[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Neurophysiology](https://durgical.com/en/physiology/areas/neuro/) › How the nervous system is organized: input, integration, output and levels

# 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.

Updated Nov 13, 2026

Also called: Functional levels of the CNS, Sensory–integrative–motor design

**See it in 3D:** [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) · [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) · [Pons](https://durgical.com/en/anatomy/pons/) · [Midbrain](https://durgical.com/en/anatomy/midbrain/) · [Thalamus](https://durgical.com/en/anatomy/thalamus/) · [Motor cortex](https://durgical.com/en/anatomy/precentral-gyrus/) · [Sensory cortex](https://durgical.com/en/anatomy/postcentral-gyrus/)

## Key points

- The neuron's parts map to its jobs: dendrites and soma receive, the axon initial segment decides (fires), the axon conducts, the terminals release transmitter.
- Most incoming sensory information is discarded as irrelevant; integration means selecting, not only adding up.
- Effectors are only three: skeletal muscle, smooth muscle and glands (exocrine and endocrine).
- Spinal cord alone can run reflexes and stepping patterns; the brainstem runs breathing, blood pressure, balance and feeding reflexes; the cortex adds memory, planning and fine control.
- Localization: a sensory level with bilateral signs suggests cord; crossed signs (cranial nerve on one side, limbs on the other) suggest brainstem; aphasia, neglect or face-arm weakness suggest 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.

## Memory aid

IN – SORT – OUT, stacked three floors high: the basement (cord) reacts, the middle floor (brainstem) keeps you alive, the top floor (cortex) thinks.

## 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.

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## Connections

## See it in 3D 7

The structures of Durgical's 3D anatomy where it happens.

- [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/)
- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)
- [Pons](https://durgical.com/en/anatomy/pons/)
- [Midbrain](https://durgical.com/en/anatomy/midbrain/)
- [Thalamus](https://durgical.com/en/anatomy/thalamus/)
- [Motor cortex](https://durgical.com/en/anatomy/precentral-gyrus/)
- [Sensory cortex](https://durgical.com/en/anatomy/postcentral-gyrus/)

## Related topics 6

- [Synaptic transmission](https://durgical.com/en/physiology/synaptic-transmission/)
- [Sensory receptors, coding and neuronal circuits](https://durgical.com/en/physiology/sensory-receptors-coding-and-neuronal-circuits/)
- [Spinal cord reflexes](https://durgical.com/en/physiology/spinal-cord-reflexes/)
- [Upper versus lower motor neuron lesions](https://durgical.com/en/physiology/upper-versus-lower-motor-neuron-lesions/)
- [Learning and memory](https://durgical.com/en/physiology/learning-and-memory/)
- [The cerebral cortex and language](https://durgical.com/en/physiology/the-cerebral-cortex-and-language/)

## Related conditions 2

- [Spinal cord injury](https://durgical.com/en/conditions/spinal-cord-injury/)
- [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://www.ncbi.nlm.nih.gov/books/NBK559056/) (StatPearls (NCBI Bookshelf), 2024); [Neuroanatomy, Brainstem](https://www.ncbi.nlm.nih.gov/books/NBK544297/) (StatPearls (NCBI Bookshelf), 2024); [Neuroanatomy, Cerebral Cortex](https://www.ncbi.nlm.nih.gov/books/NBK537247/) (StatPearls (NCBI Bookshelf), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/how-the-nervous-system-is-organized-input-integration-output-and-levels/