Central Nervous System: Structure, Function, and Integration

Central Nervous System (CNS) Overview

Functions of the CNS

• Sensory Functions: Detects internal and external sensations via sensory neurons from the Peripheral Nervous System (PNS).

• Integrative Functions: Processes sensory input, makes decisions, maintains homeostasis, and supports higher mental functions (learning, language, planning).

• Motor Functions: Sends commands to muscles and glands via motor neurons in the PNS, resulting in movement or secretion.

Basic Structure

• Brain: Located in the cranial cavity, composed of gray matter (outer) and white matter (inner), divided into four main regions: cerebrum, diencephalon, cerebellum, and brainstem.

• Spinal Cord: Long, tubular organ within the vertebral cavity, continuous with the brainstem, ending between L1 and L2 vertebrae. Contains central canal filled with cerebrospinal fluid (CSF).

• Gray Matter: Neuron cell bodies, dendrites, and unmyelinated axons.

• White Matter: Myelinated axons forming tracts for rapid communication.

• Organization: In the brain, gray matter is superficial; in the spinal cord, white matter is superficial.

Development of the CNS

• Develops from the neural tube in the embryo.

• Primary brain vesicles: forebrain, midbrain, hindbrain.

• Secondary vesicles give rise to mature brain regions (telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon).

• Neural tube cavities become brain ventricles and spinal cord central canal.

The Brain

Cerebrum

• Largest brain region; responsible for higher mental functions (learning, memory, personality, cognition, language, conscience).

• Surface features: gyri (ridges), sulci (shallow grooves), fissures (deep grooves).

• Divided into two hemispheres by the longitudinal fissure.

• Contains right and left lateral ventricles.

Lobes of the Cerebral Hemispheres

• Frontal Lobe: Anterior; bounded posteriorly by the central sulcus; includes precentral gyrus (primary motor cortex).

• Parietal Lobe: Posterior to frontal; includes postcentral gyrus (primary somatosensory cortex).

• Temporal Lobe: Lateral surface; separated by lateral fissure.

• Occipital Lobe: Posterior; separated by parieto-occipital sulcus.

• Insula: Deep to lateral fissure; visible when other lobes are separated.

Cerebral Cortex (Gray Matter)

• Outer layer; site of conscious processes (planning, interpreting sensory input).

• Divided into primary motor cortex, primary sensory cortices, and association areas.

• Fiber types:

  • Commissural fibers: Connect hemispheres (e.g., corpus callosum).

  • Projection fibers: Link cortex with other CNS parts.

  • Association fibers: Connect gyri within the same hemisphere.

• Limbic system: Involved in memory, learning, emotion, and behavior.

Motor and Sensory Cortices

• Primary Motor Cortex: Precentral gyrus; controls voluntary movement (contralateral control).

• Premotor Cortex: Planning and coordination of movement.

• Frontal Eye Fields: Control eye movements.

• Somatosensory Areas: S1 (postcentral gyrus) and S2 (posterior to S1) process touch, temperature, vibration, pressure, stretch, and joint position.

• Visual Areas: Occipital lobe; process visual input.

• Auditory Areas: Superior temporal lobe; process sound.

• Gustatory Cortex: Insula and parietal lobe; process taste.

• Vestibular Areas: Parietal and temporal lobes; process balance.

• Olfactory Cortex: Medial temporal lobes; process smell.

Association Areas

• Language Areas: Broca’s area (speech production, frontal lobe), Wernicke’s area (language comprehension, temporal/parietal lobes).

• Prefrontal Cortex: Behavior, personality, learning, memory, psychological state.

• Parietal/Temporal Association Cortices: Sensory integration, recognition, spatial awareness, attention.

Basal Nuclei

• Clusters of gray matter deep in each hemisphere: caudate nucleus, putamen, globus pallidus.

• Inhibit involuntary movement, initiate voluntary movement, and influence behavior and cognition.

Cerebral White Matter

• Myelinated axons forming tracts for rapid communication.

• Three fiber types: association, commissural, projection.

Limbic System

• Includes limbic lobe (cingulate and parahippocampal gyri), hippocampus (memory/learning), amygdala (emotion, especially fear), and fornix (white matter tract).

• Integrates memory, learning, emotion, and visceral responses.

Diencephalon

• Central brain region, hidden by cerebral hemispheres.

