Central Nervous System Overview

The central nervous system (CNS) consists of the brain and spinal cord. It is responsible for integrating sensory information, coordinating voluntary and involuntary activity, and maintaining homeostasis.

Major Divisions of the CNS

• Brain: The control center for processing sensory data, initiating responses, and higher cognitive functions.

• Spinal Cord: Relays information between the brain and the rest of the body, and mediates reflexes.

Brain Structure and Function

The brain is divided into several major regions, each with specialized functions.

Cerebrum

• Largest part of the brain, divided into two hemispheres by the longitudinal fissure.

• Responsible for integration of complex sensory and neural functions, initiation and coordination of voluntary activity.

• Each hemisphere controls the contralateral (opposite) side of the body.

Lobes of the Cerebrum

• Frontal Lobe:

  • Prefrontal Cortex: Concentration, problem solving, executive function, processing emotions, social behavior, and long-term memory.

  • Broca's Area: Usually in the left hemisphere; involved in motor activity for producing speech.

  • Motor Area: Controls voluntary muscle movement.

  • Central Sulcus: Divides frontal lobe from parietal lobe; separates motor area from sensory area.

• Parietal Lobe:

  • Processes sensory information regarding body position and spatial awareness.

  • Primary Somatosensory Cortex: Receives all sensory input from the body.

• Occipital Lobe: Vision.

• Temporal Lobe: Hearing.

Specialized Areas

• Wernicke's Area: Located at the junction of parietal and temporal lobes; integrates sensory information for understanding language (visual, auditory cues).

Cerebellum

• Separated from the cerebrum by the transverse fissure.

• Integrates sensory information about body position, coordinates skeletal muscle activity, and maintains posture.

Diencephalon

• Includes the thalamus (sensory relay), hypothalamus (homeostasis, endocrine regulation), and epithalamus (pineal gland, sleep patterns).

• Integrates sensory information and regulates autonomic and endocrine functions.

Brainstem

• Connects the brain to the spinal cord; consists of the midbrain, pons, and medulla oblongata.

• Midbrain: Regulates eye and head movement, pain suppression, and functions in the reticular system.

• Pons: Regulates breathing rate, relays information, and controls equilibrium and bladder function.

• Medulla Oblongata: Controls breathing, heart rate, vasodilation/constriction, vomiting, swallowing, coughing, sneezing.

Functional Systems of the Brain

Limbic System

• Controls emotions and memory formation.

• Amygdaloid Body: Sensory filter for input and emotional cues; can stimulate hippocampus or hypothalamus.

• Hippocampus: Involved in memory formation and sending information to the prefrontal cortex for cognitive processing.

• Hypothalamus: Stimulated by the amygdala when the body needs to respond to stimuli.

Reticular Formation

• Network of nuclei running through the brainstem; maintains alertness and consciousness.

• Reticular Activating System (RAS): Allows the brain to maintain awareness; filters sensory input to the cerebral cortex.

• Functions include regulation of sleep-wake transitions and filtering out repetitive or weak stimuli.

Memory Systems

• Short-term memory (STM): Temporary holding of information, limited to 7-8 pieces.

• Long-term memory (LTM): Limitless capacity, involves transfer from STM.

• Factors affecting transfer: emotional state, rehearsal, association, automatic memory.

• During learning, neurons change: dendritic spines change shape, synaptic terminals increase, neurotransmitter release (especially glutamate).

Protection of the CNS

Bones, Meninges, and Cerebrospinal Fluid (CSF)

• Bones: Skull and vertebrae protect the brain and spinal cord.

• Meninges: Three layers covering the CNS:

  • Dura Mater: Tough, outermost layer; forms folds (dural sinuses) that drain blood.

  • Arachnoid Mater: Web-like middle layer; subarachnoid space contains CSF.

  • Pia Mater: Delicate, innermost layer; contains blood vessels nourishing nervous tissue.

• Spinal Cord: Protected by bone, fibrocartilage disks, and meninges.

Cerebrospinal Fluid (CSF)

• Produced by choroid plexuses in the ventricles.

• Clear, colorless fluid similar to blood plasma but with less protein and different ion concentrations (more Na+, Cl-, H+; less Ca2+, K+).

• Functions: nutrition, waste removal, shock absorption, and maintaining ion balance.

• CSF circulates through ventricles, subarachnoid space, and is reabsorbed into the blood via dural sinuses.

CSF Production and Flow

1. Choroid plexuses (lined with ependymal cells) filter blood plasma to produce CSF.

2. CSF is secreted into the ventricles, enters the subarachnoid space, and circulates around the brain and spinal cord.

3. CSF is reabsorbed into the venous blood at the dural sinuses.

Hydrocephalus

• Condition where excess CSF accumulates in the brain, increasing intracranial pressure.

• Common in infants before cranial sutures fuse; may require surgical intervention (shunt).

• Can be caused by infection (e.g., Toxoplasma gondii).

Spinal Cord Structure and Function

• Continuation of the medulla oblongata; 16-18 inches long.

• Protected by vertebrae, fibrocartilage disks, and meninges.

• Gray Matter: Contains neuron cell bodies for receiving, integrating, and sending sensory/motor information.

• White Matter: Contains myelinated axons for communication between brain and body.

• 31 pairs of spinal nerves arise from the spinal cord, each with sensory (dorsal) and motor (ventral) roots.

Key Terms and Definitions

• Motor Unit: A motor neuron and all the skeletal muscle fibers it innervates.

• Ganglia: Clusters of neuron cell bodies in the PNS.

• Schwann Cells: Glial cells in the PNS that produce myelin sheaths.

• Fissure: Deep groove in the brain (e.g., longitudinal, transverse fissures).

• Sulcus: Shallow groove on the brain surface (e.g., central sulcus).

Table: Comparison of Meningeal Layers

Additional info:

• Some explanations and terminology have been expanded for clarity and completeness.

• Clinical conditions (e.g., hydrocephalus, Proteus syndrome) are included for context.

• Memory and learning processes are described with reference to synaptic plasticity and neurotransmitters.