Types of spinal neurons

The smallest neurons (soma diameters of 5 to 20 μm) are interneurons , and their cell bodies are contained within the cord. While the processes of some interneurons are confined within a single segment, others send their axons into the white matter surrounding the grey matter and ascend or descend two or more segments, interconnecting different spinal cord segments. These latter processes are termed propriospinal fibres and form the fasciculi proprii. Many of these smallest neurons participate in spinal reflexes. Others are intermediate cell stations interposed between fibre tracts descending from the brain and motor neurons projecting to cells controlling locomotion. Others again are so placed as to influence sensory transmission from lower to higher levels of the central nervous system (CNS).

Medium-sized neurons (soma diameters of 20 to 50 μm) are found in most parts of the grey matter. Most are relay (projection) cells receiving inputs from dorsal root afferents and projecting their axons to the brain. The projections are in the form of tracts , a tract being defined as a functionally homogeneous group of fibres. As will be seen, the term ‘tract’ is often used loosely because many projections originally thought to be ‘pure’ contain more than one functional class of fibre.

The largest neurons of all are the α motor neurons (soma diameters of 50 to 100 μm) for the supply of skeletal muscles. Scattered among them are smaller γ motor neurons supplying muscle spindles. In the medial part of the ventral horn are Renshaw cells , which exert tonic inhibition upon α motor neurons.

Spinal reflex arcs originating in muscle spindles and tendon organs have been described in Chapter 10 and the withdrawal reflex in Chapter 14 .

On the basis of cytoarchitectonic characteristics (e.g. neuronal size, staining characteristics, receptors, and connectivity), the spinal cord grey matter is divided into 10 layers, the laminae of Rexed that serve a descriptive but not necessarily functional purpose. Their configuration differs at various levels of the spinal cord; at some spinal cord levels, specific cell columns are recognised within the laminae, whereas in others, they are less clear ( Figure 15.3 ).

Laminae (I to X) and named cell groups at midthoracic level.

Spinal ganglia

The spinal or dorsal root ganglia are located on the dorsal root in the intervertebral foramina, where the ventral and dorsal roots come together to form the spinal nerves. Thoracic ganglia contain about 50,000 unipolar neurons, and spinocerebellar pathways serving the limbs contain about 100,000. These neurons are described as unipolar (or more correctly pseudounipolar). Their axons are morphologically indistinguishable from their dendrites because their somas are attached by a short stem axon . The individual ganglion cells are invested with modified Schwann cells called satellite cells ( Figure 15.4 ).

Dorsal root ganglion. In the bottom of the figure, note the T-shaped bifurcation of stem fibres, which explains why the neurons are described as ‘pseudounipolar’.

Central terminations of dorsal root afferents ( Figure 15.5 )

In the dorsal root entry zone close to the surface of the cord, the afferent fibres become segregated into medial and lateral divisions. The medial division comprises medium and large fibres that divide within the dorsal funiculus into ascending and descending branches. The branches swing into the dorsal grey horn and may synapse in the nucleus dorsalis (also known as the dorsal nucleus of Clarke). The largest ascending fibres run all the way to the dorsal column nuclei (gracilis/cuneatus) in the medulla oblongata, forming the bulk of the gracile and cuneate fasciculi.

Targets of primary afferent neurons in the dorsal grey horn.

The lateral division comprises small (Aδ and C) fibres, which upon entry divide into short ascending and descending branches within the Lissauer tract and synapse upon neurons of the substantia gelatinosa; some fibres synapse upon dendrites of cells belonging to the nucleus proprius. The nucleus proprius gives rise to the spinothalamic tract.

Categories of sensation

In accordance with the flowchart in Figure 15.6 , neurologists speak of two kinds of sensation, conscious and unconscious. Conscious sensations are perceived at the level of the cerebral cortex. Unconscious sensations are not perceived; they are relayed to the cerebellum.

Categories of sensation. ^a Exteroceptors can be categorised as telereceptors receiving from a distance (retina and cochlea) and somatic receptors on the body surface (touch, pain, etc.). ^b Enteroceptors ( Gr. enteron , ‘gut’) are strictly a subdivision of interoceptors, a term signifying all of the viscera. In pathological states they may produce conscious visceral/viscerosomatic sensations.

Sensory testing

Routine assessment of somatic exteroceptive sensation includes tests for the following:

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  touch, by grazing the skin with a cotton swab

  
  
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  pain, by applying the point of a pin

  
  
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  thermal sense, by applying warm or cold test tubes to the skin

In alert and cooperative patients, active and passive tests of conscious proprioception can be performed. Active tests examine the patient’s ability to execute set-piece activities with the eyes closed:

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  in the erect position, stand still, and with feet together ‘toe the line’ without swaying

  
  
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  in the seated position, bring the index finger to the nose from the extended position of the arm (finger-to-nose test)

  
  
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  in the recumbent position, place the heel of the foot on the opposite knee (heel-to-knee test)

Passive tests of conscious proprioception include the following:

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  Joint sense . The clinician grasps the thumb or great toe by the sides and moves it while asking the patient to name the direction of movement (up or down). Joint sense is mediated in part by articular receptors but mainly by passive stretching of neuromuscular spindles. (If the nerves supplying a joint are anaesthetised or if the joint is completely replaced by a prosthesis, joint sense is only slightly impaired. Alternatively, activation of spindles by means of a vibrator creates the illusion of movement when the relevant joint is stationary.)

  
  
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  Vibration sense . The clinician assesses the patient’s ability to detect the vibrations of a tuning fork applied proximal to the nail bed of the fingernail or toenail.