Spinal nerves

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 anterior/ventral ramus innervates the skin and muscle on the anterior aspect of the trunk, while the posterior/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 anterior/ventral column of central grey matter in the spinal cord. They pass through the anterior 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 posterior/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. 

Master the anatomy of the spinal nerves with our articles, video tutorials, quizzes, and labeled diagrams.

Dermatomes

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.

That’s a lot to learn! Why not test and consolidate your knowledge with the quiz below:

Spinal reflexes

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.

Stretch reflex

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.

Flexor reflex

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.