Central Nervous System (CNS)

Development and Structure of the CNS

• Neural Tube: The CNS begins as the neural tube in the embryo, which differentiates into the brain and spinal cord.

• Primary Brain Vesicles: By week 4, the neural tube forms three primary vesicles:

  • Prosencephalon (Forebrain): Develops into the cerebrum and diencephalon.

  • Mesencephalon (Midbrain): Remains as the midbrain.

  • Rhombencephalon (Hindbrain): Develops into the pons, medulla oblongata, and cerebellum.

• Secondary Brain Vesicles: By week 5, these vesicles further divide to form adult brain structures.

Meninges and Cerebrospinal Fluid (CSF)

• Meninges: Three connective tissue membranes surrounding the CNS:

  • Dura mater (most superficial)

  • Arachnoid mater (middle)

  • Pia mater (deepest)

• CSF Location: Found in the choroid plexuses, ventricles, central canal, and subarachnoid space.

• CSF Function: Cushions the brain, maintains ionic balance, and absorbs shock.

• CSF Retrieval: Lumbar puncture (spinal tap) is used to obtain CSF for diagnostic purposes.

• Epidural Space: Space outside the dura mater containing fat for cushioning the spinal cord.

• Blood-Brain Barrier: Formed by tight junctions; selectively allows nutrients and ions into the brain, absent in the hypothalamus and medulla oblongata.

Major Brain Regions and Functions

• Cerebrum: Divided into five lobes:

  • Frontal: Motor control, intelligence, reasoning, Broca's area (language production).

  • Parietal: Sensory perception, somatosensory cortex.

  • Occipital: Visual processing.

  • Temporal: Auditory processing.

  • Insula: Deep lobe, involved in taste and visceral sensation.

• Divisions:

  • Central sulcus: Separates frontal and parietal lobes.

  • Lateral sulcus: Separates parietal and temporal lobes.

  • Longitudinal fissure: Divides cerebral hemispheres.

  • Transverse fissure: Separates cerebrum from cerebellum.

• Gyri: Elevated ridges of the cerebral cortex.

• Sulci: Shallow grooves.

• Fissures: Deep grooves.

Ventricles of the Brain

• Two lateral ventricles (one in each hemisphere)

• Third ventricle (in the diencephalon, separated by the septum pellucidum)

• Fourth ventricle (between brainstem and cerebellum)

• Cerebral aqueduct: Connects third and fourth ventricles.

• Central canal: Runs through the spinal cord, contains CSF.

Diencephalon and Brainstem

• Diencephalon: Contains the thalamus (sensory relay), hypothalamus (homeostasis, endocrine control), and epithalamus (pineal gland, melatonin production).

• Brainstem: Consists of midbrain (visual/auditory reflexes), pons (respiratory control, relay), and medulla oblongata (vital centers for heart rate, respiration, BP).

Functional Areas of the Cerebral Cortex

• Precentral gyrus: Primary motor cortex (voluntary movement).

• Postcentral gyrus: Primary somatosensory cortex (sensory input, spatial discrimination).

Gray and White Matter

• Gray matter: Contains neuron cell bodies and dendrites; found in cerebral cortex, basal nuclei, and central spinal cord.

• White matter: Contains axons; found in fiber tracts (association, commissural, projection fibers) and outer spinal cord.

Basal Nuclei and Limbic System

• Basal nuclei: Groups of cell bodies in the CNS; involved in motor control and cognition.

• Limbic system: Emotional brain; includes amygdala, hypothalamus, hippocampus.

• Reticular activating system: Maintains arousal and consciousness.

Memory and Lateralization

• Short-term memory: Temporary, limited capacity (7-8 items).

• Long-term memory: Limitless capacity, strengthened by association and repetition.

• Hemispheric lateralization: Each hemisphere specializes in certain functions, but both work together.

• Decussation: Crossing over of tracts (e.g., corticospinal) at the medulla oblongata.

Cerebellum

• Located inferior to the cerebrum, posterior to the brainstem.

