W6 MHS1101 Nervous System 2

Structural Organization of Nervous System

• Central Nervous System (CNS):
  • Brain & spinal cord (processing centres)
• Peripheral Nervous System (PNS):
  • Cranial & spinal nerves (sensory input & motor response)

Brain Overview

• Contains ~97% of body’s nervous tissue
• ~1.4 kg
• ~1200 ml volume
• Regions:
  • Cerebrum
  • Cerebellum
  • Diencephalon
  • Brainstem
• Note: No correlation between brain size and intelligence

Cerebrum

• Largest part
• Controls higher functions (thought, memory, intellect)
• Divided:
  • Left and right hemispheres
  • Surface covered by grey matter (cerebral cortex)
• Gyri: Elevated ridges increasing surface area, separated by sulci (shallow) and fissures (deep)
• Lobes:
  • Frontal
  • Parietal
  • Temporal
  • Occipital
  • Insula
    • Central sulcus: Separates frontal and parietal lobes
    • Precentral gyrus: Anterior (motor control)
    • Postcentral gyrus: Posterior (sensory info)
    • Lateral sulcus: Separates frontal from temporal lobe; insula lies medial
    • Parieto-occipital sulcus: Parietal from occipital

Cerebral White Matter

• Association fibres: Connect within a hemisphere
  • Arcuate – short
  • Longitudinal fasciculi – long
• Commissural fibres: Connect two hemispheres
  • Corpus callosum
  • Anterior commissure
• Projection fibres: Link cortex to lower CNS
  • Internal capsule

Basal Nuclei

• Masses of grey matter deep in cerebrum
  • Caudate
  • Lentiform – putamen, globus pallidus, claustrum
• Functions:
  • Subconscious skeletal muscle tone
  • Coordination of learned movements (e.g. walking)
• Clinical: Increased basal nuclei activity → Parkinson’s symptoms

Cerebral Cortex Organization

• Motor areas: Precentral gyrus (primary motor cortex, pyramidal cells)
• Sensory areas: Postcentral gyrus (primary somatosensory cortex)
• Special sensory cortices: Visual, Auditory, Olfactory, Gustatory
• Association areas: Interpret data or coordinate response (premotor/somatic motor, sensory associations)
• Integrative centres: Direct complex motor activities, analytical functions (e.g., Wernicke’s & Broca’s areas)
  • Wernicke’s: Language comprehension (usually left)
  • Broca’s: Speech production (usually left)
• Prefrontal cortex: Abstract thought, prediction of consequences

Hemispheric Lateralization

• Left: Language, reading, writing, logic
• Right: Sensory analysis, spatial awareness, recognition of faces/voice inflections

Cerebellum

• Second-largest part, two hemispheres, covered by cerebellar cortex (grey matter)
• Functions: Adjust postural muscles, fine-tune conscious/subconscious movements
• Structures:
  • Folia (folds)
  • Anterior/posterior lobes
  • Vermis (midline)
  • Flocculonodular lobe
  • Arbor vitae (white matter)
  • Cerebellar nuclei
• Peduncles: Superior, middle, inferior connect cerebellum with brainstem/cerebrum/spinal cord
• Disorders: Ataxia (loss of coordination) from trauma, stroke, intoxication

Diencephalon

• Integrates sensory and motor functions
• Components:
  • Epithalamus (pineal gland – melatonin)
  • Thalamus (sensory relay)
  • Hypothalamus (emotions, autonomic function, hormones)
• Pituitary gland: Endocrine, connected to hypothalamus (infundibulum)

Thalamus

• Filters/relays sensory info to cortex/basal nuclei
• Thalamic nuclei: Relay info, separated by third ventricle/interthalamic adhesion

Hypothalamus

• Mammillary bodies: Reflex eating movements
• Infundibulum: Stalk to pituitary
• Tuber cinereum: Hormone production affecting pituitary

