Chapter 12: The Central Nervous System

Overview of the Central Nervous System

The central nervous system (CNS) is composed of the brain and spinal cord, serving as the primary control center for the body. It integrates sensory information, coordinates voluntary and involuntary responses, and is essential for higher mental functions.

• Major Areas of the Brain: The brain consists of several regions, each with specialized functions, including the cerebrum, diencephalon, cerebellum, and brainstem.

• Developmental Regions: The brain develops from five embryonic regions, which give rise to the major adult brain structures.

The Brain

The brain is divided into distinct lobes and regions, each responsible for specific motor, sensory, and integrative functions.

• Five Lobes of the Cerebral Hemisphere: Frontal, parietal, temporal, occipital, and insula. Each lobe processes different types of information.

• Motor and Sensory Distribution: Motor functions are primarily located in the frontal lobe, while sensory functions are distributed among the parietal, temporal, and occipital lobes.

• Diencephalon: Includes the thalamus, hypothalamus, and epithalamus, which relay sensory information and regulate autonomic functions.

• Cerebellum: Coordinates movement and balance.

• Brainstem: Controls basic life functions such as breathing and heart rate.

• Limbic System and Reticular Formation: Involved in emotion, memory, and arousal.

Homeostasis Part I: Role of the Brain in Maintenance of Homeostasis

The brain and endocrine system work together to maintain the body's internal balance, or homeostasis.

• Nervous System: Maintains homeostasis through rapid electrical signals and neurotransmitters.

• Endocrine System: Uses hormones for slower, longer-lasting regulation.

• Comparison: The nervous system provides immediate responses, while the endocrine system regulates long-term processes.

Higher Mental Functions

Higher mental functions such as cognition, language, and memory are localized in specific cortical areas and involve complex neural networks.

• Cognition and Language: The prefrontal cortex and language centers (Broca's and Wernicke's areas) are responsible for thought and communication.

• Cerebral Hemispheric Lateralization: Certain functions are dominant in one hemisphere (e.g., language in the left hemisphere).

• Memory Storage: Long-term memory involves the hippocampus and associated cortical areas; memory consolidation is the process of transferring short-term to long-term memory.

Protection of the Brain

The brain is protected by several physical and chemical barriers that ensure its proper function and safety.

• Cerebrospinal Fluid (CSF): Cushions the brain, removes waste, and circulates nutrients. Produced by the choroid plexus, it circulates through the ventricles and is reabsorbed into the bloodstream.

• Blood Brain Barrier: A selective barrier that prevents harmful substances from entering the brain while allowing necessary molecules to pass.

• Cranial Meninges: Three connective tissue layers (dura mater, arachnoid mater, pia mater) that protect and support the brain.

The Spinal Cord

The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. It is protected by meninges and is organized into distinct anatomical regions.

• Gross Anatomy: Extends from the foramen magnum to the lumbar region; divided into cervical, thoracic, lumbar, and sacral segments.

• Cross-Sectional Features: Includes gray matter (cell bodies) and white matter (myelinated axons).

• Spinal Meninges: Same three layers as the brain, with spaces such as the epidural and subarachnoid spaces.

• Ascending and Descending Tracts: Ascending tracts carry sensory information to the brain; descending tracts transmit motor commands from the brain.

Sensation Part I: Role of the CNS in Sensation

Sensory information is processed through a series of neurons that relay signals from receptors to the brain for interpretation.

• Central and Peripheral Roles: The peripheral nervous system detects stimuli; the CNS processes and interprets them.

• Order of Neurons:

  • First-order neurons: Carry signals from receptors to the spinal cord or brainstem.

  • Second-order neurons: Relay information to the thalamus.

  • Third-order neurons: Transmit signals to the cerebral cortex for perception.

Movement Part I: Role of the CNS in Voluntary Movement

Voluntary movement is coordinated by multiple brain regions and involves a sequence of neural pathways.

• Upper and Lower Motor Neurons: Upper motor neurons originate in the cerebral cortex; lower motor neurons connect the CNS to muscles.

• Role of Brain Regions: The cerebral cortex initiates movement, the basal nuclei regulate movement, and the cerebellum coordinates and fine-tunes actions.

• Motor Program Pathway: Involves decision-making, initiation, execution, and feedback monitoring to ensure smooth and purposeful movement.