• Coordinates subconscious proprioception, timing, and patterns of skeletal muscle contractions.

Spinal Cord Anatomy

• Gray matter: Butterfly-shaped central region (cell bodies, dendrites).

• White matter: Surrounds gray matter (axons in tracts).

• Landmarks: Foramen magnum (superior), L2 (inferior), conus medullaris (end of spinal cord), cauda equina (spinal roots below L2), filum terminale (pia mater extension anchoring cord).

• Denticulate ligaments: Extensions of pia mater anchoring cord laterally.

Spinal Cord Tracts

Homeostatic Imbalances of the CNS

• Alzheimer's disease: Progressive dementia, memory loss, beta-amyloid plaques, neurofibrillary tangles.

• Parkinson's disease: Movement disorder, dopamine deficiency.

• Coma: Persistent vegetative state, absence of EEG waves.

• Insomnia/Narcolepsy/Sleep apnea: Sleep disorders affecting quality and duration of sleep.

• Hydrocephalus: CSF drainage obstruction, increased intracranial pressure.

• Epileptic seizures: Grand mal (generalized), petit mal (absence, brief loss of consciousness).

• Meningitis: Inflammation of brain/spinal cord membranes (bacterial/viral).

• Paralysis/Poliomyelitis/Quadriplegia/Paraplegia: Motor function loss due to spinal cord injury or infection.

Peripheral Nervous System (PNS)

General Function and Organization

• Links the CNS to the outside world via sensory receptors, nerves, and ganglia.

• Sensory receptors: Detect changes in the environment and relay information to the CNS.

Types of Sensory Receptors

Somatosensory System Organization

• Receptor level: Sensory receptors detect stimulus.

• Circuit level: Ascending pathways relay information to the CNS.

• Perceptual level: Sensory input is interpreted in the cerebral cortex.

Sensory and Motor Pathways

• Sensory nerves: Enter spinal cord via dorsal root ganglion and dorsal horn.

• Motor nerves: Exit spinal cord via ventral horn to effectors (muscles/glands).

• Dermatome: Area of skin innervated by a single spinal nerve.

Peripheral Nerve Structure

Spinal Nerves and Plexuses

• 31 pairs of spinal nerves:

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

• Nerve plexuses: Interlacing networks of nerves:

  • Cervical (phrenic nerve)

  • Brachial (median, ulnar, radial, axillary nerves)

  • Lumbar

  • Sacral (sciatic nerve – longest in body)

Cranial Nerves

Reflexes

• Reflex arc components: Receptor → sensory neuron → integration center → motor neuron → effector

• Types: Inborn (intrinsic) and learned (acquired)

• Stretch reflex: Maintains muscle length (e.g., patellar reflex)

• Tendon reflex: Prevents muscle/tendon damage from excessive stretch

• Flexor reflex: Withdrawal from painful stimulus

• Ipsilateral reflex: Same side response; Contralateral reflex: Opposite side response

Autonomic Nervous System (ANS)

General Function and Organization

• Part of the motor division of the PNS; controls smooth muscle, cardiac muscle, and glands automatically.

• Main control centers: Hypothalamus (overall control), medulla oblongata (direct control of HR, BP, respiration).

Somatic vs. Autonomic Nervous System

Divisions of the ANS

• Sympathetic ("fight or flight"): Thoracic/lumbar origin; short preganglionic, long postganglionic fibers; peripheral ganglia near spinal cord; releases norepinephrine/epinephrine.

• Parasympathetic ("rest and digest"): Cranial/sacral origin; long preganglionic, short postganglionic fibers; ganglia near organs; releases acetylcholine.

• Dual innervation: Most organs receive input from both divisions, usually with antagonistic effects.

• Cooperative effects: Both divisions can work together (e.g., sexual function).

Receptors and Neurotransmitters

• Cholinergic receptors: Bind acetylcholine.

• Adrenergic receptors: Bind epinephrine/norepinephrine.

• Adrenal medulla: Releases epinephrine/norepinephrine into bloodstream.

