Central Nervous System: Structure, Function, and Protection

Central Nervous System (CNS)

Overview

The central nervous system (CNS) consists of the brain and spinal cord, serving as the primary control center for the body. It integrates sensory information, coordinates motor output, and is protected by specialized structures.

• Major Parts of the Adult Brain: Cerebrum, diencephalon, brain stem, and cerebellum.

• Ventricles: Fluid-filled cavities that circulate cerebrospinal fluid (CSF) and interconnect through the CNS.

Cerebral White Matter

Axonal Tracts and Communication

White matter in the cerebrum consists of myelinated axons bundled into tracts, facilitating communication within the brain and with the spinal cord.

• Commissural fibers: Connect the two cerebral hemispheres (e.g., corpus callosum).

• Association fibers: Connect different parts of the same hemisphere (e.g., link Wernicke's and Broca's areas).

• Projection fibers: Run vertically, connecting the cerebral cortex with lower CNS regions (e.g., corticospinal tract).

Deep Gray Matter of the Cerebrum

Basal Nuclei and Limbic Structures

Deep gray matter includes nuclei involved in motor control and memory.

• Basal nuclei (basal ganglia): Regulate voluntary movement, formed from the caudate nucleus, putamen, and globus pallidus.

• Basal forebrain nuclei: Associated with memory.

• Amygdaloid body: Part of the limbic system, involved in emotion.

Basal Ganglia Functions:

• Cooperate with the cerebral cortex for complex movement control.

• Regulate initiation, intensity, and cessation of voluntary movements.

• Degeneration leads to movement disorders: Parkinson's disease (slow, jerky movements), Huntington's disease (uncontrolled movements).

Protection of the Brain

Physical and Chemical Barriers

The brain is protected by multiple layers and mechanisms:

• The skull: Provides rigid protection.

• Meninges: Three connective tissue membranes (dura mater, arachnoid mater, pia mater).

• Cerebrospinal fluid (CSF): Cushions and nourishes the CNS.

• Blood-brain barrier: Regulates passage of substances from blood to brain tissue.

Meninges

• Dura mater: Outermost, strongest layer; consists of periosteal and meningeal layers. Forms dural venous sinuses for blood drainage.

• Arachnoid mater: Middle layer; subarachnoid space contains CSF and blood vessels. Arachnoid granulations allow CSF to enter venous sinuses.

• Pia mater: Innermost layer; delicate, highly vascularized, adheres to brain surface.

Cerebrospinal Fluid (CSF)

Formation and Function

CSF is a clear fluid that fills the subarachnoid space, ventricles, and central canal, providing mechanical and chemical protection.

• Formation: Produced by choroid plexuses in all four ventricles, mainly lateral ventricles. Composed of ependymal cells and capillaries.

• Functions:

  • Protects brain and spinal cord by cushioning against impact.

  • Nourishes CNS and removes metabolic wastes.

  • Facilitates chemical signaling between CNS regions.

Blood-Brain Barrier

Selective Permeability

The blood-brain barrier restricts passage of most substances from blood to brain, maintaining CNS homeostasis.

• Formed by: Tight junctions in capillary endothelial cells and astrocyte processes.

• Permits: Lipid-soluble substances (e.g., O2, CO2), alcohol, nicotine, anesthetics.

• Prevents: Most toxins and pathogens.

Spinal Cord

Structure and Function

The spinal cord is the main pathway for information connecting the brain and peripheral nervous system.

• Functions:

  • Sensory and motor innervation below the head via spinal nerves.

  • Ascending and descending tracts for two-way signal conduction.

  • Integration center for reflexes.

• Location: Runs through vertebral canal from foramen magnum to L1 or L2.

• Conus medullaris: Inferior end of spinal cord.

• Filum terminale: Connective tissue anchoring cord to coccyx.

