The Spinal Cord: How the Brain Communicates With the Body

The spinal cord is easy to picture as a cable running between the brain and the rest of the body. That is true, but it misses much of what the cord is actually doing.

Inside it are organised pathways carrying movement, touch, pain, temperature and information about where the body is in space. It contains the neurones involved in reflexes, contributes to autonomic control, and connects with spinal nerves at different levels. A relatively small area of damage can therefore alter several functions at once.

This is why the pattern matters when a patient develops neurological symptoms. Weakness in both legs, a band of altered sensation across the trunk and new bladder difficulty do not feel like three unrelated observations once you understand what travels through the spinal cord.

Where is the spinal cord?

The spinal cord continues from the medulla oblongata at the base of the brain and passes through the foramen magnum into the vertebral canal. The vertebral column provides bony protection, but the cord is also surrounded by the meninges: dura mater, arachnoid mater and pia mater. Cerebrospinal fluid (CSF) surrounds the cord within the subarachnoid space.

The space outside the dura is the epidural space. This is why epidural and spinal anaesthesia are not simply two names for the same procedure: epidural medication is delivered outside the dura, whereas spinal anaesthesia enters the subarachnoid space containing CSF.

The adult spinal cord does not continue all the way to the bottom of the vertebral canal. It usually tapers into the conus medullaris around the L1–L2 vertebral level. Below this, lumbar and sacral nerve roots continue down through the canal as the cauda equina. That distinction becomes clinically important because damage to the spinal cord and damage to the cauda equina can produce different neurological patterns.

The cord is organised into 31 spinal segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. A pair of spinal nerves is associated with each segment. Sensory fibres enter through posterior roots, with sensory cell bodies in the dorsal root ganglia. Motor fibres leave through anterior roots. The roots then join to form mixed spinal nerves.

This segmental organisation is where dermatomes and myotomes come from. A dermatome describes an area of skin supplied predominantly by sensory fibres from a particular spinal nerve root; a myotome describes muscles supplied predominantly by motor fibres from a particular root. They overlap, so the familiar maps are useful clinical guides rather than perfectly sharp borders drawn on the body.

What does the spinal cord look like inside?

A cross-section of the cord looks very different from a cross-section of the cerebral hemispheres. In the spinal cord, grey matter sits centrally in a butterfly or H shape and white matter surrounds it.

The posterior horns of the grey matter receive and process sensory information. The anterior horns contain lower motor neurones whose axons eventually reach skeletal muscle. Other regions of spinal grey matter contribute to autonomic function, including sympathetic and parasympathetic pathways involved in functions such as blood pressure, bladder, bowel and sexual function.

The surrounding white matter contains bundles of axons travelling up and down the cord. These are the tracts students meet in neuroanatomy. They are not separate little tubes; they are organised groups of nerve fibres occupying particular regions of the white matter. Their arrangement is what allows clinicians to use a pattern of sensory or motor loss to begin localising a lesion.

Different sensations take different routes

Imagine touching a patient’s toe with a vibrating tuning fork. Information about vibration and conscious proprioception travels mainly through the dorsal column–medial lemniscus pathway. Within the spinal cord, these fibres ascend on the same side and do not cross until they reach the medulla.

Pain and temperature follow a different route. Sensory fibres enter the cord and synapse in the dorsal horn. The next neurones cross within the spinal cord, generally over a short distance, before ascending on the opposite side within the anterolateral system, commonly taught as the spinothalamic tract.

That difference matters. A lesion affecting one side of the cord can disturb vibration and proprioception below the lesion on the same side while altering pain and temperature on the opposite side. Real patients do not always reproduce a textbook diagram perfectly, but the anatomy explains why a neurological examination tests different sensory modalities separately.

It also explains why a sensory level can be important. If altered sensation begins at a reasonably consistent level across the trunk and continues below it, spinal cord involvement needs consideration. A sensory level is not something to document casually and move on from; alongside new weakness or autonomic symptoms, it can be an important localisation clue.

Movement has to travel down the cord

Voluntary movement begins in networks that include the motor cortex, but those instructions still have to reach the spinal motor neurones connected to muscle. The corticospinal tract is a major descending pathway involved in voluntary movement.

Most corticospinal fibres cross in the lower medulla and then descend through the spinal cord. They ultimately influence lower motor neurones, whose axons leave through anterior roots and peripheral nerves to reach skeletal muscle.

This helps make sense of upper and lower motor neurone signs. Damage to descending motor pathways within the cord can produce upper motor neurone features below the lesion, including increased tone, brisk reflexes and an extensor plantar response. Damage involving anterior horn cells, nerve roots or peripheral motor fibres can produce lower motor neurone features at the affected level, with weakness, reduced reflexes and, over time, muscle wasting.

Timing matters. Immediately after a severe acute spinal cord injury, spinal shock can temporarily cause flaccidity and reduced or absent reflexes below the lesion. The expected upper motor neurone pattern may therefore not be obvious at first.

The spinal cord does not always wait for the brain

Reflexes are a good example of the spinal cord processing information locally. When a tendon is tapped, a sensory signal travels into the cord and connects with a motor pathway, allowing a response without waiting for conscious processing in the brain.

In the patellar reflex, stretching the quadriceps activates muscle spindle afferents. These enter the spinal cord and excite motor neurones supplying the quadriceps, causing the lower leg to extend. Interneurones also help reduce activity in the opposing muscles.

A withdrawal reflex is more complex but follows the same principle. Touch something painfully hot and spinal circuits can begin moving the limb away before you have consciously worked through what happened. Sensory information still travels upwards so that pain becomes conscious, but the protective response can already be underway.

