The nervous system comprises two main parts: the central nervous system, made up of the brain and spinal cord, and the peripheral nervous system, made up of all the nerves that branch off from the spinal cord.[i] Nerves are long string-like branching structures and are the body’s conduits for information, taking sensory data to the brain and conveying impulses for voluntary control of the body. The nervous system is also responsible for many of the involuntary functions, reflexes and innate physiology of the body.
Nerve cells
Nerves are made up of specialised cells called nerve cells or neurons. Each neuron has three main parts: the main body known as the soma which contains the nucleus; the dendrites – long, branched ‘input’ structures which receive impulses from other cells; and an axon – a long, thin, branched ‘output’ structure responsible for creating and passing on impulses. Nerve cells connect together to form the nerve tissue or nerves, which are long, flexible, branching structures; these supply and receive messages around the body in the form of electrical impulses. Nerve tissue is made up of these neurons, together with glia or glial cells, which support the function of the nerve cells themselves.[ii]
[i] Mai, J. K., & Paxinos, G. (Eds.). (2011). The human nervous system. Academic press.
[ii] Biga, L. M., Dawson, S., Harwell, A., Hopkins, R., Kaufmann, J., LeMaster, M., … & Runyeon, J. (2020). Anatomy & physiology. OpenStax/Oregon State University.
There are different types of neurons which provide slightly different functions. Motor neurons, as the name implies, are responsible for movement or motor control; both voluntary, deliberate movement and some reflexes. Sensory neurons are responsible for sensation and correspond to receptors for senses such as those for pressure, temperature, and pain.
The spinal cord is a bundle of nerves which extends from the brain down through most of the length of the spinal column, and pairs of nerves branch off roughly at each vertebra to supply nervous impulses for the body. Each pair of nerves is responsible for a specific area of the body, and this is determined by the point that they branch away from the spinal cord bundle. Some of the nerves that branch off from the spinal cord bunch together with others to form complex bundles and networks of nerves.
There are 31 pairs of nerves branching from the spinal cord, numbered according to the point at which they branch off: there are eight nerve pairs in the cervical spine, numbered C1 – C8 from the top down; twelve thoracic nerve pairs numbered
T1 – T12, five lumbar nerve pairs, numbered L1 – L5; and a single nerve pair known as the coccygeal nerves towards the bottom of the spine.[i]
[i] Nógrádi, A., & Vrbová, G. (2006). Anatomy and physiology of the spinal cord. In Transplantation of neural tissue into the spinal cord (pp. 1-23). Springer, Boston, MA.
The nerve roots – the point at which the nerves branch away from the spinal cord – are numbered according to the point at which they diverge from the spinal cord, but they then combine and network with other nerves from different roots, creating the complex and intricate nervous system, and those nerve groups are more commonly named according to their function.
Autonomic nervous system
The central and peripheral nervous system are also connected to a complementary involuntary functional system of nerves, known as the autonomic nervous system.[i] This is effectively our involuntarily control system for basic life functions and responses. It comprises three parts:
- Sympathetic nervous system (SNS) – the intrinsic ‘fight or flight’ reflexes, controlling our body’s innate response to stressors with changes in heart rate, blood pressure, and blood sugar. The SNS is responsible for the processes that prioritise immediate physical need over longer-term needs like digestion and rest.
- Parasympathetic nervous system (PNS) – the PNS is the system responsible for our body’s longer-term needs and processes – the ‘rest and digest’ model of function. The PNS lowers our resting heart rate and blood pressure, regulates blood sugar against the needs of the SNS, and returns function to our daily physical needs – digestion, rest, recovery.
- Enteric nervous system (ENS) – this is responsible mainly for digestive functions, controlling the muscles and processes of the gut including blood supply, secretion and absorption.[ii]
Regions of the peripheral nervous system
The areas of the skin which are supplied by different nerves are known as dermatomes, which also relate roughly to some of the internal body parts linked to the nerves. Each dermatome is the area supplied by a single nerve root. The dermatomes can vary somewhat from person to person and there can be some inconsistencies and overlap, but dermatome maps are useful for identifying which nerve serves which area of the body. Nerves have a combination of sensory and motor impulse functions, with some concentrating mainly on one or the other function.
Cranial nerves
There are twelve pairs of cranial nerves – these are nerves that branch directly from the brain, not the spinal cord.[iii] They pass through the skull in channels, and are numbered according to the point at which they diverge from the brain, from front to back.
- CNI – the olfactory nerve, a sensory nerve responsible for smell.
- CNII – the optic nerve, a sensory nerve responsible for vision.
- CNIII – the oculomotor nerve, a motor nerve linked to both voluntary and reflex eye movement.
- CNIV – the trochlear nerve, a motor nerve which allows for control of the left-right and up-down eye movements.