Brainstem

• Relays info between spinal cord & cerebrum/cerebellum
• Midbrain: Processes sight, sound, reflexes; consciousness
• Pons: Links cerebellum/brainstem; contains relay centres & nuclei for motor control
• Medulla oblongata: Connects brain to spinal cord; autonomic functions (heart rate, BP, digestion); sensory relay

Limbic System

• Establishes emotional states, links cortex with brainstem functions, memory storage/retrieval

Embryonic Development of Brain

• CNS from neural tube; rostral end forms Primary vesicles :
  • Prosencephalon (forebrain)
  • Mesencephalon (midbrain)
  • Rhombencephalon (hindbrain)
• Secondary vesicles:
  • Telencephalon (cerebrum)
  • Diencephalon
  • Mesencephalon
  • Metencephalon (cerebellum, pons)
  • Myelencephalon (medulla)

Brain Protection & Support

• Physical: Cranial bones, cranial meninges (dura, arachnoid, pia), cerebrospinal fluid
• Biochemical: Blood-brain barrier

Cranial Meninges

• Dura mater: Inner meningeal, outer periosteal; forms dural folds (falx cerebri, tentorium cerebelli, falx cerebelli)
• Arachnoid mater: Covers brain, separated from pia by subarachnoid space
• Pia mater: Bound to surface by astrocytes

Spinal Cord

• Enclosed in vertebral column & meninges, conducts info to/from brain, gives rise to spinal nerves

Gross Anatomy

• ~45 cm long, 14 mm wide; ends at L1-L2 in adults; four regions (cervical, thoracic, lumbar, sacral)
• 31 segments/nerves; divided by grooves (posterior median sulcus, anterior median fissure); central canal with CSF

Distal Structures

• Conus medullaris: Tapered end below lumbar enlargement
• Cauda equina: Bundle of nerve roots below conus
• Filum terminale: Fibrous thread anchoring cord to coccyx

Spinal Roots & Ganglia

• Anterior (ventral) root: Motor neurons
• Posterior (dorsal) root: Sensory neurons, with dorsal root ganglia

Enlargements

• Cervical: Shoulders/upper limbs
• Lumbosacral: Pelvis/lower limbs

Spinal Nerves

• Union of anterior/posterior roots; mixed (sensory & motor); each divides into rami (posterior – back, anterior – body wall/limbs)
• Named by vertebral level
• 8 pairs of cervical nerves (C1-C8), which pass through the cervical (neck) region.
• 12 pairs of thoracic nerves (T1-T12), which pass through the thoracic region.
• 5 pairs of lumbar nerves (L1-L5), which pass through the lumbar region.
• 5 pairs of sacral nerves (S1-S5), located in the sacral (buttock) region.
• 1 pair of coccygeal nerves (C0), which passes through the coccyx.

Spinal Meninges

• Three layers (continuous with cranial): dura, arachnoid, pia mater
• Functions: Protect cord, carry blood
• Arachnoid trabeculae: Collagen/elastic fibre network
• Subarachnoid space: CSF, blood vessels on pia
• Denticulate ligaments: Anchor pia to dura, stabilise cord

Internal Structure of Spinal Cord

• Gray matter: Neuron cell bodies, neuroglia, unmyelinated axons; forms “horns” (posterior – sensory, anterior – motor, lateral – visceral)
• Gray commissures: Cross-connection around central canal
• White matter: Myelinated/unmyelinated axons, forms columns (posterior, anterior, lateral)
• Commissures: Axon crossing between sides

Functional Organization

• Tracts: Bundles of axons with common origin/destination/function
  • Ascending: Sensory (to brain)
  • Descending: Motor (from brain)

Nerve Plexuses

• Interwoven nerve networks formed by anterior rami; allow redundancy in nerve supply
• Four major plexuses:
  • Cervical (neck)
  • Brachial (shoulder, upper limb)
  • Lumbar (pelvis, lower limb)
  • Sacral (pelvis, lower limb)
• Brachial plexus: C5–T1 (upper limb)
• Lumbosacral plexus: T12-L4 (pelvic/lower limb)

Lumbar Plexus

Sacral Plexus