Spinal segments

The spinal cord is divided into 31 segments (Fig. 4.1A), each with a pair of dorsal (sensory) roots and a pair of ventral (motor) roots. The dorsal and ventral roots unite on each side to form a mixed spinal nerve (spinal nerve root) (Fig. 4.1B). Each nerve root divides into a small dorsal ramus, which supplies the paravertebral muscles and provides cutaneous sensation to the back, and a large ventral ramus which innervates the limbs and trunk. The dorsal root ganglia at each spinal level contain the cell bodies of sensory neurons that innervate an area of skin called a dermatome (Fig. 4.2). They also contain the cell bodies of sensory neurons innervating muscles, tendons, ligaments and joints.

Somatic sensory pathways

Two major spinal cord pathways carry sensory information to the cerebral cortex where it can be consciously perceived. The dorsal column pathway is concerned with precisely localized touch and joint position sense. The spinothalamic tract is primarily responsible for pain and temperature sensation. Each pathway is composed of a three-neuron chain with a similar arrangement (Fig. 4.3):

The axons of the somatic sensory pathways are arranged in a precise point-to-point or somatotopic fashion (Greek: soma, body; topos, place). This means that fibres innervating adjacent parts of the body surface remain side-by-side along the full length of the pathway to the brain.

Dorsal column pathway

The dorsal column pathway is mainly responsible for precisely localized touch and proprioception (awareness of one’s own body; Latin: proprius, one’s own) including joint position sense. It contains large myelinated axons (A-alpha fibres) which transmit impulses at speeds of up to 120 metres per second. Integrity of the dorsal columns is best assessed clinically using a tuning fork (Clinical Box 4.1).

 Clinical Box 4.1:   Testing dorsal column function

Assessment of vibration sense is the best clinical test of the dorsal column pathway. A low-frequency (128 Hz) tuning fork is applied to bony prominences and the patient (with closed eyes) is asked to report when the vibration starts and stops. Assessment of light touch and proprioception is also possible, but less reliable: a wisp of cotton wool can be applied gently to different skin areas to assess light touch; whereas joint position sense can be tested by asking the patient (again, with closed eyes) to indicate whether the examiner is flexing or extending various joints. Another indication of impaired joint position sense is difficulty standing upright with the eyes closed, which is called Romberg’s sign.

Gracile and cuneate fasciculi

The dorsal columns lie between the dorsal root entry zones. On each side of the cord, the dorsal column contains two fasciculi. Closer to the midline, and present at all cord levels, is the fasciculus gracilis (Latin: gracile, slender). This transmits sensory information from the mid-thoracic (T6) level and below, including the lower limbs. The wedge-shaped fasciculus cuneatus (Latin: cuneus, wedge) is lateral to the gracile fasciculus but is only present in the upper half of the cord. It contains sensory fibres from above T6, including those from the upper limbs.

Pathway to the cerebral cortex (Fig. 4.4)

The first-order neurons of the dorsal column pathway have their cell bodies in the dorsal root ganglia. A central axonal process enters the dorsal column and, without crossing the midline, ascends uninterrupted to the medulla. These fibres terminate on second-order neurons in the dorsal column nuclei (the gracile and cuneate nuclei).

Axons emerge from the dorsal column nuclei and arch anteriorly and medially through the substance of the medulla as the internal arcuate fibres (Latin: arcuate, shaped like a bow). These axons cross the midline as the great sensory decussation. It is important to appreciate that in the dorsal column pathway all the second-order axons cross the midline together (in the great sensory decussation of the medulla).

Having crossed the midline, the second-order fibres turn sharply upwards and ascend to the thalamus as the medial lemniscus. This is a strap-like bundle which twists like a ribbon as it passes through the brain stem (Latin: lemniscus, ribbon). The medial lemniscus terminates on third-order neurons in the thalamus, which project to the primary somatosensory cortex in the parietal lobe.

Spinothalamic tract

The spinothalamic tract mediates pain and temperature sensations. Impulses are transmitted by small-diameter unmyelinated c-fibres and thinly-myelinated A-delta fibres which conduct impulses at relatively slow speeds of 0.5–15 metres per second. This pathway can be tested clinically using a sterile pin (Clinical Box 4.2).

 Clinical Box 4.2:   Testing the spinothalamic tract

The best measure of spinothalamic tract integrity is appreciation of a mild noxious stimulus. A sterile examination pin is systematically applied to different points on the skin surface, randomly switching to the opposite (blunt) end. The patient has his or her eyes closed and is asked to say if the sensation is sharp or dull. Assessment of temperature sensitivity is impractical and is not done routinely.

Pathway to the cerebral cortex (Fig. 4.5)

The first-order neurons of the spinothalamic tract have their cell bodies in the dorsal root ganglia. The central processes enter the spinal cord where they synapse on second-order neurons in the dorsal horn.

Axons of the second-order neurons then cross the midline in the ventral white commissure (in the anterior spinal cord) to reach the opposite side. Having crossed over, the second-order fibres ascend in the anterolateral spinal cord as the spinothalamic tract and pass through the brain stem as the spinal lemniscus. This fibre system runs in close proximity to the medial lemniscus and also terminates on third-order neurons in the thalamus.

Finally, third-order thalamocortical fibres project to the primary somatosensory cortex. It is important to note that in the spinothalamic tract (in contrast to the dorsal column pathway) axons of second-order neurons can be found crossing the midline at all levels of the cord, rather than in a single sensory decussation.

The spinoreticulothalamic pathway

Some spinothalamic axons contribute to a spinoreticular pathway which synapses in the reticular formation of the brain stem. From here, reticulothalamic fibres ascend to the intralaminar nuclei of the thalamus before projecting on to the limbic lobe and insula, which have visceral and emotional roles including pain perception (Clinical Box 4.3).

 Clinical Box 4.3:   Nociception versus pain

Nociception is the detection of noxious stimuli (Latin: nocēre, to harm) but its relationship to pain perception is not straightforward. For instance, soldiers injured during combat (or people involved in serious accidents) sometimes sustain devastating injuries, but feel no pain. This is termed battleground analgesia, which is regarded as a life-preserving anti-nociceptive mechanism. It is an example of stimulus-induced analgesia (another is acupuncture) and appears to rely on the release of endogenous opiates, which are similar to morphine. Conversely, some patients have intractable chronic pain syndromes with no apparent physical cause, whilst others experience pain that seems disproportionate to the degree of injury (e.g. agonising neuropathic pain following minor peripheral nerve damage). There is also a strong psychological element to pain perception. This is demonstrated by the placebo effect in which an inert substance such as a clay tablet or saline infusion can provide powerful pain relief, even in patients with serious illnesses.

Key Points

 The two somatic sensory pathways have a similar organization, consisting of first-, second- and third-order neurons arranged in a linear chain.

 The first-order neurons have their cell bodies in the dorsal root ganglia and central processes that enter the spinal cord via the dorsal (sensory) root.

 Second-order neurons have their cell bodies in the CNS grey matter (brain stem or spinal cord) and give rise to axons that cross the midline, before ascending to the thalamus.

 The third-order (thalamocortical) neurons project from the ventral posterior (VP) nucleus of the thalamus to the primary somatosensory cortex in the parietal lobe.

 In the dorsal column pathway the second-order neurons are contained in the gracile and cuneate nuclei of the medulla and their axons all cross over together in the great sensory decussation.

 In the spinothalamic tract the second-order neurons are located in the dorsal horns and their axons cross the midline (at all spinal cord levels) in the ventral white commissure.