- CNV – the trigeminal nerve, which allows for sensation around the cheeks and movement of the jaw.
- CNVI – the abducens nerve, allowing for further eye movement.
- CNVII – the facial nerve, responsible for some facial and eye movements, tear ducts and salivation, the auditory reflex, and which contributes to our sense of taste.
- CNVIII – the vestibulocochlear nerve relates to our sense of hearing and balance.
- CNIX – the glossopharyngeal nerve, is responsible for our ability to swallow and contributes to our sensation of taste.
- CNX – the vagus nerve, which is an important interface with the parasympathetic nervous system, and which affects unconscious activities like heart rate and the digestive system.[iv]
- CNXI – the accessory nerve has a role in the movement of some of the neck muscles.
- CNXII – the hypoglossal nerve is responsible for motor control of the tongue.
There is also a ‘terminal’ cranial nerve, labelled CN0 which doesn’t have a clear function in humans, although it is theorised to have some role in sensing pheromones and in the regulation of sex hormones, and which may be a largely vestigial remain from earlier in our evolution.[v]
Cervical spine
The eight nerves of the c-spine each have their own area of innervation.[vi]
- C1 – The first pair of nerves to branch off from the spinal cord as it extends away from the brain supply the head and scalp, pituitary gland, inner and middle ear and part of the impulses for the sympathetic nervous system. C1 does not have a dermatome, as other nerves are responsible for the sensory organs of the skin near the areas supplied by C1. The C1 nerve pair originate from above the C1 vertebra.
- C2 – is responsible for sensation at the back of the head.
- C3 – this is responsible for sensation at the scalp area and the sides of the face.
- C4 – this is one of the nerves that controls the diaphragm, as well as giving motor control over some shoulder movement. A spinal cord injury at C4 or above is likely to have a profound impact on breathing.
- C5 – controls and provides sensory relay for the shoulders and the upper part of the upper arm and biceps.
- C6 – controls wrist extension movements and has some control over the biceps. Its dermatome covers the thumb area.
- C7 – has some input into the triceps and wrists, with a dermatome that extends down the arm to the middle finger.
- C8 – controls the hands and gripping action of the fingers. The dermatome of C8 covers the side of the hand towards the little finger.
Thoracic Spine
- T1 and T2 supply nerve function to the top part of the chest, and some nerve function into the arms.
- T3, T4, T5 are largely responsible for supplying the chest and the upper organs of the thoracic cavity, with dermatomes covering the upper chest and back. The upper thoracic nerves play a part in breath control, but someone with a spinal cord injury at this level is likely to be able to control their breathing well.
- T6, T7, T8 provide the nerve network for some of the chest and abdomen, and belt-like dermatomes around the middle of the abdomen.
- T9, T10, T11, T12 are the nerves responsible for the lower abdomen and their dermatomes cover the mid-back and lower-abdomen.
Lumbar spine
- L1 – the first lumbar nerve is responsible for sensations around the groin and genitals, and has a role in motor control of the hips.
- L2, L3, L4 convey sensation for the front and inside of the thighs, and extend to the inner aspect of the lower legs.
- L5 nerve region covers the outside of the thighs extending into the outer aspect of the top of the lower legs. [vii]
Sacral Spine
- S1, S2, S3, S4, S5 nerves provide some nerve supply to the genitals, as well as the anus, sacrum, buttocks and the backs of the thighs. The pudendal nerve is the nerve bundle comprising nerves from S2-S4 and which is the primary nerve of the perineal area.
Coccygeal nerve pair
The spinal cord ends with one final branched pair of nerves which extend to its dermatome immediately around the area of the coccyx. They also provide part of the nerve supply for some of the pelvic organs.[viii]
Spinal cord cross-sections
The spinal cord is a long bundle of nerves that runs down the length of the spinal column, and within that bundle, different nerves are grouped together. A cross section of the spinal cord can show that there is a central bundle, left and right sections, an anterior and a posterior section.
An injury to the spinal cord, depending on how extensive the damage across the section of cord, will affect some or all nerve function below the point of injury. In partial spinal cord injuries, the section of the spinal cord injured predicts the effect it will have.
- Central cord – when only the central spinal cord is affected, the result is usual limited to muscle weakness and has a generally good prognosis.[ix]
- Anterior (front portion) cord – anterior cord syndrome – damage to the front 2/3 of the spinal cord – causes loss of motor function and significant loss of sensation, particularly of pain and temperature.[x]
- Posterior (rear portion) cord – impacts light touch sensation and can have a significant impact on coordination of movement.
