How do spinal nerves of the peripheral nervous system (pns) differ from cranial nerves (cns)?

The fourth in a series, this article explores the peripheral nervous system: the nerves that connect the central nervous system to the rest of the body. This is a Self-assessment article and comes with a self-assessment test.

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Abstract

This article – the fourth in a series about the nervous system – examines the peripheral nervous system, which is made up of nerves that connect the central nervous system to the rest of the body. It focuses on the anatomy of the spinal cord and spinal nerves. The next article in the series will continue to explore the peripheral nervous system, concentrating on the cranial nerves.

Citation: Bayram-Weston Z et al (2022) Nervous system 4: the peripheral nervous system – spinal nerves. Nursing Times [online]; 118: 6.

Authors: Zubeyde Bayram-Weston is senior lecturer in biomedical science; Maria Andrade-Sienz is honorary associate professor in biomedical science; John Knight is associate professor in biomedical science; all at the College of Human and Health Sciences, Swansea University.

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Introduction

The first article in this series introduced the nervous system, which comprises the central nervous system (CNS) and the peripheral nervous system (PNS). The next two articles focused on the CNS, exploring the structure and key functions of its organs: the brain and the spinal cord. This fourth article in the series begins to examine the anatomy and physiology of the PNS.

Anatomically, the PNS is made up of spinal and cranial nerves together with nerve endings, plexuses (branching networks of nerves) and ganglia (clusters of neural tissue that function as relay stations). The spinal nerves leave the spinal cord through spaces between the vertebra, termed intervertebral foramina. The cranial nerves originate in the base of the brain and leave though tiny holes (foramina) in the cranium. Peripheral nerves can be sensory, motor or mixed (Irimia and Van Horn, 2021).

As it consists of nerves that connect the CNS to the other regions of the body, the PNS is important for survival. Unlike the CNS, which is protected by the skull and the vertebrae, the nerves and cells of the PNS are not enclosed by bones, which makes the PNS more susceptible to damage by trauma.

“The sciatic nerve is the largest branch of the sacral plexus and runs from the top of the leg to the foot, making it the longest nerve in the human body”

Anatomy of the spinal cord

The spinal cord is cylindrical in shape, occupies around two thirds of the vertebral canal, and continues on from the medulla oblongata (inferior portion of the brain stem). It originates through the foramen magnum at the base of the skull and terminates between the first and second lumbar vertebrae (L1-L2). When a patient needs a lumbar puncture – for example to check the cerebral spinal fluid for signs of infection – it is usually performed below the third lumbar vertebra (L3) to prevent accidental damage to the spinal cord.

In adults, when the hands are placed on the hips, they line up with the fourth lumbar vertebra (L4); the spinal cord ends just above this level (L1-L2). In newborn infants, the cord extends to the third or fourth lumbar vertebra (L3-L4). During childhood, both the spinal cord and the vertebral column grow longer, in line with normal bodily growth. Elongation of the spinal cord stops around the age of 4-5 years, but growth of the vertebral column continues until 14-18 years of age (Mtui et al, 2015).

When the spinal cord is viewed externally, two enlargements can be seen. These regions have increased numbers of neurons and axons that are necessary for the innervation of the many muscles in the upper and lower limbs:

• Upper (cervical) enlargement – around the level of the fourth cervical vertebra and the first thoracic vertebra (C4-T1). Nerves that innervate muscles of the upper limbs arise from here;
• Lower (lumbar) enlargement – at, approximately, the level of the ninth and 12th thoracic vertebrae (T9-T12). Innervation of the lower-limb muscles arises from here.

Inferior to the lumbar enlargement, the spinal cord has a cone-shaped narrowing called the conus medullaris, which terminates between L1 and L2. The filum terminale arises from the conus medullaris and fuses with the arachnoid and dura mater; these anchor the spinal cord to the coccyx (VanPutte et al, 2017).

Internal anatomy of the spinal cord

In a transverse section, the spinal cord displays an outer region of white matter surrounding an inner region of grey matter (Fig 1). The grey matter forms the shape of the letter H, resembling a butterfly. It consists, primarily, of dendrites and the cell bodies of neurons, together with unmyelinated axons and supporting neuroglial cells. The arrangement of tissues gives rise to clearly visible ventral (anterior) and dorsal (posterior) horns. The grey matter tracts receive and integrate incoming and outgoing information. The ventral horns contain motor neuron cell bodies, whose axons leave through the ventral roots to innervate the muscles.

