EPPP

The nervous system divides into the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), comprising all neural tissue outside the CNS. The PNS subdivides into the somatic nervous system (voluntary movement, sensory input) and the autonomic nervous system (involuntary functions).

The autonomic nervous system contains the sympathetic division (arousal, fight-or-flight response) and parasympathetic division (rest-and-digest functions). These divisions typically have opposing effects on target organs and maintain homeostasis through balanced activation.

Neurons are the primary signaling cells of the nervous system. A typical neuron consists of dendrites (receive signals), cell body or soma (contains nucleus, integrates signals), axon (transmits action potentials), and axon terminals (release neurotransmitters). Glial cells support neuronal function and include astrocytes (metabolic support, blood-brain barrier), oligodendrocytes (CNS myelination), Schwann cells (PNS myelination), and microglia (immune function).

The resting membrane potential of approximately -70 mV results from unequal ion distribution maintained by the sodium-potassium pump. Action potentials occur when membrane depolarization reaches threshold, triggering voltage-gated sodium channels to open. The action potential follows the all-or-none principle and propagates down the axon through successive depolarization. Myelination increases conduction velocity through saltatory conduction, where action potentials jump between nodes of Ranvier.

Synaptic transmission occurs when action potentials trigger calcium influx at axon terminals, causing vesicles to release neurotransmitters into the synaptic cleft. Neurotransmitters bind to receptors on the postsynaptic membrane, producing excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs). Summation of these potentials determines whether the postsynaptic neuron fires.

Acetylcholine (ACh) functions in the neuromuscular junction, autonomic nervous system, and CNS. In the brain, ACh is involved in attention, arousal, and memory. Degeneration of cholinergic neurons in the basal forebrain contributes to Alzheimer's disease. ACh acts through nicotinic receptors (ionotropic) and muscarinic receptors (metabotropic).

Dopamine pathways include the mesolimbic pathway (reward, motivation), mesocortical pathway (cognition, executive function), nigrostriatal pathway (motor control), and tuberoinfundibular pathway (prolactin regulation). Dopamine dysfunction is implicated in Parkinson's disease (nigrostriatal degeneration), schizophrenia (mesolimbic overactivity), and addiction.

Norepinephrine, produced primarily in the locus coeruleus, modulates arousal, attention, and stress responses. Serotonin, originating mainly from the raphe nuclei, regulates mood, sleep, appetite, and aggression. Dysfunction in serotonin systems is associated with depression, anxiety, and obsessive-compulsive disorder.

Glutamate is the primary excitatory neurotransmitter in the brain, acting through AMPA, NMDA, and metabotropic receptors. NMDA receptors are particularly important for learning and memory through long-term potentiation (LTP). GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter, acting through GABA-A (ionotropic) and GABA-B (metabotropic) receptors. Benzodiazepines enhance GABA-A receptor function.

Endorphins and enkephalins are endogenous opioid peptides that modulate pain perception and reward. They act through mu, delta, and kappa opioid receptors.

The brainstem, comprising the medulla, pons, and midbrain, controls vital functions including respiration, heart rate, and consciousness. The reticular formation regulates arousal and sleep-wake cycles. The cerebellum coordinates movement, balance, and motor learning, and contributes to cognitive functions including timing and language processing.

The diencephalon contains the thalamus and hypothalamus. The thalamus relays sensory information to the cortex and regulates consciousness and attention. The hypothalamus controls the autonomic nervous system, regulates homeostatic functions (temperature, hunger, thirst), controls the pituitary gland, and influences emotional behavior.

The limbic system, involved in emotion, memory, and motivation, includes the hippocampus (declarative memory formation, spatial navigation), amygdala (emotional processing, fear conditioning), and cingulate cortex (emotional regulation, pain processing). The basal ganglia, including the caudate, putamen, and globus pallidus, coordinate voluntary movement and are involved in habit learning and reward processing.

The cerebral cortex is organized into four lobes. The frontal lobe contains the primary motor cortex, premotor cortex, and prefrontal cortex, which is responsible for executive functions including planning, decision-making, working memory, and impulse control. Broca's area in the left inferior frontal gyrus controls speech production.

The parietal lobe contains the primary somatosensory cortex and association areas involved in spatial processing, attention, and numerical cognition. The temporal lobe contains the primary auditory cortex, Wernicke's area (language comprehension), and regions important for memory and object recognition. The occipital lobe contains the primary visual cortex and visual association areas.

The corpus callosum connects the two hemispheres, enabling interhemispheric communication. Split-brain studies revealed hemispheric specialization: the left hemisphere typically dominates language and analytical processing, while the right hemisphere excels in spatial processing and holistic perception.

Neuroplasticity refers to the brain's capacity to change in response to experience. Structural plasticity involves changes in synaptic connections, dendritic branching, and neurogenesis. Functional plasticity involves reorganization of neural representations and functions.

Long-term potentiation (LTP) is a persistent strengthening of synaptic connections following high-frequency stimulation, considered a cellular mechanism for learning and memory. LTP requires NMDA receptor activation, calcium influx, and subsequent molecular cascades leading to increased AMPA receptor expression and structural synaptic changes.

Long-term depression (LTD) is a persistent weakening of synaptic connections following low-frequency stimulation, important for memory specificity and motor learning. Experience-dependent plasticity, including critical periods and sensitive periods, shapes neural development. Adult neurogenesis, primarily in the hippocampus and olfactory bulb, contributes to learning and memory throughout life.