🦴 Anatomy introductory Lesson 3 of 8 2 min read

The Nervous System

The brain, spinal cord, and nerves gather sensory information, process it, and command the body's responses using fast electrical and chemical signals.

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What you'll learn

Divide the nervous system into central and peripheral parts and their subdivisions.
Label the main structures of a neuron and describe their roles.
Explain the phases of an action potential in terms of ion movement.
Outline the steps of chemical synaptic transmission.
Match major brain regions to their principal functions.

Organization of the Nervous System

The nervous system is divided into:

Central Nervous System (CNS) — brain and spinal cord; integration and command
Peripheral Nervous System (PNS) — all nerves outside the CNS; sensory input + motor output

PNS is further divided:

Somatic — voluntary motor control of skeletal muscles; sensory from skin, joints, and muscle
Autonomic — involuntary control of viscera
- Sympathetic (“fight or flight”) — thoracolumbar; releases NE; increases HR, dilates pupils, suppresses digestion
- Parasympathetic (“rest and digest”) — craniosacral; releases ACh; decreases HR, promotes digestion

The Neuron

Neurons are the functional units of the nervous system.

Part	Function
Cell body (soma)	Contains nucleus and organelles; metabolic center
Dendrites	Receive synaptic input
Axon	Conducts action potentials away from soma
Axon terminals	Release neurotransmitters into the synapse
Myelin sheath	Insulating glial wrap; speeds conduction (saltatory conduction at nodes of Ranvier)

Action Potential

At rest, neuronal membrane potential is ~−70 mV (more negative inside due to K⁺ gradient and Na⁺/K⁺-ATPase).

Depolarization — stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in → membrane reaches ~+30 mV
Repolarization — Na⁺ channels inactivate; K⁺ channels open; K⁺ exits → membrane returns toward −70 mV
Hyperpolarization (refractory period) — briefly more negative than resting potential; ensures unidirectional propagation

Synaptic Transmission

Action potential reaches axon terminal
Ca²⁺ influx triggers fusion of synaptic vesicles with presynaptic membrane
Neurotransmitter (e.g., acetylcholine, glutamate, GABA, dopamine) is released into synaptic cleft
Neurotransmitter binds postsynaptic receptors → ionotropic (direct channel) or metabotropic (G-protein cascade)
Termination: reuptake (e.g., SERT for serotonin), enzymatic degradation (e.g., AChE), diffusion

Brain Regions

Region	Key functions
Cerebral cortex	Consciousness, sensory perception, motor control, language, memory
Cerebellum	Motor coordination, balance, fine tuning
Brainstem (midbrain, pons, medulla)	Vital functions: breathing, HR, BP; cranial nerve nuclei; arousal
Limbic system (amygdala, hippocampus)	Emotion, memory consolidation, fear
Thalamus	Sensory relay station (all senses except olfaction)
Hypothalamus	Homeostasis: temperature, hunger, thirst, circadian rhythm; controls pituitary

The hypothalamus is a key bridge to the endocrine system: it controls the pituitary gland and links nervous signals to hormone release. The autonomic nerves likewise steer the cardiovascular, respiratory, and digestive systems moment to moment.

Clinical Relevance

Stroke results from ischemic (80%; thrombotic or embolic) or hemorrhagic (20%) insult to a brain region. Symptom localization follows the affected vascular territory (e.g., MCA → contralateral face/arm weakness + aphasia if dominant hemisphere). Time-critical thrombolysis (IV tPA within 4.5 h of ischemic onset) significantly improves outcomes. The public FAST mnemonic (Face drooping, Arm weakness, Speech difficulty, Time to call emergency services) helps bystanders recognize stroke early.

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

Saltatory conduction and myelin pathology

Myelin, made by oligodendrocytes in the CNS and Schwann cells in the PNS, forces the action potential to regenerate only at the nodes of Ranvier, where voltage-gated Na+ channels are densely clustered. The impulse effectively jumps node to node (saltatory conduction), dramatically increasing conduction velocity while conserving metabolic energy. Demyelinating diseases such as multiple sclerosis (CNS) and Guillain-Barré syndrome (PNS) slow or block conduction, explaining their motor and sensory deficits.

Upper versus lower motor neurons

Upper motor neurons (UMNs) originate in the motor cortex and descend via the corticospinal tract to synapse on lower motor neurons (LMNs) in the anterior horn or brainstem nuclei; LMNs project directly to skeletal muscle. UMN lesions produce spastic paralysis, hyperreflexia, and an upgoing plantar response (Babinski sign) due to loss of descending inhibition, whereas LMN lesions cause flaccid paralysis, hyporeflexia, fasciculations, and atrophy. Localizing a deficit to the UMN or LMN pattern is a foundational step in neurologic diagnosis.

Key terms

Central nervous system (CNS): The brain and spinal cord, where sensory information is integrated and commands are issued.
Peripheral nervous system (PNS): All the nerves outside the brain and spinal cord that carry signals to and from the body.
Neuron: The excitable cell that transmits information as electrical and chemical signals; the functional unit of the nervous system.
Action potential: A rapid, self-propagating reversal of membrane voltage that carries a signal along an axon.
Synapse: The junction where a neuron passes a signal to another cell, usually by releasing a neurotransmitter.
Neurotransmitter: A chemical messenger (such as acetylcholine, glutamate, or dopamine) released at a synapse to influence the next cell.
Myelin sheath: An insulating glial wrapping around axons that speeds signal conduction.
Autonomic nervous system: The involuntary division of the PNS controlling organs, with opposing sympathetic and parasympathetic branches.

Check your understanding

5 questions · answers reveal instantly.

1.Which structures make up the central nervous system?
2.During depolarization of an action potential, which ion rushes into the neuron?
3.The sympathetic nervous system is best described as producing which response?
4.Which brain region relays most sensory information to the cerebral cortex?
5.What role does the myelin sheath play?

Citations & References

Links open publicly available educational and peer-reviewed sources.