Anterolateral system neuro PDF

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Summary

Dorsal column system Anterolateral system...

Description

 Anterolateral system (SPINOTHALAMIC PATHWAY SYSTEM)  This is the pathway for this system from the dorsal root ganglia from the receptors.  They enter the spinal cord at the posterior horn then they synapse. Usually they go one or two segments up or one or two segments down.  They cross to the other side and they cross anterior to the central canal of the spinal cord.  They ascend either in the anterior division of the anterior column of the spinal cord or they ascend in the lateral column, that’s why they called Anterolateral spinothalamic.  they go to the ventrobasal part of the thalamus which is VPL+ VPM [ventroposterior lateral and ventroposterior anterior ]  For pain (for pain it goes to intralaminar nuclei)  And from there they go to the postcentral gyrus which is the primary somatosensory area, these are the tactile sensation .  when we talk about pain we will talk more in details about its pathway regarding brain ‫مش متأكدة شو الكلمة اللي هون اخر وحدة المدقق تأكد منها‬ 

The Anterolateral (spinothalamic) pathway (the figure in the next page)

 It conveys nerve impulses for: crude touch)crude: poorly localized), pressure, pain cold and warm temperature ,itching and tickle sensation (specially tickle sensation is important for sexual sensation )

 From: the limbs, trunk, neck and posterior head of the postcentral gyrus to the cerebral cortex.

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 So in the figure, receptors for pain and temperature, crude touch , crude pressure, tickle and itching , they enter the spinal cord at the posterior horn .  In the posterior horn the (1st order neuron) synapses with (2nd order neuron) sometimes they go one segment up or one segment down then they synapse in the posterior horn  After they synapse they cross to the other side and they ascend in the lateral and anterior column to the thalamus.  So this system is different than the other system because it has broad spectrum sensation.  the other system has Mechanoreceptive sensation, no pain and temperature , has different modality  pain is nociceptive modality and temperature is thermoreceptive modality modality is the type of sensation

 so the first order neuron synapse at the level of the spinal cord.  the second order neuron synapse at the level of the thalamus.  third order neuron goes to the

primary somatosensory area in the postcentral gyrus.

Dorsal column system  well localized.  Better temporal time and special place 2|Page

faithfulness or organization or orientation ,because the speed of transmission is very high at speed 30 to 120m/sec.  Better intensity gradation (more than 100 compared to less than 20 for anterolateral) the dorsal column uses the absolute grades (0/1/2/3…100)  Repetitive stimuli are only confine to dorsal column tracts because they transmit at higher speed if it's higher speed you can have rapidly adapting sensation.

orientation (8-40 m/sec )  the anterolateral system use the letter grades (A /B/C/D ) so the letter grades has much lower grade than the absolute  transmitting too fast ,too slow they cannot discriminate different intensities of small difference  Transmit broad spectrum modalities of sensation [ nociceptor(pain) + temperature + mechanoreceptor (touch , pressure) .

 comparison between the two systems

 Trigeminothalamic Pathway

 from the face there is another system called trigeminothalamic system , trigeminal which is the fifth cranial nerve .  transmits all the sensation from both systems dorsal column and anterolateral column.  it conveys nerve impulses for most somatic sensation from face and nasal cavity, oral cavity and teeth to the cerebral cortex.  It is synonymous to both sensory systems.  then they go to the nuclei of the

 This is a comparison between these two tracts

 I

t is very important to know where is the crossing : Dorsallateral Column-Medial Lemniscal Anterior spinothalamic pathway  Dorsal column :

 fine receptors touch fine for pain pressure temperature proprioception crude the crossing for the dorsal column is at the level vibration of the medulla so if theenter damage or touch crude andpressure Stereognosis the spinal cord transection is found below the area of the medulla thefasciculus loss of sensation ipsilateral  in. the gracile oris fasciculus same side of the transection same side of the damage. goes tointhe crosses to the they synapse themedulla spinal cord crosses to  cuneate other side ascends the other side . to the thalamus then toand thelateral posterior  ventrobasal they ascendVPL,VPM in the anterior aspects of the spinal cord ,go to the postcentral gyrus of the primary :  Anterior lateral spinothalamic pathway thalamus partside andon intralamina somatosensory and If the damage is at half the level of the spinal cord the loss ventrobasal is oncortex the other the imary  this is for fasciculus cuneatus transmits the contralateral somatosensory information fromcortex. the upper half of the body  This is the gracile fasciculus transmit the information from the lower half of the body 4|Pa

 But if the damage is above the level of medulla the loss of sensation in both system is contralateral on the other side of the transection or damage ,so this differentiate these two systems.

somatic sensory cortex  Located in the postcentral gyrus  Highly organized distinct spatial orientation  Each side of the cortex receives information from the opposite side of the body contralateral aspect  Unequal representation of the body o lips have greatest area of representation followed by the face and the thumb because they have the highest density of receptors (↑ number of receptors ). o trunk and lower body have the least area.

