Myeloarchitectonics

1
Myeloarchitectonics

• Myeloarchitectonics is the schema is concerned
  with the distribution of nerve fibers in an area.
• It is the vertical pattern of connectivity that
  exists between the internal milieu and the
  extrapersonal space.
• We do not respond to every bit of sensory input
  the CNS receives.
• We have filters occurring at many stations along
  the pathways that incoming information takes at
  both conscious and subconscious levels.

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Myeloarchitectonics

• Filters are found within the spinal cord,
  brainstem, cerebellum, and cerebral sensory
  cortex, where conscious perception of sensation
  occurs.
• Motor responses to make muscles contract or
  glands secrete can be initiated at the level of
  the spinal cord (spinal reflex), the brainstem
  (medullary reflex/bulbar reflex), the cerebellum
  (cerebellar reflex), or at the motor cortex.

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Motor Pathways The Pyramidal Tracts

• The pyramidal or direct motor system is
  vertically organized.
• Pyramidal cell bodies lie in both the left and
  right motor cortices, so there is a left
  pyramidal tract and a right pyramidal tract.
• Neuronal axons descend to the brainstem or spinal
  cord to mediate the initiation and control of
  skilled voluntary movement.
• The level at which the axons exits the CNS will
  determine whether it is a corticobulbar or a
  corticospinal tract.

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Motor Pathways The Pyramidal Tracts

• The corticobulbar tract originates in the cortex
  but it terminates in the nuclei of cranial nerves
  in the pons and medulla before exiting the
  brainstem from each side.

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Motor Pathways The Pyramidal Tracts

• In humans, there are monosynaptic connections
  between corticobulbar axons and motor neurons in
  the motor trigeminal (V), facial (VII), and
  hypoglossal (XII) nuclei.
• Because corticobulbar tracts exit
  bilaterally--right tract exits both right and
  left left tract exists both left and
  right--unilateral damage to corticobulbar fibers
  on one side will produce weakness only, not
  paralysis.
• The exception is innervation to the lower facial
  muscles.

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Motor Pathways The Pyramidal Tracts

• Muscles of the lower face receive corticobulbar
  fibers from only the contralateral motor cortex.
• As a result, unilateral lesions that interrupt
  corticobulbar fibers on one side produce weakness
  only of the muscles of the contralateral lower
  part of the face.

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Motor Pathways The Pyramidal Tracts

• The corticospinal tract originates in the cortex
  and its axons descend through the brainstem on
  their way to the ventral grey horn of the spinal
  cord where they will innervate a spinal nerve.
• At the level of the medulla, most of the
  corticospinal fibers will cross (decussate) at
  the pyramids to descend on the opposite of the
  spinal cord in the lateral columnsthe lateral
  corticospinal tract.

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Motor Pathways The Pyramidal Tracts

• However, a small number of fibers will remain on
  the ipsilateral side (same) and will descend in
  the ventral columnsthe ventral corticospinal
  tract.

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Motor Pathways The Pyramidal Tracts

• In humans, the lateral corticospinal tracts
  innervate spinal nerves for distal limb muscles.
• The ventral corticospinal tracts innervate spinal
  nerves of axial and proximal muscles.

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Motor Pathways The Extrapyramidal Tracts

• The extrapyramidal or indirect motor system is so
  named because its axonal projections do not pass
  through the pyramids of the medulla.
• Instead, there is a complex system of connections
  between the cortex and other structures.

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Motor Pathways The Extrapyramidal Tracts

• Rather than initiate and guide skilled voluntary
  movement, the indirect system refines the
  accuracy of complex movements and inhibits
  unwanted movements.
• The function of the extrapyramidal tract is
  largely unconscious.
• Its tracts act automatically or involuntarily and
  are not normally subject to conscious
  modification such as that gained through practice
  or exercise.

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Motor Pathways The Extrapyramidal Tracts

• The extrapyramidal system is also passive,
  depending upon input from the brain as well as
  the skeletal muscles to function properly.
• Extrapyramidal tracts originate in diffuse areas
  of the CNS including the basal ganglia, thalamus,
  cerebellum, substantia nigra, red nucleus,
  reticular formation, and the connections between
  them.

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Motor Pathways The Extrapyramidal Tracts

• The main extrapyramidal tracts are the
  rubrospinal tract, the tectospinal tract, the
  vestibulospinal tract, and the reticulospinal
  tract.
• All of the main extrapyramidal tracts receive
  input from the cerebellum.

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The Rubrospinal Tract

• The rubrospinal tract is so named because of
  connection between the red nucleus of the
  midbrain and the spinal cord.
• Its role is to transmit impulses to skeletal
  muscles concerned with muscle tone and posture.

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The Rubrospinal Tract

• The cell bodies in the red nucleus receive
  projection axons from the cerebellum.
• Red nucleus axons cross over and then descend
  through the entire length of the opposite lateral
  white column of the spinal cord.

