II' Neurological Neuroanatomy

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II. Neurological Neuroanatomy

• Lecture 3

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Spinal Cord Tracts

• Somatosensation refers to pain, temperature,
  touch, and proprioception experienced as arising
  from the body.
• It begins with specialized receptors in the skin,
  muscles, joints, and blood vessels that convert
  sensory stimuli to neural signals and transmit
  them to the parietal lobe for conscious
  perception.

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Spinal Cord Tracts

• All somatosensory pathways are structurally
  organized in such a manner that a given sensory
  impulse enters the CNS, crosses the midline, and
  then ascends to the sensory cortex.
• This transmission of the sensory impulse is
  performed by three neurons and their fibers.

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First Order Neurons

• The first order neuron has its cell body in the
  dorsal root ganglion and its dendritic receptors
  at the periphery.

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Unipolar Neurons

• Unipolar neurons are found in the dorsal root
  ganglion (DRG).
• The cell body resides in the DRG.
• Its one pole splits into two processes.

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Unipolar Neurons

• The peripheral process heads out to the skin,
  bone, or whatever needs to be sensed.
• The central process heads into the spinal.

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Second Order Neurons

• Depending on the modality of sensation, the
  second order neuron is found either in the spinal
  cord or brainstem.

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Second Order Neurons

• The axon of the second-order neurons always cross
  the midline and ascend to the opposite thalamus.

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Third Order Neurons

• The thalamus contains the third order neurons.
• Their axons project to the primary (S1) sensory
  cortex.

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Spinal Cord Tracts

• The sensory pathways within the spinal cord are
  mixed.
• Pathways that mediate pain, temperature, and
  crude touch decussate in the spinal cord and
  proceed rostrally on the contralateral side.
• Pathways that mediate discriminative touch have
  axons that proceed up the ipsilateral side of the
  cord to decussate later at the medullary level.

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Spinal Cord Lesions

• With a spinal lesion, there will be a loss of
  discriminative touch on the ipsilateral side of
  the body below the level of the interruption and
  a loss of pain and temperature sensations on the
  contralateral side of the body.

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Spinal Reflexes

• The spinal cord is a major element in the
  regulation of sensorimotor functions.
• Many stereotyped motor responses (reflexes),
  which are largely independent of voluntary motor
  control, are generated in the spinal cord.
• They can be triggered by cortical and/or
  environmental stimuli.

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Coordinated Motor Activity

• The spinal cord also relays sensory information
  to the cerebrum, which is vital to learning
  skilled and coordinated motor activity.

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Spinal Motor Neurons

• The ventral grey horns of the spinal cord contain
  motor nerve cells which project through the
  anterior roots to activate muscles, glands, and
  joints.

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Spinal Motor Neurons

• Specifically, the spinal grey matter contains
  thousands of a- (alpha) and ?- (gamma) motor
  neurons.
• Both a- and ?-nerve cells are called lower motor
  neurons (LMN) to distinguish them from the motor
  neurons originating from the cortex.

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Spinal Motor Neurons

• The a-motor neurons are the major motor neurons
  of the spinal cord.
• They are large and numerous and each innervates
  on average more than 200 muscle fibers.

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Spinal Motor Neurons

• Their axons pass through the ventral spinal root
  to innervate the skeletal muscles responsible for
  voluntary and reflexive movements of the head,
  trunk, and extremities.
• These motor neurons are also the final common
  pathway for efferent impulses from the CNS which
  must pass through these cells before activating
  muscles.

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Spinal Motor Neurons

• The ?-motor neurons lie alongside the a-motor
  neurons in the spinal ventral horns.
• They are smaller and half as numerous.

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Spinal Motor Neurons

• The primary role of ?-motor neurons is to
  modulate skeletal muscle tone.
• Tone within skeletal muscle is controlled via a
  receptor called the muscle spindle.

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Muscle Make-Up

• Muscles consist of extrafusal and intrafusal
  fibers.
• Extrafusal fibers make up the large mass of the
  skeletal (striated) muscle.
• Striated muscles are composed of myosin filaments
  which are responsible for contractility of the
  muscle.
• Intrafusal fibers contain muscle spindles.

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Muscle Make-Up

• Muscle spindles consist 2-10 specialized muscle
  fibers enclosed in a connective tissue capsule.
• The ends of these fibers are attached to
  extrafusal muscles and contain myosin filaments
  so they are contractible.
• Whenever a skeletal muscle is stretched, the
  muscle spindles are stimulated.
• They detect changes in the length of muscle
  fibers and the rate of change at which the
  muscles are lengthening.

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Muscle Make-Up

• As the spindles stretch, either through skeletal
  muscle contraction or through innervation by a
  ?-motor neuron, a surge of sensory input is
  directed to the a-motor neurons.
• They stimulate the muscle mass to contract
  reflexively, halting the stretch of the spindles.

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Muscle Stretch Reflex

• Lets look at this muscle stretch reflex by using
  the knee jerk reflex.
• This is a classic monosynaptic or two-neuron
  reflex.
• The knee jerk reflex is initiated by tapping the
  patella (knee cap) tendon of the femoral muscle
  with a reflex hammer.

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Muscle Stretch Reflex

• A tap of the tendon stimulates the sensory
  endings of the femoral muscle spindles.
• The muscle spindles send afferent projections to
  a-motor neurons which cause a quick contraction
  (muscle jerk) of the extrafusal muscle fibers.
• The reflexive contraction of the muscle restores
  it to a resting position, decreasing the sensory
  spindle stimulation.

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Myoneural Junctions

• Acetylcholine is the major chemical messenger of
  the peripheral nervous system.
• It is released by the efferent spinal fibers at
  the myoneural junction.
• Acetylcholine regulates voluntary or reflexive
  motor movements.

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Myoneural Junction Disorders

• A diminished effect of acetylcholine occurs in
  myasthenia gravis and similar disorders
  associated with muscle weakness.
• Weakness can be caused by either excessive action
  of the enzyme acetylcholinesterase or inadequate
  release of acetylcholine at the myoneural
  junction.

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Lower Motor Neuron Disorders

• Weakness, or loss of muscle power, can also
  result from lesions to the lower motor neuron.
• Diseases such as poliomyelitis or amyotrophic
  lateral sclerosis, vascular damage, or a spinal
  cord tumor can produce lesions to the cell body
  in the spinal cord ventral horn or to its axons
  in the ventral root.

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Lower Motor Neuron Disorders

• Lesions to the lower motor neuron results in
  denervation of the innervated skeletal muscle
  fibers.
• When muscle fibers are disconnected from spinal
  motor efferents, descending cortical impulses,
  and/or reflexive sensory activity, the affected
  muscle gradually degenerates.

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Lower Motor Neuron Disorders

• With no projections of motor impulses from the
  motor neuron, the muscle fibers are completely
  paralyzed for both reflexes and voluntary muscle
  movements.
• The muscle tone becomes flaccid.
• Denervated muscle fibers pass through several
  stages.

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Lower Motor Neuron Disorders

• There is a brief period of hyperexcitability and
  spontaneous firing called fibrillation.
• Fibrillation is the contraction f individual
  denervated muscle cells contracting under the
  influence of acetylcholine circulating in the
  blood.

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Lower Motor Neuron Disorders

• If skeletal muscle cells are not reinnervated
  within 6 months, they will die and be permanently
  replaced by scar tissue.
• The muscle then atrophies or loses bulk.
• Destroyed LMN cell bodies are not replaced.
• However, destroyed LMN axons may regenerate and
  reinnervate their former muscle fiber targets.