Central and Peripheral Nervous System: Structure, Function, and Disorders

The Central Nervous System (CNS): Structure of the Brain

Overview of the CNS

The Central Nervous System (CNS) consists of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly. Cephalization refers to the evolutionary trend of increasing concentration of nervous tissue in the head, culminating in the complex human brain.

• Spinal Cord: Central cavity surrounded by gray matter, with external white matter composed of myelinated fiber tracts.

• Brain: Similar to the spinal cord but with additional gray matter in nuclei and cortex.

Ventricles of the Brain

• Continuous with the central canal of the spinal cord.

• Lined with ependymal cells and filled with cerebrospinal fluid (CSF).

• Major ventricles:

  • Paired C-shaped lateral ventricles

  • Third ventricle in the diencephalon

  • Fourth ventricle in the hindbrain, dorsal to the pons

Cerebral Hemispheres

• Form the superior part of the brain (83% of mass).

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

• Divided into five lobes: frontal, parietal, temporal, occipital, and insula.

• Major sulci:

  • Central sulcus: separates frontal and parietal lobes

  • Parieto-occipital sulcus: separates parietal and occipital lobes

  • Lateral sulcus: separates parietal and temporal lobes

Cerebral Cortex

The cerebral cortex is the superficial gray matter, accounting for 40% of brain mass. It is responsible for sensation, communication, memory, understanding, and voluntary movements. Each hemisphere controls the opposite side of the body (contralateral control).

• Functional Areas:

  • Motor areas: Control voluntary movement (e.g., primary motor cortex, premotor cortex, Broca’s area, frontal eye field).

  • Sensory areas: Conscious awareness of sensation (e.g., primary somatosensory cortex, visual and auditory areas, olfactory, gustatory, vestibular cortices).

  • Association areas: Integrate diverse information (e.g., prefrontal cortex, language areas, general interpretation area, visceral association area).

Lateralization of Cortical Function

• Lateralization: Each hemisphere has unique abilities.

• Left hemisphere: Language, math, logic.

• Right hemisphere: Visual-spatial skills, emotion, artistic skills.

Cerebral White Matter

• Deep myelinated fibers and tracts responsible for communication within the brain.

• Types:

  • Commissures: Connect corresponding gray areas of the two hemispheres (e.g., corpus callosum).

  • Association fibers: Connect different parts of the same hemisphere.

  • Projection fibers: Connect the cortex with lower brain or spinal cord centers.

Basal Nuclei

• Masses of gray matter deep within the white matter.

• Components: caudate nucleus, putamen, globus pallidus.

• Functions: Influence muscular activity, regulate attention and cognition, inhibit unnecessary movement.

Diencephalon

• Central core of the forebrain, surrounded by cerebral hemispheres.

• Consists of:

  • Thalamus: Relay station for sensory impulses; mediates sensation, motor activities, learning, and memory.

  • Hypothalamus: Main visceral control center; regulates homeostasis, emotions, body temperature, hunger, sleep, and endocrine functions.

  • Epithalamus: Contains pineal gland (secretes melatonin) and choroid plexus (produces CSF).

Brain Stem

• Consists of midbrain, pons, and medulla oblongata.

• Controls automatic behaviors necessary for survival; pathway for tracts between higher and lower brain centers.

• Associated with 10 of the 12 pairs of cranial nerves.

• Key structures:

  • Midbrain: Cerebral peduncles, corpora quadrigemina (superior/inferior colliculi), substantia nigra.

  • Pons: Connects higher brain centers and the spinal cord; relays impulses between motor cortex and cerebellum.

  • Medulla oblongata: Contains vital centers for cardiovascular, respiratory, and other autonomic functions.

Cerebellum

• Located dorsal to pons and medulla; 11% of brain mass.

• Coordinates timing and patterns of skeletal muscle contraction; involved in language and problem solving.

• Structure: Two hemispheres, three lobes each, gray matter cortex, internal white matter (arbor vitae).

• Connected to brain stem via cerebellar peduncles.

The Central Nervous System: Function and Protection of the Brain

Functional Brain Systems

• Limbic System: Emotional brain; includes amygdala (anger, fear), cingulate gyrus (emotional expression), hippocampus (memory).

• Reticular Formation: Maintains alertness (RAS), filters stimuli, controls autonomic functions.