• Four components: thalamus, hypothalamus, epithalamus, subthalamus.

Thalamus

• Main gateway to the cortex; relays and processes sensory/motor information.

• Divided into specific relay nuclei, association nuclei, and nonspecific nuclei.

Hypothalamus

• Regulates homeostasis, endocrine system (via pituitary gland), autonomic functions, body temperature, hunger, thirst, sleep/wake cycles, and behavior.

• Produces ADH and oxytocin.

Epithalamus

• Contains pineal gland (secretes melatonin for sleep/wake regulation).

Subthalamus

• Works with basal nuclei to control movement.

Cerebellum

• Coordinates movement, balance, and posture; reduces motor error.

• Two hemispheres connected by vermis; surface has folia (ridges).

• Divided into anterior, posterior, and flocculonodular lobes.

• Outer gray matter (cerebellar cortex), inner white matter (arbor vitae), deep cerebellar nuclei.

• Connected to brainstem by cerebellar peduncles.

Brainstem

• Connects brain to spinal cord; controls vital functions (heart rate, breathing), reflexes, movement, sensation, alertness.

• Three subdivisions: midbrain, pons, medulla oblongata.

Midbrain (Mesencephalon)

• Contains crus cerebri (descending tracts), tegmentum (substantia nigra, red nucleus), tectum (superior/inferior colliculi).

• Involved in movement, sensation, visual/auditory reflexes.

Pons

• Regulates breathing, sleep/wake cycles; contains motor tracts and cerebellar connections.

Medulla Oblongata

• Contains pyramids (corticospinal tracts), decussation of motor fibers, posterior columns (sensory tracts), cranial nerve nuclei, and reticular formation.

• Controls vital autonomic functions.

Reticular Formation

• Network of nuclei throughout brainstem; regulates sleep, arousal, pain, mood, autonomic functions, and motor control.

Homeostasis and the CNS

Role in Homeostasis

• Maintains stable internal environment (fluid/electrolyte balance, blood pressure, glucose, temperature, rhythms).

• Key structures: hypothalamus (main regulator), reticular formation (coordinates with hypothalamus).

Vital Organ Homeostasis

• Autonomic nervous system (ANS) controlled by hypothalamus; regulates heart rate, blood pressure, digestion.

• Vasopressor and vasodepressor centers in brainstem adjust cardiovascular function.

• Ventilation regulated by pons nuclei, influenced by cortex, limbic system, hypothalamus, and sensory input.

Body Temperature and Feeding

• Hypothalamus acts as thermostat (36.1–37.2°C); triggers cooling (sweating) or heating (shivering) mechanisms.

• Regulates hunger via orexins; integrates hormonal and neural signals for feeding and aggression.

Sleep and Circadian Rhythms

• Sleep: reversible suspension of consciousness; restores energy, clears metabolic waste.

• Circadian rhythm controlled by suprachiasmatic nucleus (SCN) in hypothalamus; influenced by light, food, hormones.

• Melatonin from pineal gland promotes sleep; orexin promotes wakefulness.

• Sleep stages: I–IV (non-REM, increasing delta waves), REM (dreaming, muscle paralysis).

Higher Mental Functions

Cognition and Language

• Cognition: processing/responding to stimuli, planning, social/moral behavior, intelligence, personality.

• Localized in association areas of frontal, parietal, temporal lobes.

• Prefrontal cortex: integrates sensory/motor info, plans behavior, develops personality (myelination completes ~25 years old).

• Damage can cause personality changes, impulsivity, cognitive deficits.

Cerebral Lateralization

• Specialization of functions in right/left hemispheres (e.g., language, emotion, attention, facial recognition).

Language

• Broca’s area (production), Wernicke’s area (comprehension); both mainly in left hemisphere.

• Right hemisphere: emotional/tonal aspects of language.

• Aphasia: language deficit from area damage (Broca’s: production; Wernicke’s: comprehension).

Learning and Memory

• Declarative Memory: Facts, consciously accessible; formed in hippocampus, stored in cortex.

• Nondeclarative Memory: Skills, subconscious; involves motor cortices, cerebellum, basal nuclei.

• Memory duration: immediate, short-term (working), long-term (consolidation via association and LTP).

• Memory can be influenced by emotion, prior experience, and is subject to distortion.