• 31 pairs of spinal nerves: Attach via dorsal and ventral roots; grouped as cervical (8), thoracic (12), lumbar (5), sacral (5), coccygeal (1).

• Cervical and lumbar enlargements: Sites where nerves for limbs arise.

• Cauda equina: Bundle of lumbar and sacral nerve roots at inferior end.

Spinal Cord Segments

• Designated by the spinal nerve emerging from each segment (e.g., segment T1 for thoracic nerve T1).

White Matter of the Spinal Cord

Organization and Fiber Types

White matter surrounds the gray matter and consists of axons organized into tracts.

• Divided into: Dorsal, ventral, and lateral funiculi.

• Fiber types:

  • Ascending fibers: Sensory information to brain.

  • Descending fibers: Motor information from brain to spinal cord.

  • Commissural fibers: Cross information from one side of spinal cord to the other.

Gray Matter of the Spinal Cord

Regions and Functions

Gray matter is the central region, shaped like an "H", containing neuronal cell bodies and some axons.

• Gray commissure: Contains the central canal.

• Dorsal horns: Sensory axons and interneurons.

• Ventral horns: Motor neuron cell bodies.

• Lateral horns: Motor neuron cell bodies (only in thoracic and superior lumbar regions).

Gray matter is further divided by innervation:

• SS: Somatic sensory

• VS: Visceral sensory

• VM: Visceral motor

• SM: Somatic motor

Protection of the Spinal Cord

Structural Barriers

• Vertebrae: Provide bony protection.

• Meninges:

  • Dura mater: Single layer (spinal dural sheath).

  • Arachnoid mater: Subarachnoid space below L1/L2 for lumbar puncture.

  • Pia mater: Delicate, extends to coccyx, forms denticulate ligaments anchoring cord.

• CSF: Cushions and nourishes spinal cord.

Sensory and Motor Pathways in the CNS

Neural Tracts and Decussation

Pathways connect the brain and body periphery, often crossing (decussating) from one side to the other.

• Consist of chains of two or three neurons.

• Arranged by body region supplied.

• Bilateral symmetry: pathways occur on both sides of CNS.

Ascending Pathways

• Dorsal column–medial lemniscal pathway: Fine touch, pressure, conscious proprioception.

• Spinothalamic pathway: Pain, temperature (non-discriminative sensations).

• Spinocerebellar pathway: Proprioceptive information.

Descending Pathways

• Direct (pyramidal) pathways: Corticospinal tracts for precise, skilled voluntary movements.

• Indirect pathways: Originate in brainstem nuclei, control muscle tone, balance, and visual tracking (e.g., tectospinal, vestibulospinal, rubrospinal, reticulospinal tracts).

Clinical Application: Lumbar Puncture (Spinal Tap)

Procedure and Purpose

A lumbar puncture is performed below L1/L2 to access the subarachnoid space for CSF sampling, avoiding spinal cord injury.

Disorders of the CNS

Common Pathologies

• Spinal cord damage: Paresthesia (loss of sensation), paralysis (loss of motor function), paraplegia (lower limb paralysis), quadriplegia (all limb paralysis).

• Brain dysfunction: Stroke (CVA), Alzheimer's disease, congenital malformations (anencephaly, spina bifida, cerebral palsy).

Table: Types of Cerebral White Matter Tracts

Type of Tract	Function	Example
Commissural fibers	Connects left and right hemispheres	Corpus callosum
Association fibers	Connects regions within the same hemisphere	Wernicke's and Broca's areas
Projection fibers	Connects cortex with lower CNS regions	Corticospinal tract

Key Equations and Terms

• Decussation: Crossing of neural pathways from one side of the CNS to the other.

• CSF Formation: $\text{CSF production} = \text{filtration of plasma through choroid plexus}$

Check for Understanding

• What would you expect to see in individuals with a dysfunctional reticular activating system?

• Where is cerebrospinal fluid produced? What structure collects CSF from the meninges to return it to the heart?