Notes on CNS Structure and Function

Learning Objectives

  • Topographical organization of motor neurons in the ventral horn

    • Medial to lateral organization: Trunk, proximal, and distal motor neurons
    • Ventral to dorsal organization: Extensors to flexors

  • Major divisions and subdivisions of the brain

    • Forebrain (Prosencephalon): Divided into Telencephalon and Diencephalon
    • Midbrain (Mesencephalon)
    • Hindbrain (Rhombencephalon): Comprising Metencephalon (pons, cerebellum) and Myelencephalon (medulla)

  • Primary cortical regions

    • Motor cortex: Precentral gyrus (primary motor)
    • Somatosensory cortex: Postcentral gyrus (primary somatosensory)
    • Visual cortex: Located at the calcarine fissure
    • Auditory cortex: Superior temporal gyrus

  • Meningeal layers and spaces

    • Dura mater: Tough, outermost layer
    • Arachnoid mater: Web-like, middle layer
    • Pia mater: Adheres directly to the brain, innermost layer

  • Flow of venous blood within dural venous sinuses

    • Superior sagittal sinus, inferior sagittal sinus, straight sinus, transverse sinus, sigmoid sinus where veins drain into the sinuses

The Nervous System

  • Central Nervous System (CNS): Comprises the brain and spinal cord
  • Peripheral Nervous System (PNS): Comprised of peripheral and cranial nerves

Spinal Cord Features

  • Cervical Enlargement: Spinal levels C5-T1, forming the brachial plexus
  • Lumbar Enlargement: Spinal levels L1-S3, forming the lumbosacral plexus
  • Conus Medullaris: Terminal spinal cord; located at vertebral levels L1-L2
  • Cauda Equina: Collection of spinal roots extending below the conus medullaris

Spinal Segments

  • A total of 31 spinal segments and pairs of spinal nerves:
    • Cervical (C): 8
    • Thoracic (T): 12
    • Lumbar (L): 5
    • Sacral (S): 5
    • Coccygeal (Co): 1

Spinal Segment Organization

  • Dorsal Horn: Contains sensory neurons that receive input
  • Ventral Horn: Mostly motor neurons responsible for motor output
  • Dorsal and Ventral Roots:
    • Dorsal Root: Sensory nerve processes bringing information into spinal cord
    • Ventral Root: Carries efferent motor fibers to muscles

Organization of Nerves

  • Median Nerve Pathway: Branching from spinal nerves contributing to the brachial plexus
    • Traced through various nerve divisions, responsible for upper limb motor and sensory function

Cerebrum

  • Divisions:
    • Telencephalon (two hemispheres, five major lobes: frontal, parietal, temporal, occipital, insular)
    • Diencephalon (includes thalamus as sensory relay center and hypothalamus maintaining homeostasis)

Somatotopic Mapping

  • Adjacent receptive surfaces in the body are mapped to adjacent fibers in pathways leading to neighboring cortical regions
  • Motor and Sensory Homunculus represent the relative amount of cortical tissue dedicated to sensorimotor function of various body parts

Meningeal Layers and Spaces

  • Dura Mater:

    • Outermost, tough layer with meningeal and periosteal layers
    • Specializations: Falx cerebri, falx cerebelli, tentorium cerebelli

  • Arachnoid Mater:

    • Middle layer, web-like, containing cerebrospinal fluid in the subarachnoid space
    • Arachnoid granulations allow CSF reabsorption into venous system

  • Pia Mater: Closely adheres to the brain, innermost layer

Dural Venous Sinuses

  • Comprised of specialized channels that allow venous drainage from the brain to the internal jugular vein
  • Major sinuses include superior sagittal sinus, inferior sagittal sinus, straight sinus, transverse sinus, and sigmoid sinus

Clinical Correlates

  • Epidural and Subdural Hematomas:
    • Epidural Hematoma: Usually arterial bleeding between skull and dura mater
    • Subdural Hematoma: Typically venous bleeding between dura and arachnoid mater
  • Lumbar Puncture: Employed to sample cerebrospinal fluid (CSF) for diagnostics, typically performed at L3-L4 or L4-L5 intervertebral spaces