 Cellular Organization of the Cortex  Six separate layers of neurons in cerebral cortex with layer I (outside) near the surface of the cortex and layer VI (inside)deep within the cortex.  Incoming signals enter layer IV and spread both up and down.  Layers I and II receive diffuse input from lower brain centers.  Layer II and III neurons send axons to closely related portion of the cortex presumably for communicating between similar areas.  Layer V and VI send axons to more distant parts of the nervous system, layer V to the brainstem and spinal cord, layer VI to the thalamus. layer 6, 5 are different they go from there to the either brainstem and spinal cord layer 5 more distant to the thalamus layers 2 and 3 they send impulses presumably for communication between similar area

I + II Diffuse lower input III

IV

Related brain areas

Incoming signals

V To brainstem and spinal cord Va

To thalamus

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 Within the layers the neurons are also arranged in columns.  Each column serves a specific sensory modality (i.e., stretch, pressure, touch).  Different columns interspersed among each other. So they can have certain communication between each other o interaction of the columns occurs at different cortical levels which allows the beginning of the analysis of the meaning of the sensory signals . because sensor signals might be interrelated.

 Function of the Somatic Sensory Cortex Destruction of somatic area I results in:  loss of discrete localization ability.  inability to judge the degree of pressure . how much the pressure is, but still if this is lost still you might feel that there is sensation but you lose its localization

 inability to determine the weight of an object.  inability to determine the shape or form of objects, called Astereognosis. and this is bad for blinds because of they use this kind of sensation for reading . A ‫للنفي‬

 inability to judge texture.

 Somatic Association Areas  Located behind the somatic sensory cortex in the parietal area of the cortex . behind each primary area there are lateral lobe behind it there are the secondary or association somatic area, it has higher level of interpretation, it interprets the meaning of the primary sensation .  Association area receive input from somatic sensory cortex, ventrobasal nuclei of the thalamus (VPL ,VPM) , visual and auditory cortex. Ventral posteromedial (VPM) and the ventral posterolateral (VPL).

 Function is to decipher sensory meaning. give a meaning for that kind of sensation , for sensation it makes a good localization, orientation from where is this area compared to the other area and forming three-dimensional position 6|Page

 Loss of these areas results in the inability to recognize complex objects and loss of self (Amorphosynthesis). That person unable to form a shape of the objects he sense

 Special Aspects of Sensory Function  Thalamus has some ability to discriminate tactile sensation. because some of the neurons they end in the thalamus

 Thalamus has an important role in the perception of pain and temperature. so if there is damage to the postcentral gyrus still that person can feel pain and can feel temperature but very little regarding tactile sensation.

Corticofugal fibers : descending fibers come from the cerebral cortex downward to the spinal cord or down to the brainstem areas negative feedback fibers , function:  fibers from the cortex to the sensory relay areas of thalamus, medulla and spinal cord.  these fibers are inhibitory, they can suppress the sensory input.  function to decrease the spread of a signal and sharpen the degree of contrast and adjust the sensitivity of the system. o they can remove the adaptation (negative feedback) and they might go to the level of sensory sensation to increase its sensitivity ,contrast increase and sharpness .

 This is some kind of coding

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Coding for the intensity of the stimulus as percieved at the cerebra cortex:

The two points discrimination:

if the stimulus is weak then it is able to it has a lateral inhibitory pathway stimulate the central part of the cerebral so these 2 systems inhibits laterally cortical cells here if this number of cortical cells are stimulated then the impulse is (so in the center we have inhibition perceived as weak stimulus ,if the stimulus is by lateral inhibitory circuits) and in stronger it will diverge and able to stimulate the two red peaks ( lateral larger number of cortical cells ,then that stimulation ). person will interpret this as a higher intensity , it is going to be interpreted as very strong stimulus so coding for intensity of the stimulus is perceived as a number of neurons that are stimulated also the number of impulses(frequency of action potential) . een stimulus strength (power law) e

  

stronger stimulus is perceived as stronger, due to the higher number of impulses. The red curve represents the number of impulses. Note that the power law does not hold at either very weak or very strong stimulus strength.

highest sensitivity : any change in the strength will have too much change in the number of .

 Processing of position sense  When you’re extending the limb (moving to higher degree of extension) the impulses of some neurons in the thalamus (1,2,3) decreases, on the other hand the impulses of neuron 4 and 5 increases >> the system knows this is an extension  The opposite happens regarding to flexion. (the impulses of some neurons in the thalamus (1,2,3) decreases, on the other hand the impulses of neuron 4 and 5 increases)

EXAMPLE : At degree 140 :the level of impulses at neuron5= 40 / at neuron4 ≈50 ,at these numbers our system knows that this is 140 degree of extension At degree 80 has the same number at neurons 1+2 but a different number neuron 3 so our system knows that this is flection.  this combination is specific for each degree .  When you change the angle of the joint, some neurons transmit more impulses while other neurons transmit less impulses; this combination of the number of impulses in these neurons(maybe 100 neurons), your system will interpret it as flexion or extension.

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