Brain Waves and EEG

• EEG (Electroencephalogram): Records electrical activity of the brain.

• Types of brain waves:

  • Alpha: Relaxed, awake state.

  • Beta: Alert, active thinking.

  • Theta: Common in children, abnormal in adults.

  • Delta: Deep sleep, brain damage if present in awake adults.

• EEGs help diagnose brain disorders (e.g., epilepsy, tumors).

Consciousness and Sleep

• Consciousness: Awareness of self and environment; graded as alertness, drowsiness, stupor, coma.

• Sleep: Two types—NREM (four stages) and REM (dreaming, muscle inhibition).

• Sleep cycles alternate between NREM and REM; regulated by hypothalamus.

• Sleep disorders: narcolepsy, insomnia, sleep apnea.

Memory

• Short-term memory (STM): Temporary, limited capacity.

• Long-term memory (LTM): Limitless capacity, more stable.

• Transfer from STM to LTM influenced by emotional state, rehearsal, association, and automatic memory.

• Categories: Fact (declarative) memory (explicit information) and Skill (procedural) memory (motor skills).

• Memory mechanisms involve synaptic changes, neurotransmitter release, and long-term potentiation (LTP).

Protection of the Brain

• Meninges: Three connective tissue membranes—dura mater, arachnoid mater, pia mater.

• Cerebrospinal Fluid (CSF): Cushions, nourishes, and protects the brain; produced by choroid plexuses.

• Blood-Brain Barrier: Selective barrier protecting the brain from harmful substances; formed by capillary endothelium, basal lamina, and astrocyte feet.

The Central Nervous System: The Spinal Cord and Disorders

Spinal Cord Anatomy

• Extends from foramen magnum to L1 vertebra.

• Protected by bone, meninges, and CSF.

• 31 pairs of spinal nerves attach by dorsal (sensory) and ventral (motor) roots.

• Key features: conus medullaris, filum terminale, denticulate ligaments, cauda equina.

• Gray matter (internal): dorsal, ventral, and lateral horns; gray commissure.

• White matter (external): ascending, descending, and transverse tracts organized into funiculi (columns).

Spinal Cord Pathways

• Ascending (sensory) pathways: Carry sensory information to the brain (e.g., spinothalamic, spinocerebellar tracts).

• Descending (motor) pathways: Deliver motor commands from the brain (e.g., corticospinal, rubrospinal tracts).

• Pathways are typically paired, decussate (cross over), and involve two or three neurons.

Spinal Cord Disorders

• Trauma: Concussion, contusion, hemorrhage, edema.

• Cerebrovascular accidents (strokes): Blockage of blood flow; treated with tissue plasminogen activator (TPA).

• Degenerative diseases: Alzheimer’s, Parkinson’s, Huntington’s, ALS (Lou Gehrig’s disease), poliomyelitis.

• Paralysis: Flaccid (lower motor neuron damage), spastic (upper motor neuron damage), paraplegia, quadriplegia.

The Peripheral Nervous System (PNS): Sensory Receptors, Nerves, and Plexuses

Sensory Receptors

• Specialized to respond to environmental changes (stimuli).

• Classified by stimulus type:

  • Mechanoreceptors: Mechanical force (pressure, vibration, stretch).

  • Thermoreceptors: Temperature changes.

  • Photoreceptors: Light (vision).

  • Chemoreceptors: Chemicals (taste, smell, blood chemistry).

  • Nociceptors: Potentially damaging stimuli (pain).

• Classified by location:

  • Exteroceptors: External stimuli (skin, special senses).

  • Interoceptors: Internal stimuli (viscera, blood vessels).

  • Proprioceptors: Degree of stretch in muscles, tendons, ligaments.

• Classified by structural complexity:

  • Simple receptors: Free or encapsulated dendritic endings (e.g., Merkel cells, Meissner’s corpuscles).

  • Complex receptors: Sense organs (e.g., eyes, ears).

Nerves

• Bundles of peripheral axons; may be sensory, motor, or mixed.

• Enclosed in connective tissue wrappings; contain blood and lymphatic vessels.

• Cranial nerves: 12 pairs, attach to brain, innervate head and neck (except vagus nerve).

• Spinal nerves: 31 pairs, attach to spinal cord, all mixed nerves, innervate body except head and part of neck.