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The Tectospinal Tract

• The tectospinal tract is so named for its
  connection between the tectum of the midbrain and
  the spinal cord.
• Its function is to transmit impulses that control
  movements of the head in response to visual
  stimuli.

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The Tectospinal Tract

• It originates in the superior colliculus of the
  midbrain, crosses to the opposite side, descends
  in the anterior white column and enter the
  ventral gray horns.

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The Vestibulospinal Tract

• The vestibulospinal tract is so named because it
  runs from the vestibular nuclei in the medulla to
  the spinal nerves.
• The axons of the tract descend uncrossed on down
  through the anterior white column.

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The Vestibulospinal Tract

• Through this tract, the vestibular
  apparatus--which detects whether the body is on
  an even keel--exerts its influence on those
  muscles that restore and maintain upright
  posture.
• It also has a role in maintaining the normal
  position of the head.
• The head will be pulled reflexively back to an
  erect position if you lose balance.

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The Reticulospinal Tracts

• The reticulospinal tracts originate in the
  reticular formation of the medulla and pons.
• These two motor circuits govern the posture of
  the limbs and the tone of their muscles.

21
The Reticulospinal Tracts

• The pontine reticulospinal tract receives input
  from many sources, including the cortex.
• It stimulates anti-gravity reflexes and mediates
  extensor tone.
• The medullary reticulospinal tract does the
  opposite, is the antagonist, of the pontine
  tract.
• It relieves the antigravity muscles of reflex
  control and mediates flexor tone.

22
Sensory Pathways

• The two major pathways for tactile sensory
  transmission are the spinothalamic tract and the
  dorsal (posterior) column pathway.
• The spinothalamic tract has two main bundles
  the lateral column bundle and the ventral column
  bundle.
• Pain and temperature information is conveyed
  through the lateral spinothalamic tract.
• Light touch and pressure information is conveyed
  through the ventral spinothalamic tract.

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Lateral Spinothalamic Tract

• Information related to pain comes from free nerve
  ending receptors in the skin.
• The cell bodies of the first order pain neurons
  are located in the dorsal root ganglia.

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Lateral Spinothalamic Tract

• Their axons synapse with the second order neurons
  located in the dorsal grey horn.
• Second order axons cross to the contralateral
  lateral white column and ascend to the thalamus.

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Lateral Spinothalamic Tract

• The third order neuron is found in the ventral
  posterior nucleus of the thalamus.
• From their its axons project to S1 (areas 3, 1,
  and 2) of the somatosensory cortex.
• In S1, recognition of sensation takes place, but
  mapped to specific regions.
• These areas of the parietal lobe have pronounced
  somatotopic organization.

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Lateral Spinothalamic Tract

• That is, information from specific parts of the
  body are mapped to the brain in a way that
  reflects of the arrangement of the body.
• For example, information for the head is received
  in the most lateral and ventral part of the
  postcentral gyrus near where it curves in at the
  lateral fissure.

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Lateral Spinothalamic Tract

• From that point up toward the superior
  longitudinal fissure, the postcentral gyrus
  receives information from the neck, arms, hands,
  trunk, abdomen, and hip.

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Ventral Spinothalamic Tract

• Information related to light touch or pressure
  comes from encapsulated nerve ending receptors
  in the skin.
• The cell bodies of the first order pain neurons
  are located in the dorsal root ganglia.
• Their axons synapse with the second order neurons
  located in the dorsal grey horn.
• Second order axons cross to the contralateral
  ventral (anterior) white column and ascend to the
  thalamus.
• The third order neuron is found in the ventral
  posterior nucleus of the thalamus.
• From their its axons project in a somatotopic
  manner to S1 where recognition of
  non-discriminative touch occurs.

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Dorsal Column Pathway

• The dorsal column pathway is the responsible for
  carrying information about discriminative touch,
  pressure, and proprioception to the brainstem.
• As we have discussed, there are two dorsal
  columns, or fasciculi, on each side of the spinal
  cord.

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Dorsal Column Pathway

• The fasciculus gracilis which lies more medially
  (1), carries lower body information (from about
  the waist down).

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Dorsal Column Pathway

• The fasciculus cuneatus which lies more laterally
  (2) carries upper body information (from about
  the waist up to the back of the head).

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Dorsal Column Pathway

• Information related to discriminative touch or
  pressure comes from subcutaneous
  mechanoreceptors.
• Information related to proprioception comes from
  muscle spindles and joint receptors.

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Dorsal Column Pathway

• The cell bodies of the first order neurons are
  located in the dorsal root ganglia.
• Their axons enter the spinal cord and form the
  bulk of the dorsal columns.

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Dorsal Column Pathway

• The axons from these first-order neurons pass
  upward in the fasciculus gracilis or fasciculus
  cuneatus.
• Each terminates at the second-order neuron of the
  nucleus gracilis or cuneatus in the medulla.