The grey matter in the dorsal horns is largely made up of the cell bodies of interneurons, which process signals from the axons of sensory neurons entering though the dorsal root. Overall, the spinal cord communicates with the body through 31 pairs of spinal nerves, which arise from both sides. In the centre of the grey matter is a small space called the central canal; it extends the entire length of the spinal cord and is filled with cerebrospinal fluid (Pavlina and Ross, 2018; Leijnse and D’Herde, 2016).

The white matter contains myelinated axons of neurons. Tracts between the white matter and the brain conduct sensory information: ascending tracts carry impulses from the spinal cord to the brain, and descending tracts carry impulses from the brain to the spinal cord.

Spinal nerves and cranial nerves can be:

• Afferent – they carry information from the body to the brain;
• Efferent – they carry information from the brain to muscles and glands;
• Mixed – they contain both afferent and efferent fibres.

Each spinal nerve divides into two branches: dorsal roots and ventral roots. Sensory information, carried by the afferent axons of the spinal nerves, enters the cord via the dorsal roots; motor commands, carried by the efferent axons, leave the cord via the ventral roots. These two branches fuse at the intervertebral foramen, forming a mixed spinal nerve that has both afferent sensory and efferent motor axons (Hansen, 2014).

“Spinal nerves do not go directly to skin and muscle; instead, with the exception of T1-T12, they form complicated nerve networks called plexuses”

Nerve endings

Afferent nerve endings respond to mechanical, thermal or chemical stimulation. These nerve fibres conduct action potentials to the CNS. Regardless of whether this sensory information reaches a conscious level, the pathway is termed sensory. No matter how efferent nerve endings innervate muscles or glands, because they relate to movement, they are called motor nerve endings.

Afferent nerve endings relate to the senses, which are divided into the special and the general. The special senses are olfaction, vision, hearing, balance and taste. The remaining senses are categorised as general and there are three major types:

• Sensory nerve endings in the dermal layer of the skin – these are high in density and respond to pain, temperature, touch and pressure;
• Sensory nerve endings in the viscera (internal organs in the chest, abdomen and pelvis) – these respond to chemical and mechanical stimuli, such as temperature and the stretching of tissue, causing us to feel visceral pain, nausea, hunger and fullness;
• Some sensory receptors in muscles, joints and tendons (termed proprioceptors) – these generate information that ensures awareness of posture and movement.

Efferent nerve endings communicate with muscles and glands. They resemble the chemical synapses that allow communication between neurons. The release of neurotransmitter substances in efferent nerve endings acts on the target cells (the muscle or glands) to elicit the desired effect (Kiernan and Rajakumar, 2014).

Spinal nerves

The spinal cord appears to be segmented because 31 pairs of spinal nerves emerge at regular intervals to gain access to the periphery and communicate with specific regions of the body. Unlike cranial nerves, spinal nerves do not have a special name; instead, a letter and number identify each one. The spinal cord is anatomically described according to the vertebrae in that section (Fig 2). The 31 pairs of nerves are made up of:

• Eight pairs of cervical nerves (C1-C8), which pass through the cervical (neck) region;
• Twelve pairs of thoracic nerves (T1-T12), which pass through the thoracic region;
• Five pairs of lumbar nerves (L1-L5), which pass through the lumbar region;
• Five pairs of sacral nerves (S1-S5), which are located in the sacral (buttock) region;
• One pair of coccygeal nerves (C0), which passes through the coccyx.

Spinal nerves do not go directly to skin and muscle; instead, with the exception of T1-T12, they form complicated nerve networks called plexuses. A plexus is a site of intermixing branches of the spinal nerves. There are four major plexuses formed next to the spinal cord:

• Cervical plexus in the neck region;
• Brachial plexus in the shoulder region;
• Lumbar plexus in the lower-back region;
• Sacral plexus in the buttock region.

As the spinal cord has terminated, these large nerves are no longer considered part of it. The roots of the lower spinal nerves leave between vertebrae and appear in the vertebral canal as structures resembling wispy hair. These fine nerves are collectively referred to as the cauda equina, meaning ‘horse’s tail’ (Thibodeau, 2019; Tortora and Derrickson, 2014).

