Spinal nerves are an integral part of the peripheral nervous system (PNS). They are the structures through which the central nervous system (CNS) receives sensory information from the periphery, and through which the activity of the trunk and the limbs is regulated. Also they transmit the motor commands from the CNS to the muscles of the periphery.
They are composed of both motor and sensory fibres, as well as autonomic fibres, and exist as 31 pairs of nerves emerging intermittently from the spinal cord to exit the vertebral canal.
|Origins||Dorsal and ventral roots of the spinal cord|
|Regional divisions||8 cervical
|Function||Receive sensory information from the periphery and pass them to the CNS
Recieve motor information from the CNS and pass them to the periphery
|Clinical relations||Nerve root impingement, disk protrusion, disk herniation, spinal stenosis, spinal nerve impingement|
This article will discuss the anatomy and function of the spinal nerves.
Before diving into the anatomy of the spinal nerves, let's list most common terms used in neuroanatomy in order to easily orientate in the matter.
|Ventral||Anterior, towards the front|
|Dorsal||Posterior, towards the back|
|Rostral||On the forwards side (towards the nose)|
|Caudal||On the lowermost end (towards the tail)|
|Cranial||On the top side, towards the skull|
|Ipsilateral||On the same side|
|Contralateral||On the opposite side|
|Bilateral||On both sides|
Ventral and Dorsal Roots
Each spinal nerve contains a mixture of motor and sensory fibres. They begin as nerve roots that emerge from a segment of the spinal cord at a specific level. Each spinal cord segment has four rootlets: a dorsal and a ventral rootlets on both right and left sides. Each of these roots individually is composed of approximately eight nerve rootlets.
The rootlets unite to form a dorsal or ventral root of a spinal nerve. The ventral root contains efferent nerve fibres, which carry stimuli away from the CNS towards their target structures. The cell bodies of the ventral roots are located in the central grey matter of the spinal cord. Motor neurons controlling skeletal muscle, as well as preganglionic autonomic neurons are located in the ventral roots.
The dorsal root contains afferent nerve fibres, which return sensory information from the trunk and limbs to the CNS. The cell bodies of the dorsal roots are not located in the central grey matter in the spinal cord, but instead in a structure called the dorsal root ganglion. The dorsal and ventral roots join to form the spinal nerve proper, containing a mixture of sensory, motor, and autonomic fibers.
There are 31 bilateral pairs of spinal nerves, named from the vertebra they correspond to. For the most part, the spinal nerves exit the vertebral canal through the intervertebral foramen below their corresponding vertebra. Therefore, there are 12 pairs of thoracic spinal nerves, 5 pairs of lumbar spinal nerves, 5 pairs of sacral spinal nerves, and a coccygeal nerve.
The cervical spinal nerves differ from this pattern. C1-C7 spinal nerves emerge from the vertebral canal above the corresponding vertebra, with an eighth pair of cervical spinal nerves emerging below the C7 vertebra, meaning there are a total of 8 pairs of cervical spinal nerves while there are only 7 cervical vertebrae. The sacrum differs from the rest of the vertebral column in that its individual vertebrae are fused together, thus there are no intervertebral foramina. The spinal nerves instead pass through the sacral foramina.
From the level of C1 down as far as the level of L1/L2, the spinal nerve roots have a short distance to travel to their corresponding intervertebral foramen. Caudal to the level of L1/L2, the spinal cord tapers into a structure called the conus medullaris where the remaining spinal nerve rootlets exit the spinal cord at this level.
These pairs of spinal nerves have to pass a longer distance to exit the vertebral canal and form a structure within it that closely resembles a horse’s tail: the cauda equina. The dural and subarachnoid layers of meninges surrounding the spinal cord in the vertebral canal cover the spinal nerve roots as they pass towards the intervertebral foramen, effectively forming a meningeal sleeve. They fuse with the nerve to become the outer coating of the spinal nerve, the epineurium.
Path of Spinal Nerve
Having exited the vertebral canal, the spinal nerve divides into two branches: a larger anterior or ventral ramus, and a smaller posterior or dorsal ramus. Generally speaking, the ventral ramus innervates the skin and muscle on the anterior aspect of the trunk, while the dorsal ramus innervates the post-vertebral muscles and the skin of the back. The nerve fibres supplying the upper limb are from the anterior rami, which have been redistributed within a network of nerves, called the nerve plexus. The anterior rami of the upper cervical spinal nerves form the cervical plexus (supplies the anterior neck). The lower cervical and first thoracic anterior rami form the brachial plexus (supplies upper limb). The lower lumbar and upper sacral anterior rami form the lumbosacral plexus (supplies lower limb). The thoracic anterior rami remain segmental becoming the intercostal nerves in the intercostal spaces.
The rami communicantes, which translate as ‘communicating branches’, are responsible for relaying autonomic signals between the spinal nerves and the sympathetic trunk. Spinal nerves can have a grey ramus communicans and a white ramus communicans. Grey rami communicantes exist at all levels of the spinal cord. They carry postganglionic nerve fibres from the paravertebral ganglia in the sympathetic chain to their target organ. White rami communicantes only exit the spinal cord between the levels of T1-L2. They carry preganglionic nerve fibres from the spinal cord to the paravertebral ganglia in the sympathetic chain.
The spinal nerves also give off a meningeal (sinuvertebral) branch, which provides sensory and vasomotor innervation to the spinal meninges.
