Functional Organization of the Nervous System

How to stydy the anatomy of the brain:

  • Histological staining

  • Radiological → in alive patients

  • Gross

How to study physiology:

  • Studying the lesions→ abnormal functions

  • Special techniques:

    • Electrical Study of PNS → Electrodiagnostic study

    • Electrical study of CNS: Electroencephalogram

    • Brainstem auditory evoked potential

    • Visual evoked potential

    • PET Scan: Glucose uptake in each part of nervous system

    • Functinal MRI: oxygen utilization/ blood flow of the brain.

Functional Organization: The Big Picture

  • Ascending Pathway: Sensory

  • Descending: Motor & Autonomic

  • Reflexive Circuit: go to level of spinal cord and leave without processing in the brain.

Sensory System:

  • Somatosensory sensation: mostly mechanosensation and proprioception (and joint position)

  • different kinds of receptor are designed to sense specific stimulus.

  • ex. Meissner’s corpuscle

  • ex. Free nerves are for pain and temperature.

  • requires different types of axon to transfer electric signal to brain ( A alpha, A beta, C, etc.)

  1. Special Sensory System

2 Major Sensory Pathways:

  • Posterior column sensory pathway (Dorsal column-medial leminiscal pathway)

    •  Pathway (see slide 9): stimulation → conveyed through axon → spinal cord → barely enter dorsal horn but travel to posterior part/ dorsal column of spinal cord (of greath lengths = very long and neuron is unipolar) → synapse with 2nd order neuron (multipolar shape) → project its axon to another side (opposite projection: decussation) → project up to Thalamus → primary somatosensory cortex in thalamus.

    • Gracile Fascicle: sensation from arm (latin = thin)

    • Cuneate Fascicle: sensation from leg (latin = wedge)

      • Gracile nucleus and Nucleate nucleus: 2nd order neuron in the medulla.

    • Medial Leminiscus

    • Thalamocortical tract.

Spinothalamic sensory pathway

Pain & Temperature: different pathways: sense by free nerve endings → synapse with neuron locally at that particular level (+-1 level) → cell bodies in PNS lies in: Dorsal Horn or Dorsal Root Ganglia?

Thalamus & Cortex

  • 2 cortical area mapping as specific areas of the cortex responsible for perception: does not represent real representation of the body: Somatotopic Organization/Sensory Homunculus.

  • Cortical processing after homunculus will be perception part: wide network cortical processing not within a single area.

Facial Somatosensation

  • Trigeminal Nerve and Ganglion

  • enter via brainstem

Special Sensation

  • each has their own anatomical pathway, no spinal cord involved

  • most run through thalamus and synapse with 2nd order neuron

2. MOTOR CONTROL

  1.     Voluntary Motor System:

    • Frontal lobe: controlled movement output

      • Anterior: more abstract and complex concept of movement (planning)→ primary motor cortex

    • Motor Homunculus

    • Betz Cell/ Giant Pyramidal Neuron

    • Pathway (see slide 16): Primary motor cortex → Precentral Gyrus → Posterior limb of internal capsule → Pyrimidal decussation (90% of all trunks) → projects downward → lateral column of spinal cord (lateral cortico spinal tract) → reaches level (ex. C8) will synapse with lower motor unit neuron which is anterior horn cells.

    • Facial Movement: lower motor neurons in cranial nucleus. Controls facial muscles, tongue, and pharynx, and eye movement. Pyramidal neurons send axon to synapse with/at facial nucleus?? at Pons??? and with what and Medulla??? (left image slide 19) . At lower motor neuron level → decussates. Coticobulbar tract: bilaterally projects to both sides of the nucleus.

    • Ocular movement: pyramidal neuro does not lie with others at the primary motor cortex but at Frontal Eye Field. Different pathway from other tracts.

    • Refinement of motor signal: cerebellar: precision and smoothen muscle, Basal Ganglia: optimize movement (reduce unnecessary and add deficit)

    • Withdrawal Reflex: skin: receive dangerous stimulus (step on sharp objects): free nerve → processing in spinal cord at that level within a few neurons

    • consciously processing noxious stimulus → pass through spinal cord

    • Deep Tendon Reflex: Tap tendon forcefully/ any skeletal muscle → activate deep tendon organ → sense change of muscle tension → activate golgi tendon organ/ muscle spindles → sends signal into spinal cord → if triceps: around C7 level: monosynaptic reflex → contraction of triceps.

  1. Autonomic Signal

  • controls homeostasis of body from head to toe (ex. smooth muscles of pupils and bronchus dilatation, rate of involuntary breathing, HR, vasodilation, peristalsis of GI tract, secretion of enzymes, endocrine gland regulation, kidney function, bladder function. functions we can’t voluntarily control in the body.

  • Second order neuron for sympathetic system : in thoracic and lumbar level

  • Second order neuron for Parasym: lies in brain stem (Cranial Level) and Sacral Level

  • Slide 26 left image: ganglion beside spinal cord (sympathetic trunk) but not for parasymathetic system.

  • Parasymoathetuc nucleus lies within the brainstem ex. pupillary constriction function (parasym) 2nd neuron lies at dorsal part of mid brain, pupil dilation (sympathetic)

  • Every organ has to be controlled by both sympathetic and parasympathetic (autonomic system)

  1. Integrative System

    • integrate signals from many areas in the cortex

    • Limbic System: includes subcortical grey matter area