Central nervous system

The central nervous system (CNS) is the primary control center of the body, integrating sensory information and coordinating responses. It comprises the brain and spinal cord, which together regulate voluntary and involuntary functions. Understanding the CNS is essential for studying neurophysiology and neurological disorders.

Anatomy of the Central Nervous System

Definition

The central nervous system consists of the brain and spinal cord. It processes sensory input, generates motor commands, and controls cognitive, emotional, and autonomic functions. The CNS is protected by bones, meninges, cerebrospinal fluid, and the blood-brain barrier.

Major Components

The CNS is composed of two principal structures:

• Brain: Located in the cranial cavity, responsible for higher-order functions, sensory processing, and motor coordination
• Spinal cord: Extends from the brainstem through the vertebral canal, transmitting signals between the brain and the rest of the body

Protective Structures

The CNS is safeguarded by several protective mechanisms:

• Skull and vertebral column: Rigid bony structures that provide mechanical protection
• Meninges: Three connective tissue layers—dura mater, arachnoid mater, and pia mater—surround the CNS
• Cerebrospinal fluid: Cushions the CNS, provides nutrients, and removes waste products
• Blood-brain barrier: Regulates the passage of substances between the bloodstream and CNS tissue

Brain

Cerebrum

The cerebrum is the largest part of the brain and is divided into two hemispheres. It is responsible for voluntary motor activity, sensory perception, cognition, and language. The cerebral cortex contains gray matter, while underlying white matter facilitates communication between regions.

• Frontal lobe: motor control, decision-making, planning
• Parietal lobe: sensory perception and spatial awareness
• Temporal lobe: auditory processing and memory
• Occipital lobe: visual processing

Diencephalon

The diencephalon is located beneath the cerebrum and consists of several important structures:

• Thalamus: sensory relay center
• Hypothalamus: autonomic regulation and endocrine control
• Pineal gland: regulates circadian rhythms
• Subthalamus: involved in motor control

Brainstem

The brainstem connects the cerebrum and cerebellum to the spinal cord, regulating vital functions such as respiration, heart rate, and consciousness. It consists of three main regions:

• Midbrain: involved in vision, hearing, and motor control
• Pons: relays signals between the cerebrum and cerebellum and manages autonomic functions
• Medulla oblongata: controls cardiovascular and respiratory centers and reflex activities

Cerebellum

The cerebellum is located posterior to the brainstem and is primarily responsible for coordination, balance, and fine-tuning of movements. It consists of two hemispheres connected by the vermis and is divided into lobes based on functional regions:

• Anterior lobe: regulates posture and gait
• Posterior lobe: coordinates voluntary movements
• Flocculonodular lobe: maintains balance and eye movements

Spinal Cord

Gross Anatomy

The spinal cord is a cylindrical structure extending from the medulla oblongata to the conus medullaris, located within the vertebral canal. It is segmented into cervical, thoracic, lumbar, sacral, and coccygeal regions, each giving rise to spinal nerves that innervate specific body regions. The conus medullaris marks the terminal end, from which the filum terminale and cauda equina extend.

Microscopic Structure

The spinal cord contains gray matter at its center, surrounded by white matter. Gray matter is organized into dorsal (sensory) and ventral (motor) horns, while white matter contains ascending sensory and descending motor tracts. Supporting glial cells, including astrocytes and oligodendrocytes, maintain neuronal health and myelination. The central canal runs longitudinally, carrying cerebrospinal fluid.

Protective Structures and Blood Supply

The spinal cord is protected by the vertebrae, meninges, and cerebrospinal fluid. Its vascular supply includes the anterior spinal artery, posterior spinal arteries, and segmental medullary arteries, which ensure adequate perfusion of both gray and white matter regions.

