How the nervous system is organized: input, integration, output and levels
- Concept
- Exam priority: medium
Nerves bring in information, the brain and cord sort it and decide, and nerves carry the orders out to muscles and glands.
Also called: Functional levels of the CNS, Sensory–integrative–motor design
See it in 3D: Spinal cord · Medulla oblongata · Pons · Midbrain · Thalamus · Motor cortex · Sensory cortex
What it is
The nervous system works as a chain: receptors turn stimuli into signals, central neurons combine and filter them, and motor neurons drive muscles and glands. This chain is built in three stacked levels: the spinal cord, the subcortical brain (brainstem, thalamus, hypothalamus, basal ganglia, cerebellum) and the cerebral cortex. Knowing which level does what is the basis of lesion localization.
Key facts
- Input side: somatic receptors (skin, muscle, joints), special senses (eye, ear, taste, smell, vestibule) and visceral receptors.
- Output side: somatic motor neurons to skeletal muscle and autonomic neurons to smooth muscle, heart and glands.
- Synapses are where integration happens: they can pass, block, amplify or redirect a signal.
- Memory is stored mainly as lasting changes in synaptic strength (facilitation), so a pathway used often becomes easier to use.
- Higher levels do not replace lower ones; they command, refine and inhibit the patterns that lower levels already contain.
The three functional levels
| Level | What it can do on its own | Typical sign when damaged |
|---|---|---|
| Spinal cord | Withdrawal and stretch reflexes, stepping rhythms, bladder and sweating reflexes | Sensory level, paraparesis, sphincter loss |
| Subcortical (brainstem, thalamus, hypothalamus, cerebellum, basal ganglia) | Breathing, blood pressure, posture and balance, feeding and emotional reflexes, sensory relay | Crossed signs, cranial nerve palsies, coma, ataxia |
| Cerebral cortex | Stores memories, plans and selects actions, fine motor control, language, conscious perception | Aphasia, neglect, cortical sensory loss, face-arm-predominant weakness |
Clinical notes
- Spinal cord injury: below the lesion, cord reflexes survive and later become exaggerated because descending control is lost.
- Ischemic stroke of the cortex removes refined control and conscious sensation but leaves brainstem functions such as breathing intact.
Test yourself
Think of the answer, then tap the question to see it.
Which part of a typical motor neuron normally starts the action potential, and why there?
The axon initial segment: it has the highest density of voltage-gated Na⁺ channels, so its threshold is lowest and summed synaptic input fires the cell there.
A patient has a right facial palsy with weakness of the left arm and leg. Which level of the nervous system is most likely affected?
The brainstem (here the right pons): crossed signs, a cranial nerve on one side and limbs on the other, point to the brainstem.
After a complete thoracic cord transection, why can a pinprick on the foot still cause leg withdrawal?
The withdrawal reflex is wired within the cord segments below the lesion; it needs no input from the brain, which only modulates it.
What are the three kinds of effector the nervous system controls?
Skeletal muscle, smooth muscle (with cardiac muscle under autonomic control) and glands, both exocrine and endocrine.
Connections
See it in 3D7
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Neuroanatomy, Spinal Cord (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Brainstem (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Cerebral Cortex (StatPearls (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.