UWO  --  Neuroscience Graduate Program  

Neuroscience 9500: Notes for J. A. Kiernan's lecture.

•      1. The methods used to acquire knowledge of the ways in which different parts of the nervous system are functionally interconnected. (Click here for shortcut.)
•      2. The somatic sensory pathways as an example of related systems of interconnected neurons. (Click here for shortcut.)

Neuroanatomical tracing methods

The file neurmeth.pdf (click to download) contains somewhat longer accounts of neuroanatomical methods and somatosensory pathways than this web page. You need Adobe Acrobat or the free Adobe Acrobat Reader program to read this file, which is a shortened version of the accounts of these subjects in Barr's The Human Nervous System (9th edition, 2008) [ WL101.B268h.2009 in the UWO Sciences Library ].

Traditional approaches have provided most of our knowledge of connectivity in the human brain and spinal cord. These include:

Clinical consequeces of lesions that destroy groups of neuronal cell bodies or sever fibre tracts Staining anterograde degeneration in sections of the brain, post mortem.    Includes Marchi method (degenerating myelin black> and    stains for normal myelin (negative in degenerated tracts). Electrical stimulation, especially of sites in the cerebral cortex in conscious patients undergoing neurosurgical procedures under local anaesthesia

Modern approaches have generally confirmed the traditional body of knowledge of human neuroanatomy.

Imaging techniques such as computerized tomography (CT) and nuclear magnetic resonance imaging (MRI) show extent of lesions at the same time that they cause disability, allowing more accurate clinicopathological correlation than is possible with post mortem examination. Functional imaging is based on changes in blood flow, oxygen usage or electrical activity, and can show parts of the brain that are relatively more metabolically active than others. Techniques include PET, fMRI, electroencephalography and magnetoencephalography.

Experiments with animals are needed to provide detailed neuroanatomical information such as exact cells of origin and sites of termination of axons.

Degeneration methods include silver stains that are selective for early degenerative changes in axons (rather than myelin). These are seldom used today. Axonally transported tracers can be    Retrograde: Taken up by synaptic terminals and moved to the cell body. Examples are horseradish peroxidase,  peroxidase- or fluorochrome-labelled lectins, and several simple fluorescent dyes. Some viruses such as pseudorabies, are retrogradely transported. They propagate in the neuron, are transferred to its presynaptic terminals and then transported to the cell bodies of the presynaptic neurons.    Anterograde: Taken up by cell bodies and dendrites and transported along axons to synaptic terminals. Various lectins are used this way, and there are fluorochromes that can be microinjected into individual neuronal cell bodies

The file basicneuro.pdf (click to download) is a 35-page summary of the anatomy and functional connectivity of the human central nervous system. Somatic sensory pathways are reviewed on pages 23-25. The pathways are also reviewed in the second half of the file neurmeth.pdf. You need Adobe Acrobat or the free Adobe Acrobat Reader program to read these files.

Neuronal signals from skin and deeper structures are segregated in the spinal cord. Transmission to the thalamus and cerebral cortex may occur through the spinothalamic tract or through the dorsal funiculi and medial lemniscus.

For pain, temperature and the less discriminative aspects of touch, neurons in the dorsal horn have axons that cross in the spinal cord and ascend as the spinothalamic tract, which is laterally situated in the spinal cord and brain stem.

For discriminative touch and for conscious proprioception, the axons of primary sensory neurons ascend ipsilaterally in the dorsal funiculus (either gracile or cuneate fasciculus) and end in the gracile or cuneate nucleus. Fibers arising in these nuclei cross in the medulla and ascend in the medial lemniscus, which is near the midline in the medulla and shifts to a lateral location in the midbrain.
      For conscious proprioception from the lower limb there is an additional pathway, involving the dorsal spinocerebellar tract and nucleus Z in the medulla. Consequently a lesion that transects the gracile fasciculus in the cervical spinal cord does not cause complete loss of position sense in the lower limbs. Proprioceptive sensation is lost if the lesion is larger, extending ventrally and laterally into the territory of the dorsal spinocerebellar tract.

Reminder - Positions of tracts in spinal cord (Use your web browser's Back button (right mouse click in Netscape) to return from the picture to this web page.)

Click here to download sompaths.exe (1266393 bytes).  This is a free-standing executable slide show. (It needs MS Windows 95 or later. Download the file to a directory on your hard drive. Run it by typing sompaths, either at the Run prompt (shown when you click the Start icon at the bottom of the screen) or at a command prompt (found in the Accessories folder).

