Ascending Spinal Pathways carry information from the Spinal Cord to higher levels of the Central Nervous System.
Large myelinated neurones with cell bodies in the dorsal root ganglia give rise to axon collaterals that project through the dorsal column-medial lemniscal pathway, which carries information about touch and vibration to the dorsal column nuclei (cuneate and gracile nuclei) in the caudal medulla.
The dorsal column nuclei process the information by introducing a degree of spatial contrast (edge detection) and this information is passed to the contralateral ventro-basal thalamus, which in turn processes and relays somatosensory signals to the primary receiving areas of the cerebral cortex in the post-central gyrus.
The Spinothalamic pathway processes nociceptive signals carried by unmyelinated sensory axons in the ipsilateral dorsal horn, and the output neurones have long axons that ascend to the ventral postero-lateral thalamic nuclei.
The Anterolateral System consists of the spinothalamic tract and other pathways carrying nociceptive signals that synapse en route to the posterior thalamus.
The spino-cerebellar tracts carry information about the position of limbs to the cerebellum. This is an ipsilateral pathwaythat is used in the coordination of movements and balance.
Key Words: Dorsal Columns, Dorsal column nuclei, medial lemniscus, thalamus: dorsal column-medial lemniscal system concerned with touch and vibration.
Spino-thalamic and Spino-reticular pathways concerned with pain and temperature sensation.
Spino-cerebellar pathways concerned with cerebellar inputs from proprioceptive neurones.
Fine Discriminative Touch Sensation
The dorsal columns contain the axon collaterals of neurones that sense touch, and project rostrally to the dorsal column nuclei at the caudal end of the medulla. In lower segments of the cord the dorsal columns are thin, and these fibres remain in a medial position. Axons from higher segments of the cord are added to the lateral edge of the dorsal columns. In the cervical region there are two distinct components of the dorsal columns - the fasciculus gracilis (medial) and fasciculus cuneatus (lateral).
Information about injurious stimuli and temperature are relayed in the dorsal horn of the same side of the body. Within a few segments, the axons of these neurones then pass to the opposite (contralateral) lateral columns and project to the thalamus in the spinothalamic tract, which is the 'classical' pain pathway. Other ascending tracts carrying information about nociception pass rostrally in the antero-lateral system and other smaller pathways.
Discriminative Touch Sensation
The sense of Fine Touch is a highly sensitive discriminative sensation, most highly developed in the fingers, lips and genitalia.
The sensory receptors that mediate fine touch sensation all have myelinated axons and complex end organs, that allow them to distinguish moving or vibratory stimuli from maintained pressure or stretch of the skin. The former group are rapidly adapting receptors, whereas the receptors that maintain their discharge of action potentials throughout a period of mechanical stimulation are called slowly adapting receptors.
Sensory receptors have overlapping receptive fields (the area of skin served by a single axon and its branches) that are smallest in the fingers and lips, where they account for the ability to discriminate between mechanical stimuli that are close together. In skin areas where discrimination between two mechanical stimuli is poor (such as the skin of the back), the receptive fields are large.
The central pathways of receptors concerned with fine touch include the dorsal columns of the same side of the body (ipsilateral), the dorsal column nuclei (n. gracilus and n. cuneatus).
Information is relayed through the dorsal column nuclei and these axons pass through the brainstem and project to the contralateral thalamic nuclei. This relay station sends its axons to the primary somatosensory receiving area of the cerebral cortex (the post-central gyrus).
In each of these relay stations there is a process of contrast enhancement, which helps to provide more precise information concerning the location of touch stimuli.
Proprioception and Kinaesthesia requires input from Vibration receptors, Muscle Spindles, Golgi Tendon Organs and Joint receptors. The central pathways are similar to those for fine touch.
Pain and Temperature Sensation
Image source: docstoc.com
The neurones of the spino-thalamic tract carry information cross the midline near the central canal, before ascending in the lateral columns and proceeding to the thalamus. The cross-over occurs in the segment of entry and several segments rostral to that. These are sometimes called the 'second-order' neurones.
The diagram opposite shows the spinothalamic tract, and its pathways from the dorsal horn through the lateral brainstem, passed the central grey matter around the aqueduct of the midbrain into the thalamus. There, they synapse on thalamic neurones that project to the cortex and other areas of the CNS.
Pain and Temperature Sensation
Pain is a subjective response to injurious or inflammatory (noxious) stimuli. Noxious sensations are carried largely by unmyelinated nerve fibres with bare nerve endings.
A network of neurones in the superficial layers of the dorsal horn processes this information, along with other afferent inputs which can modulate the signal that passes into ascending pain pathways projecting to the posterior thalamus.
The classical anatomical description of the ascending pain pathway is the spinothalamic tract, which originates in the dorsal horn and ascends in the contralateral lateral columns to the thalamus. We now know that there are a number of other ascending pain pathways, including the spino-reticular tract, the spino-mesencehalic pathway, and the post-synaptic dorsal column pathway.
The classical anatomical view was that the dorsal columns consisted of axon collaterals of primary afferent neurones, and transmitted innocuous sensations. But modern evidence is that the dorsal columns do have an ascending post-synaptic system concerned with pain, and explains why tractotomies (which divide the anterolateral sytem) are not always successful in producing permanent pain relief.
The dorsal horn contains a netork of neurones that alter the strength of the nociceptive signal passed to ascending nociceptive pathays. The Gate Theory describes the importance of a balanced input between small and large fibre activity.
Other factors that modulate the pain pathway within the dorsal horn are descending pathways from the brainstem (medulla and pons), midbrain (periaqueductal gray matter) and other areas of the central nervous system.
Image source: dartmouth.edu
Left: Proprioceptive afferents from the lower limbs synapse in Clarke's Column (segments T1-L2), and the second order neurones travel ipsipaterally in the lateral columns to the cerebellum.
Right: Afferents from the upper limb travel rostrally in the dorsal columns and synapse in the accessory cuneate nuclei, which project to the same side of the cerebellum. This pathway for proprioceptive information from upper limbs is sometimes called the cuneocerebellar tract.
The Spino-Cerebellar Tracts carry sensory information from the limbs and trunk and informs the cerebellum about the position of parts of the body, particularly the limbs. This information is used in the execution of movements and posture, and in the control of balance; this is not regarded as a major pathway for conscious perception of the position of the limbs.
Sensory axons arising in muscle spindles, tendon organs and joint receptors in the lower limbs and trunk synapse in Clarke's column, the origin of the dorsal spino-cerebellar tracts, which carry these messages to the cerebellum. Proprioceptive afferents from the upper limbs project to the cuneate nuclei, and the second order neurones in this pathway project to the ipsilateral side of the cerebellum.
Clarke's nucleus extends from C8 - L2/L3 and is sometimes called Clarke's Column, or the Nucleus Dorsalis. Afferents from the upper limb therefore enter the cord above the rostral end of the column. As Clarke's nucleus does not extend above C8, the dorsal spinocerebellar tract does not convey information from the upper limb.