Emotion

• Combination of visceral motor, somatic motor, and subjective feelings.

• Amygdala: analyzes emotional significance, forms associations, influences behavior via connections to cortex and basal nuclei.

Protection of the Brain

Physical Protection

• Skull (cranial cavity) provides external protection.

• Three meninges: dura mater (outer, tough), arachnoid mater (middle, web-like), pia mater (inner, delicate, follows brain contours).

• Spaces: epidural (potential), subdural (potential), subarachnoid (contains CSF and blood vessels).

• Hematomas (epidural, subdural, subarachnoid) can increase intracranial pressure; require surgical intervention.

Dura Mater

• Two layers: periosteal (attached to skull), meningeal (above arachnoid).

• Forms dural sinuses (drain CSF and blood), falx cerebri, tentorium cerebelli, falx cerebelli.

Arachnoid Mater

• Thin, elastic, spider-web-like; anchored to pia by trabeculae.

• Arachnoid granulations return CSF to bloodstream via dural sinuses.

Pia Mater

• Thin, permeable, follows brain surface; allows exchange between CSF and brain extracellular fluid.

Ventricles and Cerebrospinal Fluid (CSF)

• Four ventricles: right/left lateral, third, fourth; continuous with central canal of spinal cord.

• CSF: clear, plasma-like fluid; cushions brain, maintains temperature, removes waste, provides buoyancy.

• Produced by choroid plexuses (capillaries + ependymal cells); circulates through ventricles and subarachnoid space; reabsorbed via arachnoid granulations.

• Hydrocephalus: excess CSF due to overproduction, blockage, or impaired reabsorption; increases intracranial pressure.

Blood Brain Barrier

• Formed by endothelial cells with tight junctions, basal lamina, and astrocytes.

• Restricts passage of most substances; allows water, gases, lipid-soluble molecules, and select nutrients (glucose, amino acids) via transporters.

• Protects brain from toxins/pathogens but limits drug delivery.

The Spinal Cord

Functions

• Relay Station: Communicates between brain and body below head/neck.

• Processing Station: Integrates some information (spinal reflexes).

Protection

• Spinal meninges (dura, arachnoid, pia) continue from brain; dura lacks periosteal layer.

• Pia mater forms denticulate ligaments anchoring cord.

• Spaces: epidural (real, contains fat/veins), subdural (potential), subarachnoid (CSF-filled).

External Anatomy

• Begins at foramen magnum, ends at L1-L2 (conus medullaris).

• Filum terminale anchors cord to coccyx.

• Cervical/lumbar enlargements: sites of nerve root attachment.

• Cauda equina: bundle of nerve roots below conus medullaris.

Internal Anatomy

• Butterfly-shaped gray matter (deep), surrounded by white matter (superficial).

• Central canal (CSF-filled) runs through center.

• Gray commissure connects right/left sides.

Spinal Gray Matter

• Anterior Horn: Somatic motor neurons (skeletal muscle control).

• Posterior Horn: Sensory processing (somatic and visceral).

• Lateral Horn: Autonomic motor neurons (present in thoracic/lumbar regions).

• Anterior (motor) and posterior (sensory) roots form spinal nerves; dorsal root ganglion houses sensory neuron cell bodies.

Spinal White Matter

• Organized into funiculi (posterior, lateral, anterior), each containing ascending (sensory) and descending (motor) tracts.

• Tracts are bilaterally symmetrical.

Ascending (Sensory) Tracts

• Posterior Columns: Fasciculus gracilis (lower limbs), fasciculus cuneatus (upper limbs/trunk); carry touch and proprioception.

• Spinocerebellar Tracts: Joint position/muscle stretch to cerebellum.

• Anterolateral System: Spinothalamic tracts; carry pain, temperature, crude touch.

Descending (Motor) Tracts

• Corticospinal Tracts: Control skeletal muscles below head/neck; decussate in medulla.

Sensation and the CNS

General Sensory Pathways

• Stimulus detected by PNS neurons, sent to CNS for interpretation.

• Integration in cerebral cortex produces conscious perception.

• Special Senses: Vision, hearing, taste, smell, balance (processed by specialized organs and CNS pathways).

• General Senses: Touch, stretch, pain, temperature (detected by skin, muscles, organs).