- Left or right-sided spinal cord – one-sided injury is known as Brown-Sequard Syndrome. It is usually caused by traumatic injury, and may only affect movement and sensation in that side of the body, leaving the other side with completely or near-completely normal function.[xi]
Spinal cord injuries (SCI)
Injury to the spinal cord affects the nerves from that point downward, so the higher the injury, the more impact on function. People with very high c-spinal cord injuries may need full or partial respiratory support, with loss of control or sensation from the neck down. Spinal cord injuries at lower points, i.e. below the level of the nerves that are most responsible for breathing and the upper limbs may retain upper body function but lose motor control and sensation for their lower limbs, affecting sexual function, continence, and digestion.
The completeness of an SCI, i.e. how much of the spinal cord is damaged also plays a large part in determining the impact of the SCI on physical function. A complete spinal cord injury means that there is effectively no nerve communication from the brain beyond the point of injury, and can cause immediate health problems, as well as predisposing the injured person to long-term health challenges and care needs.
[i] Karemaker JM. (2017) An introduction into autonomic nervous function. Physiol Meas.
[ii][ii] Lake JI, Heuckeroth RO. (2013) Enteric nervous system development: migration, differentiation, and disease. Am J Physiol Gastrointest Liver Physiol.
[iii][iii] Monkhouse, S. (2005). Cranial nerves: functional anatomy. Cambridge University Press.
[iv] Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in psychiatry, 9, 44. https://doi.org/10.3389/fpsyt.2018.00044
[v] Sonne, J., Reddy, V., & Lopez-Ojeda, W. (2017). Neuroanatomy, cranial nerve 0 (terminal nerve).
[vi] Bland, J. H., & Boushey, D. R. (1990, August). Anatomy and physiology of the cervical spine. In Seminars in arthritis and rheumatism (Vol. 20, No. 1, pp. 1-20). WB Saunders.
[vii] Waxenbaum, J. A., Reddy, V., Williams, C., & Futterman, B. (2017). Anatomy, back, lumbar vertebrae.
[viii] Wooten, C. (2015). Anatomy of the Coccygeal plexus. In Nerves and Nerve Injuries (pp. 659-661). Academic Press.
[ix] Brooks, N. P. (2017). Central cord syndrome. Neurosurgery Clinics, 28(1), 41-47.
[x] Pearl, N. A., & Dubensky, L. (2020). Anterior cord syndrome.
[xi] Shams, S., & Arain, A. (2021). Brown Sequard Syndrome. In StatPearls [Internet]. StatPearls Publishing.
Whether someone has a high spinal injury with complex long-term care needs, or a lower spinal injury with the ability to live completely independently, an SCI can be profoundly life changing. Care packages specialising in supporting people with complex care needs can mean living a full and satisfying life, even with extra needs. Whatever the impact of a spinal cord injury, professional help and peer support groups can help as life goes on.
[1] Mai, J. K., & Paxinos, G. (Eds.). (2011). The human nervous system. Academic press.
[1] Biga, L. M., Dawson, S., Harwell, A., Hopkins, R., Kaufmann, J., LeMaster, M., … & Runyeon, J. (2020). Anatomy & physiology. OpenStax/Oregon State University.
[1] Nógrádi, A., & Vrbová, G. (2006). Anatomy and physiology of the spinal cord. In Transplantation of neural tissue into the spinal cord (pp. 1-23). Springer, Boston, MA.
[1] Karemaker JM. (2017) An introduction into autonomic nervous function. Physiol Meas.
[1][1] Lake JI, Heuckeroth RO. (2013) Enteric nervous system development: migration, differentiation, and disease. Am J Physiol Gastrointest Liver Physiol.
[1][1] Monkhouse, S. (2005). Cranial nerves: functional anatomy. Cambridge University Press.
[1] Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in psychiatry, 9, 44. https://doi.org/10.3389/fpsyt.2018.00044
[1] Sonne, J., Reddy, V., & Lopez-Ojeda, W. (2017). Neuroanatomy, cranial nerve 0 (terminal nerve).
[1] Bland, J. H., & Boushey, D. R. (1990, August). Anatomy and physiology of the cervical spine. In Seminars in arthritis and rheumatism (Vol. 20, No. 1, pp. 1-20). WB Saunders.
[1] Waxenbaum, J. A., Reddy, V., Williams, C., & Futterman, B. (2017). Anatomy, back, lumbar vertebrae.
[1] Wooten, C. (2015). Anatomy of the Coccygeal plexus. In Nerves and Nerve Injuries (pp. 659-661). Academic Press.
[1] Brooks, N. P. (2017). Central cord syndrome. Neurosurgery Clinics, 28(1), 41-47.
[1] Pearl, N. A., & Dubensky, L. (2020). Anterior cord syndrome.
[1] Shams, S., & Arain, A. (2021). Brown Sequard Syndrome. In StatPearls [Internet]. StatPearls Publishing.