This is why reflexes are useful clinically. A tendon reflex depends on several parts of the pathway being intact: sensory input, the relevant spinal segment, motor output, peripheral nerve and neuromuscular connection. Descending pathways from the brain also influence the response. Reflexes therefore make more sense when interpreted alongside power, tone, sensation and the rest of the neurological examination.

The spinal cord needs a continuous blood supply

Like the brain, spinal cord tissue is metabolically active and vulnerable to interruption of its blood supply. A single anterior spinal artery supplies much of the anterior two-thirds of the cord, while paired posterior spinal arteries contribute to the posterior part. This longitudinal supply is reinforced by segmental vessels at different levels.

The artery of Adamkiewicz, the largest anterior radiculomedullary artery, is an important contributor to perfusion of the lower thoracic and lumbosacral cord. Its origin varies considerably between people, which is one reason its anatomy matters during thoracic and thoracoabdominal aortic procedures.

Spinal cord ischaemia can therefore produce a neurological deficit even when there has been no direct injury to the vertebral column. In anterior spinal artery syndrome, motor pathways and pain and temperature pathways can be affected while posterior-column functions such as vibration and proprioception are relatively preserved. Again, the point is not to memorise a perfect syndrome: it is to see how vascular territory and tract anatomy can produce a recognisable pattern.

The level of a lesion changes what you see

Suppose a patient develops weakness in both legs and altered sensation rising towards the abdomen. Their arms are unaffected. That immediately gives you more anatomical information than the word “weakness” alone.

A cervical cord lesion can affect the arms and legs and, when sufficiently high or severe, may compromise respiratory function. A thoracic lesion can affect the trunk and legs while leaving the arms unaffected. Lower cord, conus medullaris and cauda equina lesions can affect lower-limb function alongside perineal sensation, bladder, bowel and sexual function in different combinations.

The spinal cord also carries autonomic pathways. A cord lesion can therefore affect blood pressure regulation, temperature regulation, bladder and bowel control and sexual function as well as movement and sensation.

This is where nursing observations become much more useful. Instead of recording “legs weak”, establish what has changed. Is weakness unilateral or bilateral? Is it getting worse? Can the person lift each limb against gravity? Has their gait changed? Is there numbness, tingling or a sensory level? Are bladder or bowel symptoms new? What is happening to tone and reflexes? The pattern helps the wider team understand where the problem may be and how urgently it is evolving.

A high acute spinal cord injury can also produce neurogenic shock through loss of sympathetic tone, resulting in hypotension with inappropriate bradycardia. This is different from spinal shock, which refers to the temporary reduction or loss of reflex and motor activity below an acute cord lesion. Similar names, different physiological problems.

When the pattern needs urgent escalation

Spinal cord dysfunction has many possible causes. Trauma, compression, malignancy, infection, inflammation and vascular disease can all affect the cord or structures around it. Anatomy helps you recognise the pattern; it does not tell you the cause on its own.

After trauma, new weakness, altered sensation or another neurological abnormality that could be due to spinal cord injury requires urgent assessment. NICE recommends CT in adults when spinal imaging is indicated and, if a neurological abnormality could be attributable to spinal cord injury, MRI after CT even when the abnormality is not visible on CT.

Compression can also occur without trauma. In somebody with current or previous cancer, new limb weakness, difficulty walking, numbness or sensory loss, radicular pain, or new bladder or bowel dysfunction can indicate metastatic spinal cord compression (MSCC). NICE treats suspected MSCC with neurological signs or symptoms as an oncological emergency, with MRI required as soon as possible and always within 24 hours.

At the lower end of the canal, severe low back pain radiating into the leg together with new bladder, bowel or sexual dysfunction, or new perineal numbness, requires immediate assessment for cauda equina syndrome according to local pathways.

The nursing role is not to identify the exact damaged tract before escalating. It is to notice that the neurological picture has changed, establish the pattern as clearly as the situation allows, document it accurately and escalate promptly. Bleepbook’s UK Nursing Observations: NEWS2, Neurological and Neurovascular Checks looks more closely at neurological observations in practice.

Bring the anatomy back to the patient

The spinal cord is not simply a wire between the brain and the body. It is an organised part of the central nervous system where sensory, motor and autonomic pathways sit close together, spinal nerves connect at defined levels and local circuits can generate reflex responses.

That organisation is what makes localisation possible. A patient with weakness in one hand, a patient with bilateral leg weakness and a sensory level, and a patient with saddle numbness and new urinary dysfunction are giving you very different anatomical stories.

You do not need to name every tract at the bedside. You do need to recognise when movement, sensation, reflexes and autonomic function are changing together. Once you understand what the spinal cord carries, those observations stop looking separate.

References

Hardy, T.A. (2021) ‘Spinal cord anatomy and localization’, Continuum, 27(1), pp. 12–29. PubMed.

Watson, C., Paxinos, G. and Kayalioglu, G. (2022) ‘The tracts, cytoarchitecture, and neurochemistry of the spinal cord’, The Anatomical Record. PubMed.

Taterra, D. et al. (2019) ‘Artery of Adamkiewicz: a meta-analysis of anatomical characteristics’, Neuroradiology, 61, pp. 869–880. PubMed.

National Institute for Health and Care Excellence (NICE). Spinal injury: assessment and initial management (NG41).

National Institute for Health and Care Excellence (NICE). Suspected neurological conditions: recognition and referral (NG127).

National Institute for Health and Care Excellence (NICE). Spinal metastases and metastatic spinal cord compression (NG234).