Each spinal nerve cord segment communicates with its corresponding body segment though the paired segmental spinal nerves. These neurons carry nerve impulses from the skin into the spinal cord and brain when the cell bodies of the sensory neurons located in the dorsal root ganglia and their main projections enter the spinal cord via the dorsal spinal roots.

Innervation of the skin, muscles and surrounding connective tissue is segmental: each root supplies a region of skin called a dermatome. The nerve supply in neighbouring dermatomes overlaps; knowing which spinal cord segment supplies each dermatome (Fig 3) makes it possible to locate damaged regions of the spinal cord (Thibodeau, 2019).

The sciatic nerve is the largest branch of the sacral plexus and runs from the top of the leg to the foot on the posterior side, making it the longest nerve in the human body. Injury to the sciatic nerve results in sciatica – the most common form of back pain – and is usually caused by nerve compression or irritation. The sciatic nerve may be injured by:

• A herniated (slipped) disc;
• A dislocated hip;
• Osteoarthritis of the lumbosacral spine;
• Pathological shortening of the thigh muscle;
• Pressure from the uterus during pregnancy;
• Inflammation;
• Irritation (De Fine et al, 2017).

As discussed in the previous article in this series, injury to the sciatic nerve can cause pain, weakness and sensory loss in the lower back and along the thigh and leg; this is due to radiculopathy. Assessment of the origin of sciatic pain usually uses neuroradiological scans – such as computed tomography (CT) and magnetic resonance imaging (MRI) – to define the compression site. Treatments include painkillers, physiotherapy and decompression surgery to treat compressed nerves in the lower spine (Al-Khodairy et al, 2007).

Conclusion

This article has started to explain the role of the PNS in the nervous system, and looked at the anatomy and function of the spinal cord and spinal nerves in particular. The next article in this series will continue to explore the PNS’ structures, focusing specifically on the cranial nerves.

Key points

• The peripheral nervous system comprises the nerves that connect the central nervous system to the rest of the body
• Thirty-one pairs of spinal nerves leave the spinal cord through spaces between the vertebrae
• Spinal nerves can be sensory, motor or mixed
• Sensory nerve endings respond to stimulation, and motor nerve endings communicate with muscles and glands
• Injury to the sciatic nerve can cause pain, weakness and sensory loss

Also in this series

• Nervous system 1: introduction to the nervous system
• Nervous system 2: the central and peripheral nervous system I
• Nervous system 3: the central and peripheral nervous system II
• Nervous system 5: the peripheral nervous system – cranial nerves
• Nervous system 6: the autonomic nervous system – anatomy and function

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References

Al-Khodairy A-WT et al (2007) Sciatica in the female patient: anatomical considerations, aetiology and review of the literature. European Spine Journal; 16: 6, 721-731.

De Fine M et al (2017) Sciatic nerve palsy following total hip replacement: are patients personal characteristics more important than limb lengthening? A systematic review. BioMed Research International; 8361071.

Hansen JT (2014) Netter’s Clinical Anatomy. Saunders.

Irimia A, Van Horn JD (2021) Mapping the rest of the human connectome: atlasing the spinal cord and peripheral nervous system. NeuroImage; 225, 117478.

Kiernan JA, Rajakumar N (2014) Barr’s The Human Nervous System: An Anatomical Viewpoint. Lippincott Williams and Wilkins.

Leijnse JN, D’Herde K (2016) Revisiting the segmental organization of the human spinal cord. Journal of Anatomy; 229: 3, 384-393.

Mtui E et al (2015) Fitzgerald’s Clinical Neuroanatomy and Neuroscience. Elsevier.

Pawlina W, Ross MH (2018) Histology: A Text and Atlas – With Correlated Cell and Molecular Biology. Wolters Kluwer.

Patton KT, Thibodeau GA (2019) Anthony’s Textbook of Anatomy and Physiology. Elsevier.

Tortora GJ, Derrickson B (2014) Principles of Anatomy and Physiology. John Wiley & Sons.

VanPutte CL et al (2017) Seeley’s Anatomy and Physiology. McGraw-Hill.

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