General Effects

Homeostatic Imbalances

• Permanent hypertension: Overactive sympathetic division, often due to stress.

• Orthostatic hypotension: Dizziness upon standing due to poor sympathetic response.

Special Senses

Overview

• Special senses include vision, gustation (taste), olfaction (smell), hearing, and balance.

Vision

• Accessory structures: Eyebrows, eyelids, conjunctiva, lacrimal apparatus, extrinsic eye muscles.

• Lacrimal apparatus: Lacrimal glands, ducts, punctum, canaliculi, nasolacrimal duct; produces and drains tears.

• Extrinsic eye muscles: Controlled by cranial nerves III (oculomotor), IV (trochlear), VI (abducens).

• Cornea: Avascular, easily transplanted.

• Layers of the eye:

  • Fibrous (sclera, cornea)

  • Vascular (choroid, ciliary body, iris)

  • Neural (retina)

• Optic disc: Blind spot, no photoreceptors, origin of optic nerve (II).

• Macula lutea/fovea centralis: Area of greatest visual acuity, contains cones.

• Photoreceptors: Rods (dim light, black/white), cones (color, high acuity).

• Visual transduction: Conversion of light to electrical signals in retina (photoreceptors → bipolar cells → ganglion cells → optic nerve).

• Visual pathway: Signals decussate at the medulla oblongata; primary visual cortex in occipital lobe; association area interprets form, color, depth, motion.

• Light path: Cornea → aqueous humor → lens → vitreous humor → retina.

• Humors: Aqueous (anterior segment), vitreous (posterior segment); aqueous humor drains via Canal of Schlemm.

• Refraction: Bending of light; accommodation for near vision involves lens bulging, pupil constriction, eyeball convergence.

• Visible spectrum: 400–700 nm; cones sensitive to red (560 nm), green (530 nm), blue (500 nm).

• Adaptation: Light adaptation (entering bright light), dark adaptation (entering darkness).

• Common disorders: Myopia (nearsighted, concave lens), hyperopia (farsighted, convex lens), presbyopia (aging lens), astigmatism (unequal curvature), glaucoma (increased intraocular pressure), cataracts (clouded lens), color blindness (cone loss), night blindness (rod dysfunction).

Gustation (Taste) and Olfaction (Smell)

• Olfactory receptors: Olfactory hairs in nasal cavity; supporting cells secrete mucus; pathway projects to primary olfactory cortex and limbic system.

• Gustatory receptors: Located in taste buds; five tastes: sour (acids), bitter (caffeine/morphine), sweet (sugars/alcohol), salty (metals), umami (glutamate).

• Gustatory cortex: Located in the insula.

• Cranial nerves for taste: VII (facial, anterior 2/3 tongue), IX (glossopharyngeal, posterior 1/3), X (vagus, epiglottis/pharynx).

• 80% of taste is due to smell.

Hearing and Equilibrium

• Sound pathway: Pinna → external auditory canal → tympanic membrane → ossicles (malleus, incus, stapes) → oval window → cochlea (Organ of Corti).

• Organ of Corti: Receptor for hearing, located in cochlea; hair cells transduce sound.

• Cranial nerve: Vestibulocochlear (VIII); cochlear branch for hearing, vestibular branch for balance.

• Auditory pathway: Cochlear nerve → thalamus → primary auditory cortex (temporal lobe).

• Equilibrium receptors:

  • Crista ampullaris: In semicircular canals; detects rotational acceleration (dynamic equilibrium).

  • Macula: In utricle and saccule; detects head position relative to gravity (static equilibrium).

• Pharyngotympanic (Eustachian) tube: Equalizes middle ear pressure.

• Sound localization: Achieved by comparing input from both ears.

• Common disorders: Conduction deafness (sound transmission issue), sensorineural deafness (neural damage), vertigo (balance disorder), motion sickness, dizziness, night blindness.

Additional info:

• Some content was logically grouped and expanded for clarity and completeness.

• Tables were recreated to summarize tract functions, cranial nerves, and receptor types.

• Definitions, examples, and clinical correlations were added for exam preparation.