Central Nervous System: Brain and Spinal Cord – Structure and Function

Major Brain Parts

Overview of Brain Regions

The brain is divided into several major regions, each with distinct anatomical structures and functions. Understanding these regions is essential for comprehending the organization and function of the central nervous system (CNS).

• Medulla oblongata: Controls autonomic functions such as breathing and heart rate.

• Thalamus: Relay station for sensory information to the cerebral cortex.

• Pons: Connects different parts of the brain and assists in regulating breathing.

• Hypothalamus: Maintains homeostasis; regulates hunger, thirst, temperature, and endocrine functions.

• Midbrain: Involved in vision, hearing, and motor control.

• Epithalamus: Contains the pineal gland; involved in circadian rhythms.

• Corpora Quadrigemina: Four colliculi involved in visual and auditory reflexes.

• Cerebral columns: (Additional info: Likely refers to white matter tracts in the cerebrum.)

• Cerebellum: Coordinates voluntary movements and balance.

• Cerebrum: Largest part; responsible for higher brain functions.

• Diencephalon: Includes thalamus, hypothalamus, and epithalamus.

• Hypophysis (Pituitary gland): Endocrine gland controlling various hormones.

Cerebrospinal Fluid (CSF)

Production and Flow of CSF

Cerebrospinal fluid is a clear, colorless body fluid found in the brain and spinal cord. It is produced by the choroid plexus, a network of capillaries in the ventricles of the brain.

• Functions of CSF:

  • Cushions and protects the brain against trauma.

  • Provides buoyancy, reducing the effective weight of the brain.

  • Helps remove metabolic waste from the brain.

  • Maintains a constant environment for the CNS.

• Flow of CSF:

  • Produced by choroid plexuses in the ventricles.

  • Flows through the ventricles and into the subarachnoid space via median and lateral apertures.

  • Some CSF circulates through the central canal of the spinal cord.

  • CSF is absorbed into the dural venous sinuses via arachnoid villi.

Example: Blockage of CSF flow can lead to hydrocephalus, a condition characterized by increased intracranial pressure.

White and Gray Matter in the CNS

Definitions and Functions

• Gray matter: Contains neuron cell bodies and unmyelinated processes; involved in processing and integration.

• White matter: Contains myelinated axons; responsible for communication between different CNS regions.

In the cerebrum, white matter allows communication between the cerebral hemispheres and between the cortex and spinal cord.

Types of White Matter Tracts

• Commissural tracts: Connect corresponding gray areas of the two hemispheres (e.g., corpus callosum).

• Association tracts: Connect different parts of the same hemisphere.

• Projection tracts: Connect the cortex with lower brain or spinal cord centers.

Spinal Cord Anatomy

Gross Anatomy

• Begins at the foramen magnum and ends at the conus medullaris (approx. L1-L2).

• 31 pairs of spinal nerves attach via dorsal and ventral roots.

• Two enlargements: cervical (upper limb innervation) and lumbar (lower limb innervation).

• Cauda equina: Bundle of nerve roots below the conus medullaris.

Gray Matter of the Spinal Cord

• Butterfly-shaped in cross-section.

• Divided into anterior (ventral), posterior (dorsal), and lateral horns.

• Contains cell bodies of motor and sensory neurons.

White Matter of the Spinal Cord

• Organized into columns (funiculi): anterior, posterior, and lateral.

• Contains ascending (sensory), descending (motor), and transverse (commissural) tracts.

• Each funiculus contains several tracts, each with axons of similar origin and destination.

Major Sensory and Motor Pathways

Ascending (Sensory) Pathways

• Transmit sensory information from the body to the brain.

• Typically involve three neurons: first-order (receptor to spinal cord), second-order (spinal cord to thalamus), third-order (thalamus to cortex).

Descending (Motor) Pathways

• Transmit motor commands from the brain to the body.

• Typically involve two neurons: upper motor neuron (cortex to spinal cord), lower motor neuron (spinal cord to effector).

• Many pathways decussate (cross) to the opposite side of the CNS.

Functional Regions of the Cerebrum

Key Cortical Areas

• Primary motor cortex: Voluntary movement control.

• Premotor cortex: Planning of movements.

• Primary somatosensory cortex: Receives sensory input from the body.

• Somatosensory association cortex: Interprets sensory information.

• Primary visual cortex: Processes visual information.

• Visual association cortex: Interprets visual stimuli.

• Broca's area: Speech production.

• Wernicke's area: Language comprehension.

• Primary auditory cortex: Processes auditory information.

• Auditory association cortex: Interprets sounds.

Somatotopy

Somatotopy refers to the point-for-point correspondence between an area of the body and a specific point on the central nervous system. The homunculus is a visual representation of this mapping in the cortex.

Thalamus and Hypothalamus

Thalamus

• Located in the superior diencephalon.

• Acts as a relay station for sensory information to the cortex.

• Composed of multiple nuclei.

Hypothalamus

• Maintains homeostasis via autonomic and endocrine control.

• Regulates hunger, thirst, temperature, and circadian rhythms.

• Controls the pituitary gland and hormone release (e.g., ADH, oxytocin).

Corpus Callosum

The corpus callosum is a large commissural tract of white matter connecting the left and right cerebral hemispheres, allowing communication between them.

Meninges of the Brain and Spinal Cord

Three Layers

• Dura mater: Outermost, toughest layer.

• Arachnoid mater: Middle, web-like layer.

• Pia mater: Innermost, delicate layer adhering to the CNS surface.

Table: Major White Matter Tracts of the Spinal Cord

Tract	Type	Function	Decussation
Posterior column (dorsal columns)	Sensory (ascending)	Fine touch, proprioception	Medulla
Spinothalamic tract	Sensory (ascending)	Pain, temperature	Spinal cord
Corticospinal tract	Motor (descending)	Voluntary movement	Medullary pyramids
Spinocerebellar tract	Sensory (ascending)	Proprioception to cerebellum	Does not decussate (mostly)

Additional info: Some details, such as the specific nuclei of the thalamus and hypothalamus, and the full list of tracts, have been inferred or expanded for academic completeness.