Types of Fibers
Somatic efferent fibers originate in the ventral column of central grey matter in the spinal cord. They pass through the ventral root of the spinal nerve. They are responsible for motor innervation of the skeletal muscles.
Somatic afferent fibers carry sensory information from the skin, joints and muscle to the dorsal column of grey matter in the spinal cord. These fibres pass through the dorsal root ganglion.
Visceral efferent fibers are autonomic fibres that supply the organs. They are divided into sympathetic and parasympathetic fibres. Sympathetic fibres originate from the thoracic spinal nerves as well as L1 and L2. Parasympathetic nerves come from the S2, S3, and S4 spinal nerves only to supply the pelvic and lower abdominal viscera. The remainder of the parasympathetic nerves come from extensions of the cranial nerves into the thoracic and abdominal cavities.
Visceral afferent fibers carry sensory information through the dorsal root ganglion and to the dorsal column of grey matter in the spinal cord.
|Somatic efferent||Type: motor
Pass thorugh: ventral root
Innervate: skeletal muscles
|Somatic afferent||Type: sensory
Pass through: dorsal root
Innervate: skin, joints and muscles
|Visceral efferent||Type: autonomic motor and secretory (sympathetic and parasympathetic)
Sympathetic: T1-T12, L1, L2
|Visceral afferent||Type: autonomic sensory
Pass thorugh: dorsal root
Dermatomes are a defined area of skin to which the sensory component of a spinal nerve is distributed to a specific spinal cord segment. All dermatomes from the shoulders down relay their sensory information back to the CNS through spinal nerves.
Myotomes are similar in function to dermatomes, but carry motor stimuli. They are responsible for segmental innervation of skeletal muscle. An example of this is the diaphragm muscle, which is innervated by the C3, C4, and C5 spinal nerves; collectively they form the phrenic nerve.
Spinal Nerves Function
A reflex is an involuntary response that occurs at a subconscious level in response to a sensory stimulus. Reflex pathways are composed of afferent neurons relaying sensory information from sensory receptors to the CNS, and efferent neurons conveying the motor stimulus back to the effector muscle or gland. Interneurons are also present between the afferent and efferent neurons, in all but the simplest reflexes.
When a muscle is stretched, it responds by contracting. The stretch reflex is one of the most simple reflexes and is known as a monosynaptic reflex arc since there is no interneuron between the efferent and afferent neurons. The afferent and efferent signals are relayed at the level of a single spinal nerve.
Stretch receptors are located within muscles. They are composed of sensory nerve endings which attach to the central region of specialised muscle cells called intrafusal fibres. The intrafusal muscle fibres are oriented parallel to the long axis of the muscle in groups called muscle spindles. When the muscle spindle is stretched, a sensory afferent signal is relayed from the sensory nerve endings to the CNS. They synapse on alpha motor neurons, which innervate the extrafusal or bulk of contractile fibres of the muscle. As well as synapsing directly on the alpha motor neuron for the same muscle, the afferent neurons also synapse on interneurons of antagonist muscles to inhibit their action.
For example, in the quadriceps reflex (patellar tendon reflex), the afferent signal from the stretch in the patellar tendon is sent back to the spinal cord, where the afferent neuron synapses on the alpha motor neuron of the quadriceps muscle causing it to contract. Simultaneously, the afferent neuron synapses on the interneuron of the hamstring muscles, which are antagonists to the quadriceps, causing them to relax.
The flexor reflex is commonly referred to as the withdrawal reflex, and occurs in response to a harmful (noxious) sensory stimulus, such as pain. This is a polysynaptic reflex, involving one or more interneurons. Afferent fibres carrying cutaneous sensory information are carried through the spinal nerve from the dermatome responsible for the stimulus. They synapse on interneurons in the grey matter of the spinal cord, which then excites alpha motor neurons of the flexor muscles in the limb. As this requires the coordinated action of more than one level of the spinal cord, interneurons distribute the signal accordingly. Activation of the flexor reflex in a weight bearing limb can also occur to take the weight off the stimulated limb.
Nerve root impingement
Nerve root impingement occurs where a spinal nerve has become obstructed or compressed as it emerges from the vertebral canal through the intervertebral foramen. There are a number of reasons as to why a spinal nerve may become impinged. These are mostly due to issues relating to the bony and cartilaginous structures surrounding the nerves as they emerge, as with the natural aging process.
Disk Protrusion occurs where the cartilaginous intervertebral disk separating adjacent vertebral bodies from each other bulges laterally. The intervertebral discs are composed of an outer fibrous cartilage called the annulus fibrosus, which surrounds a gelatinous nucleus pulposus centrally. Disk Herniation occurs when the nucleus pulposus protrudes out through the surrounding annulus fibrosus. In both disk protrusion and herniation, the spinal nerve can be compressed. Poor manual handling practises are often the cause of intervertebral disk injuries.
Spinal Stenosis is an age related condition caused by the natural degeneration of vertebrae which narrows the vertebral canal. In this case, the width of the intervertebral foramen may also narrow, impinging on the spinal nerve passing through it.
Spinal Nerve Impingement
Spinal nerve impingement can cause great discomfort, causing weakness in muscles innervated by the impacted spinal nerve as well as pain and numbness in the associated dermatome. In some instances, spinal nerve impingement can result in a radiating burning sensation. If this occurs at the level of the lower lumbar spinal nerves, it can result in a condition called sciatica, in which pain sensations radiate down the lower back into the gluteal region and posterior thigh.