• Spinal nerves split into dorsal (back) and ventral (front/trunk/limbs) rami.

Plexuses

• Networks where ventral rami of spinal nerves crisscross and redistribute fibers.

• Major plexuses:

  • Cervical: C1–C4 (neck, diaphragm via phrenic nerve).

  • Brachial: C5–T1 (shoulder, upper arm).

  • Lumbar: L1–L4 (abdominal wall, anterior/medial thigh).

  • Sacral: L4–S4 (pelvis, leg except anteromedial thigh; sciatic nerve).

• Thoracic nerves (T2–T12) do not form plexuses; become intercostal nerves.

The Peripheral Nervous System: Reflexes and Pain

Reflexes

• Rapid, predictable, involuntary motor responses to stimuli.

• Reflex arc components:

  1. Receptor

  2. Sensory neuron

  3. Integration center (CNS)

  4. Motor neuron

  5. Effector (muscle or gland)

• Classifications:

  • By development: innate (genetic) or acquired (learned)

  • By response: somatic (skeletal muscle) or visceral (autonomic)

  • By circuit: monosynaptic (single synapse) or polysynaptic (multiple synapses)

  • By integration site: spinal or cranial

• Examples:

  • Stretch reflex: Patellar (knee-jerk), involves muscle spindles.

  • Flexor (withdrawal) reflex: Response to pain, polysynaptic, ipsilateral.

  • Crossed extensor reflex: Contralateral response to pain (e.g., stepping on sharp object).

  • Superficial reflex: Plantar reflex; abnormal response (Babinski’s sign) indicates corticospinal damage.

• Reflexes can be modified by conscious effort and other neural inputs (EPSPs, IPSPs, presynaptic facilitation/inhibition).

Pain

• Warns of actual or impending tissue damage; difficult to measure clinically.

• Detected by nociceptors; tissue damage releases chemicals (prostaglandins, bradykinins, histamine, potassium) that activate pain fibers.

• Pain signals transmitted via glutamate and substance P.

• Pain threshold: Similar for all people; pain tolerance: Variable, influenced by age, culture, mental state.

• Chronic pain can lead to hyperalgesia (amplified pain) and phantom limb pain.

• Pain modulation involves descending pathways releasing epinephrine, norepinephrine, endorphins, and enkephalins to inhibit pain transmission.

Table: Major Brain Waves and Their Characteristics

Wave Type	Frequency (Hz)	State	Notes
Alpha	8–13	Relaxed, awake	Low amplitude, synchronous
Beta	14–30	Alert, active	Higher frequency, irregular
Theta	4–7	Children, some sleep	Abnormal in awake adults
Delta	0.5–4	Deep sleep	High amplitude, present in brain damage if awake

Table: Major Spinal Cord Tracts

Tract	Direction	Function
Spinothalamic	Ascending	Pain, temperature, crude touch
Spinocerebellar	Ascending	Proprioception to cerebellum
Corticospinal (pyramidal)	Descending	Voluntary motor control
Rubrospinal	Descending	Flexor muscle control
Reticulospinal	Descending	Balance, posture

Key Equations and Concepts

• Ohm's Law (for nerve conduction): $V = I \times R$ Where V = voltage (potential), I = current, R = resistance.

• Resting Membrane Potential: $V_m = V_{in} - V_{out}$ Where $V_m$ is the membrane potential, $V_{in}$ is the potential inside the cell, $V_{out}$ is the potential outside.

• Nernst Equation (for equilibrium potential): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$ Where R = gas constant, T = temperature, z = charge, F = Faraday's constant, [ion] = concentration.

Example: Patellar (Knee-Jerk) Reflex

• Stimulus: Tap on patellar tendon stretches quadriceps muscle.

• Receptor: Muscle spindle detects stretch.

• Sensory neuron: Transmits impulse to spinal cord.

• Integration center: Monosynaptic synapse in spinal cord.

• Motor neuron: Conducts impulse to quadriceps muscle.

• Effector: Quadriceps contracts, causing leg extension.

Additional info: This study guide covers the structure and function of the CNS and PNS, including major brain regions, spinal cord anatomy, sensory and motor pathways, reflexes, and pain mechanisms, as well as key clinical disorders and protective mechanisms.