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Dorsal Column Pathway

• The axons from the second-order neurons (nucleus
  gracilis or nucleus cuneatus) cross to the
  opposite side of the medulla and ascend to the
  thalamus through the medial lemniscus of the
  medulla, pons, and midbrain.

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Dorsal Column Pathway

• They will synapse with the third order neuron in
  the ventral posterior lateral nucleus of the
  thalamus, whose axon will convey information
  about position, movement, location of a stimulus
  to S1.

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Behavioral Specialization

• We have subdivided the entire cortical surface
  into five zones on the basis of cellular typology
  and we have looked at the vertical connections
  between them and the outside world.
• We will now examine the behavioral specialization
  associated with these zones.

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Behavioral Specialization

• The idiotypic or primary areas constitute the
  first cortical relay for input from the
  modality-specific nuclei.
• Whether the stimulus is visual, auditory, or
  somatosensory, the primary sensory areas are
  responsible for stimulus recognition
• They do not detect stimulus features per se, but
  instead look for stimulus constancies/consistencie
  s of size, shape, and position in a gestalt
  sense.

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Behavioral Specialization

• The primary sensory areas recognize relevant
  attributes of the stimulus forms and categorize
  them into more stable and permanent template
  representations to which subsequent stimulus
  attributes can be matched.
• If primary sensory areas are damaged, the
  individual is essentially cortically deaf, blind,
  or unable to feel pain, temperature, touch, or
  positional movement, although some gross
  recognition of all sensory stimuli is preserved
  at the thalamic level.

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Behavioral Specialization

• The primary motor area is a bit different in that
  it is the last step in the execution of discrete,
  precise movements in the manipulation of
  extrapersonal space.
• If these upper motor neurons are damaged,
  paralysis, weakness, or clumsiness of limbs on
  the contralateral side may be manifested.

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Behavioral Specialization

• The second stage in the analysis of sensory
  information processing takes place within the
  modality-specific (unimodal) isocortical areas.
• Unimodal sensory areas serve as perceivers of
  the stable sensory templates constructed in the
  primary areas.
• It is in the unimodal association areas that the
  experience of the stimulus occurs.
• The unimodal areas act as obligatory relays for
  the intracortical transfer of sensory information
  from primary areas to other parts of the cortex.

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Behavioral Specialization

• Consequently, there are two major classes of
  behavioral deficits reflective of disruption of
  unimodal cortex function.
• Lesions directly within the unimodal region (say
  AA) give rise to complex perceptual deficits,
  termed agnosias, in that modality (auditory),
  while elemental sensation remains in tact.
• Lesions that interfere with specific output
  fibers from unimodal areas deprive certain
  heteromodal, paralimbic, or limbic regions of
  information from that sensory modality.

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Behavioral Specialization

• Again, the motor association areas are a bit
  different.
• They are assembling the information needed by M1
  for movement execution.
• The motor association area is often described as
  the macroprogrammer of more global actions of
  multiple movements.
• Damage to this area results in specific
  disturbances of movement without accompanying
  weakness, clumsiness, or dystonia.

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Behavioral Specialization

• The heteromodal areas almost never receive their
  sensory information directly from the primary
  areas.
• Instead, unimodal association areas act as
  obligatory relays.
• In the heteromodal stage of processing, the
  attributes of separate modalities can not be
  inter-related for elaboration.
• Analysis of sensory experiences no longer
  confined to a single modality.
• Modality specificity is lost in favor of
  intermodal association.

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Behavioral Specialization

• There is no longer a distinction between what is
  motor and what is sensory.
• Two essential transformation are likely to occur
  in heteromodal areas.
• Neural templates for intermodal associations are
  formed for many cognitive processes, such as
  language.
• Extensively processed sensory information can not
  interact with limbic-paralimbic input.

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Behavioral Specialization

• Mood and drive can now influence the manner in
  which the self and the world are experienced and
  thought and experience can now influence mood.
• Damage to heteromodal areas in the parietal,
  temporal, occipital region, result in complex
  disorders such as anomia, alexia, dysgraphia,
  acalculia, but in the absence of a generalized
  language comprehension deficit.
• In addition to these cognitive integrative
  deficits, affective disturbances can also emerge
  in mood alterations and agitated confusional
  states.

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Behavioral Specialization

• When damage occurs to prefrontal heteromodal
  areas, there are dramatic alterations in
  comportment and personality.
• Some individuals may become puerile, slovenly,
  inappropriately jocular, grandiose, and
  irritable.
• Others may become apathetic and have profound
  slowness of thought processes.
• Patients may also show an erosion in foresight,
  judgment, insight, as well as in abstract
  reasoning abilities.

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C. Behavioral Specialization

• They may jump to premature conclusions, or become
  excessively stimulus bound.
• Marked disruptions are also evident in planning
  and sequencing of complex behavior, in the
  ability to multi-task, and in the capacity to
  grasp the gist of a complex situation.
• In contrast, motor dexterity, perceptual
  abilities, memory, language and most other
  cognitive faculties remain in tact.