Neural Pathways and Circuits

Motor Pathways

Motor pathways transmit signals from the brain to muscles, enabling voluntary and involuntary movements. Key tracts include:

• Corticospinal tracts: Responsible for voluntary motor control, particularly fine movements of the limbs
• Extrapyramidal tracts: Involved in posture, balance, and coordination of reflexive movements

Sensory Pathways

Sensory pathways carry information from the body to the brain for processing. Major tracts include:

• Spinothalamic tract: Conveys pain, temperature, and crude touch sensations
• Dorsal column-medial lemniscus pathway: Transmits fine touch, vibration, and proprioception

Reflex Arcs

Reflex arcs are neural circuits that mediate rapid, automatic responses to stimuli without conscious input:

• Monosynaptic reflexes: Involve a single synapse between sensory and motor neurons, e.g., the patellar reflex
• Polysynaptic reflexes: Involve one or more interneurons, e.g., withdrawal reflex from a painful stimulus

Development and Embryology

Neural Tube Formation

The CNS originates from the neural tube, formed during the third week of embryogenesis. The anterior portion develops into the brain, while the posterior portion gives rise to the spinal cord. Proper closure of the neural tube is essential to prevent congenital anomalies such as spina bifida and anencephaly.

Brain Vesicle Development

The anterior neural tube forms three primary brain vesicles: prosencephalon, mesencephalon, and rhombencephalon. These vesicles further differentiate into secondary vesicles, giving rise to the cerebrum, diencephalon, midbrain, pons, medulla, and cerebellum.

Spinal Cord Formation

The caudal portion of the neural tube elongates to form the spinal cord. Somites differentiate into vertebrae, and neuroblasts form the central and peripheral neural structures. The central canal and meninges develop concurrently to support and protect the cord.

Clinical Implications of Developmental Defects

Abnormal neural tube or vesicle development can lead to congenital CNS disorders, including spina bifida, hydrocephalus, and cortical malformations. Early prenatal detection and intervention can mitigate long-term neurological deficits.

Physiology and Function

Control of Voluntary Movements

The CNS coordinates voluntary movements through motor pathways originating in the motor cortex, cerebellum, and basal ganglia. Signals travel via descending tracts to motor neurons in the spinal cord, which then activate skeletal muscles for precise and coordinated actions.

Autonomic Regulation

The CNS regulates autonomic functions, including heart rate, blood pressure, digestion, and respiratory rhythm. The hypothalamus, brainstem, and spinal cord integrate sensory input and modulate sympathetic and parasympathetic outputs to maintain homeostasis.

Sensory Processing

Sensory information from the periphery is transmitted to the CNS, where it is integrated and interpreted. The CNS processes visual, auditory, tactile, proprioceptive, and nociceptive stimuli, allowing perception, reflexive responses, and adaptive behavior.

Cognition, Emotion, and Behavior

The CNS supports higher-order functions such as thinking, memory, language, emotion, and behavior. The cerebral cortex, limbic system, and associated neural networks are critical for learning, decision-making, emotional regulation, and social interactions.

Clinical Significance

Traumatic Injuries

Trauma to the CNS can result in acute or chronic neurological deficits. Examples include traumatic brain injury, spinal cord injury, and diffuse axonal injury, which may lead to motor, sensory, cognitive, or autonomic impairments depending on the site and severity of damage.

Infections

Infectious processes can affect the CNS and lead to significant morbidity:

• Meningitis: Inflammation of the meninges caused by bacterial, viral, or fungal pathogens
• Encephalitis: Inflammation of the brain parenchyma, often viral in origin

Degenerative Disorders

Progressive CNS diseases lead to neuronal loss and functional decline:

• Alzheimer’s disease: Neurodegeneration affecting memory and cognition
• Parkinson’s disease: Degeneration of dopaminergic neurons, resulting in motor dysfunction
• Multiple sclerosis: Demyelination of CNS axons causing sensory, motor, and cognitive deficits

Vascular Disorders

Vascular abnormalities in the CNS can cause ischemia or hemorrhage:

• Stroke: Interruption of cerebral blood flow leading to neuronal death
• Subarachnoid hemorrhage: Bleeding into the subarachnoid space, often from aneurysmal rupture

Neoplasms

Primary or metastatic tumors can affect CNS structure and function, leading to neurological deficits, increased intracranial pressure, or seizures. Common CNS tumors include gliomas, meningiomas, and metastatic lesions from systemic cancers.

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