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