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

The Central Nervous System: The Brain and Spinal Cord

Overview

The central nervous system (CNS) consists of the brain and spinal cord, serving as the primary control center for the body’s functions and behaviors. The CNS processes sensory information, coordinates movement, and is responsible for higher mental functions such as reasoning, memory, and emotion.

The Brain

General Characteristics

• Neurons: The human brain contains approximately 100 billion neurons, which are specialized cells for transmitting nerve impulses.

• Mass: The average brain weighs about 1.6 kg in males and 1.45 kg in females. Brain size is generally proportional to body size, but does not directly correlate with intelligence.

• Division: The brain is divided into hemispheres (left and right) and further into lobes (frontal, parietal, temporal, occipital).

• Complexity: The complexity of neural connections and organization, rather than sheer size, determines the brain’s processing power.

Mind and Brain Function

• Consciousness: The mechanisms by which the brain produces the mind and consciousness remain largely unknown.

• Research Methods: Historically, much of our understanding of brain function has come from studying individuals with brain injuries (e.g., accident or stroke victims).

• Modern Imaging: Advanced imaging techniques such as Positron Emission Tomography (PET) and Functional Magnetic Resonance Imaging (fMRI) allow scientists to observe brain activity in real time.

Major Subdivisions of the Brain

• Cerebrum: The largest part of the brain, responsible for higher mental functions including conscious thought, intelligence, and voluntary movement.

• Cerebellum: The second largest part, involved in coordination of repetitive body movements and balance.

• Diencephalon: Includes the thalamus (sensory relay and processing), hypothalamus (hormone production, emotion, autonomic function), and pituitary gland (major endocrine gland).

• Brain Stem: Connects the brain to the spinal cord and processes information between them. Includes the midbrain (mesencephalon), pons, and medulla oblongata, which regulate vital autonomic functions such as heart rate, blood pressure, and digestion.

Brain Protection

• Cranial Meninges: The brain is protected by three layers of connective tissue called meninges: dura mater, arachnoid mater, and pia mater. These layers are continuous with the spinal meninges and help protect the CNS from trauma.

Functional Organization of the Cerebral Cortex

• Motor Areas: Control voluntary movements. The primary motor cortex initiates movement, while the premotor cortex is involved in planning and coordinating learned, patterned movements (e.g., playing piano).

• Language Areas: Broca’s area (usually in the left hemisphere) controls speech production; Wernicke’s area is involved in language comprehension and interpretation.

• Sensory Areas: The primary somatosensory cortex receives sensory input from the body, localizing sensations. The auditory cortex (in the temporal lobe) processes sound information.

• Association Areas: Integrate information from different modalities, supporting complex functions such as reasoning, planning, and personality (prefrontal cortex).

Brain Lateralization

• Hemispheric Specialization: The left hemisphere is typically dominant for language, math, and logic, while the right hemisphere is associated with creativity, spatial skills, intuition, and appreciation of art and music.

• Dominance: About 90% of people are left-hemisphere dominant (often right-handed). In some individuals, dominance is reversed or equal, which may be associated with ambidexterity or dyslexia.

The Spinal Cord

Structure and Function

• Location: The spinal cord is housed within the vertebral column and passes through the vertebral foramina.

• Functions: Serves as a major reflex center and conducts signals to and from the brain.

• Meninges: Like the brain, the spinal cord is protected by specialized membranes (meninges) and is supplied with blood.

Internal Anatomy

• Gray Matter: Central region shaped like a butterfly or 'H', containing nerve cell bodies.

• White Matter: Peripheral regions containing myelinated sensory and motor neuron fibers.

• Length: The spinal cord is about 45 cm long in adults.

External Anatomy

• Conus Medullaris: The tapered end of the spinal cord, typically between L1 and L2 vertebrae.

• Filum Terminale: A fibrous extension of the pia mater anchoring the spinal cord to the coccyx.

• Cauda Equina: A bundle of spinal nerve roots at the lower end of the spinal cord, resembling a horse’s tail.

Spinal Nerves

• Connection: Each segment of the spinal cord is connected to a pair of spinal nerves, which are surrounded by connective tissue layers (endoneurium, perineurium, epineurium).

• Dermatomes: Specific regions of skin are innervated by particular spinal nerves, known as dermatomes.

Homeostatic Imbalances of the CNS

Brain and Meningeal Disorders

• Meningitis: Inflammation of the meninges, which can be caused by bacterial (e.g., Streptococcus pneumoniae, Staphylococcus, Haemophilus influenzae) or viral infections.

• Encephalitis: Inflammation of the brain, often viral and spread by insects such as mosquitoes or ticks.

Traumatic Brain Injuries

• Concussion: Caused by a blow to the head; the brain moves within the skull, potentially leading to cognitive problems even without visible damage.

• Contusion: Involves bleeding and bruising of brain tissue, with effects depending on the location of injury.

Spinal Nerve Disorders

• Shingles: Caused by reactivation of the Varicella zoster virus (chickenpox), leading to painful blisters along dermatomes.

Prion Diseases

• Prions: Infectious proteins that cause neurodegenerative diseases such as scrapie (sheep), mad cow disease (cattle), and Creutzfeldt-Jakob disease (humans).

• Mechanism: Prions are misfolded forms of normal brain proteins (PrP). They induce conformational changes in normal proteins, propagating the disease.

• Discovery: Stanley Prusiner discovered prions in the early 1980s and won the Nobel Prize in Medicine in 1997.

Table: Comparison of Brain Regions and Functions

Region	Main Function
Cerebrum	Conscious thought, intelligence, voluntary movement
Cerebellum	Coordination, balance, repetitive movements
Diencephalon (Thalamus)	Sensory relay, learning, memory
Diencephalon (Hypothalamus)	Hormone production, emotion, autonomic function
Brain Stem	Vital autonomic functions, information relay

Example: PET and fMRI in Brain Research

• PET: Used to measure metabolic activity in different brain regions by detecting radioactive tracers.

• fMRI: Measures changes in blood flow, indicating neural activity during tasks or rest.

Additional info: The notes have been expanded to include definitions, examples, and a comparison table for clarity and completeness.