Nervous System

Nervous System Overview - Brainy Blueprint

• Central Nervous System (CNS): Brain (cerebrum, cerebellum, brainstem) & Spinal Cord. Function: Integration, processing, command initiation.
• Peripheral Nervous System (PNS): Cranial nerves (12 pairs) & Spinal nerves (31 pairs). Function: Relays sensory information to CNS & motor commands from CNS.
  • Somatic Nervous System (SNS): Voluntary control of skeletal muscles. Neurotransmitter: Acetylcholine (ACh) at neuromuscular junction.
  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, glands.
    • Sympathetic Division: "Fight or flight" responses (e.g., ↑ heart rate, pupil dilation). Thoracolumbar outflow.
    • Parasympathetic Division: "Rest and digest" functions (e.g., ↓ heart rate, ↑ digestion). Craniosacral outflow.

⭐ The nervous system originates from the ectoderm, with neurulation being a key developmental stage forming the neural tube.

Neurohistology & Neurophysiology - Tiny Transmitters

• Neurons: Functional units.
  • Structure: Soma, Dendrites (input), Axon (output), Synapse.
  • Action Potential: Resting -70mV. Na⁺ influx (depolarization), K⁺ efflux (repolarization).
• Glial Cells: Support neurons.

  Cell Type	Location	Key Function(s)
  Astrocytes	CNS	BBB, support, K⁺ buffer
  Oligodendrocytes	CNS	Myelination (multiple axons)
  Microglia	CNS	Phagocytosis
  Ependymal cells	CNS	Line ventricles, CSF production
  Schwann cells	PNS	Myelination (single axon segment), regenerate
  Satellite cells	PNS	Support ganglia neuron bodies

• Neurotransmitters: Chemical signals.
  • Excitatory: Glutamate, ACh.
  • Inhibitory: GABA, Glycine.
  • Modulatory: Dopamine, Serotonin, NE.

⭐ Myelination: Oligodendrocytes (CNS) myelinate multiple axons; Schwann cells (PNS) myelinate a single axon segment.

Central Nervous System - Command Central

• Brain: Command center. Major divisions:
  • Cerebrum: Lobes (Frontal-motor, Parietal-sensory, Temporal-auditory/memory, Occipital-visual); higher functions, consciousness.
  • Cerebellum: Motor learning, coordination, posture, balance. Vermis, hemispheres.
  • Brainstem (Midbrain, Pons, Medulla): Autonomic control; vital reflexes; cranial nerve (CN III-XII) nuclei.
• Spinal Cord: From medulla to L1-L2 (conus medullaris), then cauda equina. Transmits signals; spinal reflexes.
  • Gray matter (H-shape: horns), White matter (funiculi: tracts).
• Meninges: Protective layers: Dura, Arachnoid (CSF in subarachnoid space), Pia mater.
• CSF: By choroid plexus; in ventricles & subarachnoid space. Buoyancy, protection, homeostasis.

⭐ MCA Stroke: Common; contralateral hemiplegia (face/arm > leg) & sensory loss. Aphasia (Broca's/Wernicke's - dominant hemisphere) or neglect (non-dominant).

Peripheral Nervous System & Clinical Links - Network Navigators

• Connects CNS to limbs & organs. Includes cranial nerves, spinal nerves, ganglia.
• Cranial Nerves (CN): 12 pairs. Key examples:

  No.	Name	Type	Foramen (Key)	Key Lesion/Function
  III	Oculomotor	M	SOF	"Down & out" gaze, ptosis
  V	Trigeminal	B	SOF/Rot/Ovale	Facial sensation, mastication; Trigeminal neuralgia
  VII	Facial	B	IAM/Stylomast	Facial expression; Bell's palsy
  X	Vagus	B	Jugular	Swallowing, speech; Hoarseness

• Spinal Nerves: 31 pairs (8C, 12T, 5L, 5S, 1Co). Form plexuses.
  • Brachial Plexus (C5-T1): Innervates upper limb.
    • Upper trunk (C5-C6) injury → Erb's palsy (waiter's tip).
    • Lower trunk (C8-T1) injury → Klumpke's palsy (claw hand).
  • Lumbosacral Plexus (L1-S4): Innervates lower limb. Sciatica common.

⭐ Erb's palsy (C5-C6 injury) results in an arm that hangs by the side, medially rotated, with the forearm pronated ("waiter's tip" or "policeman's tip" hand).

High‑Yield Points - ⚡ Biggest Takeaways

• Broca's area (motor speech) in inferior frontal gyrus; Wernicke's area (sensory speech) in superior temporal gyrus.
• CN nuclei: III, IV in midbrain; V-VIII in pons; IX-XII in medulla.
• Circle of Willis: AComA is a common site for berry aneurysms.
• Key dermatomes: T4 at nipple, T10 at umbilicus, L4 over knee, S1 lateral foot.
• Corticospinal tract (motor) decussates at pyramids; Spinothalamic tract for pain & temperature.
• Cerebellar lesions: ipsilateral ataxia, intention tremor, nystagmus.