
oval shaped capsule with 20-70 concentric layers of an onion, with a nerve terminal in the middle
when the capsule is compressed, energy is transferred to the nerve terminal, its membrane is deformed
and mechanosensitive channels open.
current flowing through the channel generates a receptor potential, that is depolarizing.
if depolarizing is large enough, the axon will fire an AP.
But the capsule layers are slick with vsicous fluid in between them.---> therefore,if stimulus pressure is
maintained, the layers transfer the stimulus energy in such a way that the axon terminal is no longer
deformed and receptor potential dissipates.
when pressure is released, the events reverse themselves, the terminal depolarizes again and may fire
another action potential.----> rapidly adapting
When pac.corpuscle is stripped away of its capsule, the naked nerve terminal becomes much less
sensitive to vibrating stimuli and more sensitive to steady pressure.
--> therefore, it is the layered capsule that makes the Pac. corpsucle sensitive to vibrating, high frequency
stimuli and unresponsive to steady pressure
Primary Afferent axons
-skin is richly innervated by axons
-Primary Afferent axons: axons bringing information from somatic sensory receptors to the spinal cord or
they enter the spinal cord through the dorsal roots, their cell bodies lie in the dorsal root ganglia
have widely diff diameters, their size correlates with teh type of sensory receptor to which they are
axons from sensory receptors are designated as Aalpha, Abeta,Agamma and C, ordered in decreasing
axons of similar size but innervating msucles and tendosn are called Groups 1, II and III and IV
sensory skinnenrves lck tehalrgets Aalpha group of axons
Group C or IV axons are UNMYELINATED axons
all the rest are myelinated
the smallest axons called C fibers have no myelination and are smaller in diameter
C fibers mediate pain and temperature sensation
they are the slowest of axons,conduct at about 0.5-2 m/sec
touch sensation are mediated by large Abeta axons which can conduct up tp 75m/sec
THE SPINAL CORD
many spinal nerves communciate with the CNS via the spinal cord
spinal cord is encased inthe bony vertebral coloumn
Segmental organization of the spinal cord
arrangement of paired dorsal and ventral roots is repeated 30 times down the length of human spinal cord
each spianl nerve cosnists of dorsal root and ventral root axons
each spinal nerve passes through the notches between the vertebrae of teh spinal cord
there are as many spinal nerves as there are notches between vertebrae
the 30 spinal segments are divided into 4 groups:
the segmental organization of the spinalnerves and the sensory innervation of the skin are related
dermatome: area of skin innervated by the right and left dorsal roots of a single spinal segment
therefore, there is a one to one correspondence between dermatomes and spinal segments
organization of teh dermatomes is best revealed when one bends over to stand on both hands and feet
when a dorsal root is cut, the correspondign dermatome on that side of body doesnot lose all sensation.
residual somatic sensation
adjacent dorsal roots innervate overlappign areas....> to lose all sensation in one dermatome, 3 adjacent
dorsal roots must be cut
On the other hand, skin that is innervated by axons of one dorsal root...> shingles in which the neurons
ofa singel root ganglion become infected with virus.
spinal cord in an adult ends at about the level of third lumbar vertebra
bundles of spinal nerves streaming down within the lumabr and sacral vertebra column are called cauda
equina ;"horses tail"
teh cauda equina courses down the spinal coloumn with in a sack of dura mater filled with CSF (cerebral
used to collect CSF for medical diagnosis
a needle i sinserted at the midline
if teh needle is inserted alittle off centre, a nerve can be touched...> causes a sensationof sharp pain in
Mechanoreceptors also vary in the persistence of their responses to long lasting stimuli. Meissners and pacinian corpuscles respond quickly at first but then stop firing even though the stimulus continues ---these receptors are called rapidly adapting ---local / government mp"s are rapidly adapting! Pacinian corpuscles are most sensitive to vibrations of baout 200-300 hz. Vibration and pacinian corpuscle selectivity of a mechanoreceptor axon depends on primarily the structure of its special ending. When pac. corpuscle is stripped away of its capsule, the naked nerve terminal becomes much less sensitive to vibrating stimuli and more sensitive to steady pressure. -> therefore, it is the layered capsule that makes the pac. corpsucle sensitive to vibrating, high frequency stimuli and unresponsive to steady pressure. Group c or iv axons are unmyelinated axons all the rest are myelinated the smallest axons called c fibers have no myelination and are smaller in diameter.