Somatic Sensory Pathways

• Posterior Columns/Medial Lemniscal System: Fine touch, proprioception; first-order neuron ascends ipsilaterally, synapses in medulla, decussates, ascends to thalamus, then cortex.

• Anterolateral System: Pain, temperature, crude touch; first-order neuron synapses in spinal cord, second-order neuron decussates, ascends to thalamus, then cortex.

Role of the Cerebral Cortex

• Thalamus relays sensory input to primary somatosensory cortex (S1, postcentral gyrus).

• Somatotopy: body parts mapped to specific cortex regions (sensory homunculus).

• S1 sends information to association areas (S2) for further processing (e.g., tactile learning).

Pain Processing

• Thalamus relays pain to S1/S2, basal nuclei, limbic system, hypothalamus, prefrontal cortex.

• Brain modulates pain perception (e.g., placebo effect via endorphin release).

Special Senses Processing

• Vision: Thalamus to occipital cortex; association areas for recognition/spatial awareness.

• Hearing: Brainstem to thalamus to temporal cortex; association with language areas.

• Taste: Medulla to thalamus to insula/parietal cortex; integrates with hypothalamus/limbic system.

• Smell: Direct to limbic cortex, then thalamus, prefrontal cortex, hypothalamus.

• Balance: Brainstem, cerebellum, thalamus, cortex.

Movement and the CNS

Voluntary Movement

• Planned/coordinated by motor cortex, basal nuclei, cerebellum, spinal cord.

• Upper Motor Neurons: Cell bodies in cortex/brainstem; axons descend to spinal cord, synapse on interneurons/lower motor neurons.

• Lower Motor Neurons: Cell bodies in anterior horn; axons exit spinal cord to innervate skeletal muscle.

Motor Pathways

• Corticospinal Tracts: Originate in cortex, descend through internal capsule, decussate in medulla, control contralateral body.

• Corticonuclear Tracts: Originate in cortex, synapse on cranial nerve nuclei, control head/neck muscles.

Role of the Cerebral Cortex

• Primary motor cortex and association areas select and initiate motor programs.

• Somatotopic organization (motor homunculus) reflects importance of hands, face, tongue.

Role of the Basal Nuclei

• Modify activity of upper motor neurons; inhibit inappropriate movements, initiate voluntary movements.

• Substantia nigra (dopaminergic neurons) enhances basal nuclei output; damage causes movement disorders (e.g., Parkinson's disease).

Role of the Cerebellum

• Monitors ongoing movement, integrates sensory/motor input, corrects motor error (difference between intended and actual movement).

• Influences upper motor neurons via cortex; essential for smooth, coordinated movement.

• Damage causes ataxia (jerky, uncoordinated movement).

Table: Major Brain Divisions and Functions

Division	Main Structures	Primary Functions
Cerebrum	Frontal, parietal, temporal, occipital lobes; insula	Higher mental functions, sensation, movement, language, memory
Diencephalon	Thalamus, hypothalamus, epithalamus, subthalamus	Sensory relay, homeostasis, endocrine regulation, biological rhythms
Cerebellum	Anterior, posterior, flocculonodular lobes	Coordination, balance, motor learning
Brainstem	Midbrain, pons, medulla oblongata	Vital functions, reflexes, movement, sensation, alertness

Key Terms and Definitions

• Gyri: Elevated ridges on the brain surface.

• Sulci: Shallow grooves between gyri.

• Fissures: Deep grooves separating brain regions.

• Ventricles: CSF-filled cavities in the brain.

• Choroid Plexus: Structure producing CSF in ventricles.

• Somatotopy: Mapping of body regions to specific brain areas.

• Homunculus: Visual representation of somatotopic organization.

• Decussation: Crossing over of nerve fibers (e.g., in medulla).

• Motor Program: Coordinated set of muscle actions for a movement.

• Ataxia: Lack of muscle coordination due to cerebellar damage.

Selected Equations and Concepts

• Resting Membrane Potential: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

• Ohm's Law (for neurons): $V = IR$

• Action Potential Propagation: $\text{Speed} \propto \sqrt{\text{axon diameter}}$

Summary

• The CNS integrates sensory input, coordinates motor output, and maintains homeostasis through complex interactions among its regions.

• Protection is provided by the skull, meninges, CSF, and blood brain barrier.

• Higher mental functions, sensation, and movement depend on specialized structures and pathways within the CNS.