Nervous System

The human nervous system is the body’s electrical and chemical communication network. About 86 billion neurons connect through roughly 150 trillion synapses to coordinate everything you do: every thought, every muscle movement, every sensation, every breath, every heartbeat. This study note covers the structural divisions (CNS, PNS, somatic, autonomic, sympathetic, parasympathetic), the cellular components (neurons, glia), how action potentials and synapses actually work, and what happens when the system fails.

Structural Divisions

Anatomically, the nervous system divides into two parts.

• Central nervous system (CNS). The brain and spinal cord. The processing center where information is integrated, memories are stored, decisions are made, and motor commands originate.
• Peripheral nervous system (PNS). Every nerve outside the CNS. Cranial nerves, spinal nerves, and the autonomic ganglia. The PNS carries sensory information from the body to the CNS, and motor commands from the CNS to the body.

Functional Divisions

Functionally, the PNS divides further into voluntary and involuntary subsystems.

Somatic Nervous System

Controls voluntary movement of skeletal muscle. When you decide to lift your arm, motor neurons in your spinal cord send signals through somatic motor nerves to your arm muscles. Somatic sensory nerves carry touch, pain, temperature, and proprioception back to the CNS.

Autonomic Nervous System

Controls involuntary functions: heart rate, blood pressure, digestion, sweating, pupil dilation, glandular secretion. You do not consciously direct any of it. The autonomic system itself divides into two branches that usually oppose each other.

• Sympathetic (fight-or-flight). Activated under stress, exercise, or perceived threat. Increases heart rate and blood pressure, dilates pupils and bronchi, redirects blood from gut to muscle, releases adrenaline and noradrenaline. Prepares the body for action.
• Parasympathetic (rest-and-digest). Activated during relaxation, eating, sleeping. Slows heart rate, increases digestion, constricts pupils, conserves energy. The main effector is the vagus nerve, which alone carries about 75% of parasympathetic outflow.

Most organs receive input from both branches and respond to whichever is currently dominant. A healthy autonomic system shifts smoothly between them depending on the situation.

The Neuron

The functional unit of the nervous system is the neuron. A typical neuron has four distinct parts.

• Dendrites. Branching extensions that receive signals from other neurons. A single neuron may have thousands of dendritic inputs.
• Cell body (soma). Contains the nucleus and most organelles. Integrates incoming signals.
• Axon. A long thin projection that carries the outgoing electrical signal. Axons can be over a meter long in tall humans (the sciatic nerve from spinal cord to toe).
• Axon terminals. The endings where the axon meets the next cell, forming a synapse. Most synapses release neurotransmitters into the synaptic cleft.

Many axons are wrapped in myelin, a fatty insulating sheath produced by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS). Myelin dramatically speeds up signal conduction — myelinated axons conduct at 100 m/s, unmyelinated at 0.5-2 m/s. This is why multiple sclerosis (a demyelinating disease) produces such severe neurological symptoms.

Action Potentials

Neurons communicate via action potentials — brief electrical pulses that travel down the axon.

1. Resting potential. A neuron at rest has a membrane potential of about -70 mV, with the inside negative compared to the outside. This gradient is maintained by the Na⁺/K⁺ ATPase pump and selective ion channels.
2. Depolarization. If incoming signals push the membrane potential above the threshold (around -55 mV), voltage-gated Na⁺ channels open. Sodium rushes in, the membrane potential briefly reverses to about +30 mV.
3. Repolarization. Sodium channels close. Voltage-gated K⁺ channels open. Potassium flows out, restoring the negative interior.
4. Hyperpolarization and recovery. The membrane briefly overshoots to about -80 mV. The Na⁺/K⁺ ATPase pump restores the original ion balance. The neuron is ready for another action potential.

The entire action potential lasts about 1-2 milliseconds. A myelinated axon then conducts it down the length of the axon via saltatory conduction (jumping between nodes of Ranvier) at speeds up to 100 m/s.

Synapses and Neurotransmitters

Action potentials end at the axon terminal, where the neuron meets the next cell at a synapse. Most synapses are chemical: the arriving action potential triggers vesicles full of neurotransmitter to release into the synaptic cleft. Neurotransmitters bind receptors on the next cell, opening ion channels and either exciting (depolarizing) or inhibiting (hyperpolarizing) that cell.

The major neurotransmitters:

• Glutamate — the main excitatory neurotransmitter in the CNS
• GABA — the main inhibitory neurotransmitter; benzodiazepines and alcohol act here
• Acetylcholine — neuromuscular junction; many CNS roles; nicotine acts on its receptors
• Dopamine — reward, motivation, motor control; Parkinson’s disease is dopamine loss in the substantia nigra
• Serotonin — mood, sleep, appetite; SSRIs increase its synaptic availability
• Noradrenaline — arousal, attention, fight-or-flight response

Glial Cells — the Other 50%

Until the 1980s, neurons got most of the attention. We now know that glial cells outnumber neurons in many parts of the brain and do far more than passive support.

• Astrocytes regulate the blood-brain barrier, supply nutrients to neurons, and recycle neurotransmitters.
• Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin.
• Microglia are the brain’s resident immune cells, clearing debris and dead cells, and orchestrating responses to injury and infection.
• Ependymal cells line the ventricles and produce cerebrospinal fluid.

Related study notes: Homeostasis, Protein, Enzyme.

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