Memory tricks for neurons, signals, and nervous system divisions

CNS (Central Nervous System): brain + spinal cord — housed within bone (skull and vertebral column). Integration and command center. PNS (Peripheral Nervous System): all nervous tissue outside the CNS — cranial nerves (12 pairs), spinal nerves (31 pairs), ganglia, sensory receptors. PNS has two functional divisions: Sensory (afferent) division carries signals TO the CNS. Motor (efferent) division carries signals FROM the CNS. Motor division splits further: Somatic nervous system (voluntary — skeletal muscle) and Autonomic nervous system (involuntary — smooth muscle, cardiac muscle, glands).

Dendrites: branching extensions — receive signals from other neurons or sensory receptors. Increase in number with complexity. Cell body (soma): contains nucleus and organelles — integrates incoming signals. If threshold reached at axon hillock → fires action potential. Axon: single long process — conducts action potentials away from cell body. One per neuron. Axon hillock: where axon joins cell body — decision point, lowest threshold. Myelin sheath: lipid insulation around axon from Schwann cells (PNS) or oligodendrocytes (CNS). Speeds conduction by forcing AP to jump between nodes of Ranvier (saltatory conduction). Axon terminals (synaptic knobs): release neurotransmitters into synaptic cleft.

Resting membrane potential: -70 mV (K+ leaks out, Na+ pumped out by Na+/K+ ATPase). Depolarization: stimulus → Na+ channels open → Na+ rushes IN → membrane goes from -70 to +30 mV. Repolarization: Na+ channels inactivate → K+ channels open → K+ rushes OUT → returns toward -70 mV. After-hyperpolarization: K+ channels slow to close → briefly below -70 mV (~-80 mV). Refractory period: absolute (Na+ channels inactivated — no new AP possible) → relative (need larger-than-normal stimulus). All-or-none: once threshold is reached, the AP is always the same size — frequency encodes stimulus strength, not amplitude.

Action potential reaches axon terminal → voltage-gated Ca2+ channels open → Ca2+ flows in → Ca2+ triggers synaptic vesicle fusion with presynaptic membrane → neurotransmitter released into synaptic cleft by exocytosis → NT diffuses across cleft → binds postsynaptic receptors → ion channels open or close → EPSP (excitatory, depolarizing) or IPSP (inhibitory, hyperpolarizing) generated in postsynaptic cell. NT then removed: reuptake into presynaptic terminal (most NTs), enzymatic degradation (ACh by acetylcholinesterase), or diffusion. Summation of EPSPs and IPSPs at axon hillock determines if AP fires.

CNS Glia: Astrocytes (most numerous CNS glia — blood-brain barrier, metabolic support, K+ buffering), Oligodendrocytes (produce CNS myelin — one cell myelinates multiple axon segments), Microglia (CNS immune cells — phagocytose debris and pathogens, activated in neuroinflammation), Ependymal cells (line brain ventricles and central canal of spinal cord — produce and circulate cerebrospinal fluid). PNS Glia: Schwann cells (produce PNS myelin — one cell per one segment, enable nerve regeneration), Satellite cells (surround and support cell bodies in ganglia). Glia do NOT generate action potentials — but actively regulate neural activity.

Brainstem (medulla, pons, midbrain): vital functions — breathing, heart rate, blood pressure, swallowing, vomiting. Reticular activating system (consciousness). CN III-XII originate here. Diencephalon: thalamus (relay station — all sensory info except smell passes through) + hypothalamus (master homeostasis center — temperature, hunger, thirst, circadian rhythm, pituitary control). Cerebellum: coordination, balance, fine-tuning movements. Receives copy of all motor commands. Damage → ataxia (clumsy, uncoordinated movement). Cerebrum: largest part — conscious thought, voluntary movement, language, memory, sensory perception. Divided into 4 lobes: frontal (motor, personality), parietal (sensory), temporal (hearing, memory), occipital (vision).

The spinal cord has a butterfly-shaped gray matter (cell bodies) surrounded by white matter (axon tracts). Dorsal (posterior) horn: receives sensory input — pain, temperature, touch enter here. Dorsal root carries sensory fibers (afferent). Dorsal root ganglion: contains cell bodies of sensory neurons. Ventral (anterior) horn: motor output — alpha motor neurons here control skeletal muscle. Ventral root carries motor fibers (efferent). Bell-Magendie Law: dorsal roots = sensory, ventral roots = motor. Spinal nerves form when dorsal and ventral roots merge — mixed (both sensory and motor). 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Sympathetic (fight or flight): thoracolumbar origin (T1-L2). Short preganglionic, long postganglionic. NE at target (except sweat glands = ACh). Effects: ↑HR, ↑BP, bronchodilation, ↓digestion, ↑blood glucose, pupil dilation, piloerection. Parasympathetic (rest and digest): craniosacral origin (CN III, VII, IX, X + S2-S4). Long preganglionic, short postganglionic. ACh throughout. Effects: ↓HR, ↑digestion, bronchoconstriction, pupil constriction, bladder contraction. Vagus nerve (CN X): carries 75% of parasympathetic fibers — heart, lungs, GI tract. Both divisions active simultaneously — the balance determines organ response. ANS control is largely subconscious — hypothalamus is the primary control center.

Reflexes are rapid, involuntary, predictable responses to stimuli — processed at the spinal cord level without waiting for brain input. Five components: Receptor: detects stimulus. Afferent (sensory) neuron: carries signal to spinal cord. Integration center (spinal cord): processes signal. Efferent (motor) neuron: carries response signal. Effector: muscle or gland that responds. Stretch reflex (patellar/knee-jerk): monosynaptic — one synapse between afferent and efferent. Only monosynaptic reflex. Withdrawal reflex: polysynaptic — pulls limb away from pain. Crossed extensor reflex: simultaneous withdrawal of one limb and extension of other (weight shift). Reflexes test spinal cord integrity — hyperreflexia (UMN lesion), hyporeflexia (LMN lesion).