Both the spinothalamic tract and the medial lemniscus end in the VPL nucleus of the thalamus.  This thalamic nucleus projects to the primary somesthetic cortex of the postcentral gyrus, where the contralateral half of the body is represented as an upside-down homunculus.

Diagram summarizing the somatic sensory pathways

The somatic sensory pathways for the head involve the trigeminal sensory nuclei and their projections to the contralateral VPM thalamic nucleus.
      Primary afferent fibers for touch end in the pontine trigeminal nucleus. Pain and temperature fibers descend in the spinal trigeminal tract before ending in the caudal part of its nucleus. This pathway will be discussed later, in connection with the central connections of cranial nerve V.

Lesions in the spinal cord and brain stem can affect the somesthetic pathways separately, causing dissociated sensory loss.

What sensory loss results from a destructive in the lateral part of the medulla, involving the left spinothalamic tract and sensory nuclei of the left trigeminal nerve (but sparing other somatic sensory pathways)?    Click here for the answer (but think first!)

The main pathways are supplemented by others, especially for pain, which involve the reticular formation and thalamic nuclei other than the VP_L or VP_M .

The cerebral cortex is necessary for localizing the source of a painful stimulus and for the recognition of objects by touch.

A note about nuclei in sensory pathways.

The synapses between the main neurons in a pathway are not simply "relays," which would serve no useful purpose. Through connections from other parts of the CNS, and with the involvement of local interneurons, the incoming signals are modified for onward transmission.
For example:

In the gracile and cuneate nuclei, lateral inhibition (feed-forward and feedback types) sharpens the perception of the most strongly stimulated part of a receptive field. There is also remote inhibition by corticobulbar fibres in these nuclei.

In the dorsal horn of the spinal cord, large and small diameter axons contact inhibitory interneurons and tract cells that send pain signals to the thalamus. Balance between the two types of input constitutes the gate control mechanism for pain.

Pain is a big subject! It is mentioned only briefly in the lecture and in these notes. A direct pathway from certain dorsal horn neurons (Waldeyer cells) to the mediodorsal (MD) thalamic nucleus is also involved. One of several targets of neurons in MD is the anterior part of the cingulate gyrus, on the medial surface of the cerebral hemisphere. The anterior cingulate cortex recognizes pain as a nasty sensation caused by injury or disease. Surgical lesions are sometimes made in the anterior part of the cingulate gyrus to relieve suffering that cannot be relieved in any other way. After such surgery the patient still feels the pain but does not find the sensation intolerable.

Descending pathways that influence sensation

The proper interpretation of the outside world and the state of the body itself would be impossible if every impulse in every sensory axon were to be brought eventually to the cerebral cortex. An editing system is necessary, so that the cortex can select the sensory information worthy of conscious attention while leaving more humble duties to the spinal cord, brain stem, and cerebellum. The editing function is carried out by descending fibres that terminate in the sites of origin of the ascending tracts. Some of these descending pathways are shown in a diagram (Click here).

Descending tracts modify activity in the ascending systems at three levels:

1.  In the dorsal horn of the spinal gray matter. This is the site of termination of large numbers of corticospinal fibers, mostly from the postcentral gyrus. Other axons ending in the dorsal horn come from the reticular formation and from the gracile and cuneate nuclei. One of the reticulospinal projections, the raphespinal tract, is notable for inhibiting the upward transmission of signals concerned with pain. It originates in the raphe nuclei, in the midline of the medulla, and the unmyelinated serotonergic axons of the tract are lateral to the tip of the dorsal horn. The raphe nuclei are themselves stimulated by neurons in the periaqueductal gray matter of the midbrain. Electrical stimulation of the periaqueductal gray causes prompt relief of pain and has occasionally been done clinically for this purpose. A curious observation is that stimulation for a few minutes can produce analgesia lasting several hours.

2.  In the brain stem. Large numbers of fibers descend from the somatosensory area of the cerebral cortex to the gracile and cuneate nuclei. They are presumed to influence the medial lemniscus system. Corticobulbar fibers also end in the trigeminal sensory nuclei.
The suffix "-bulbar" refers to the termination of axons in the brain stem. The term "corticonuclear" is sometimes used for axons of cortical neurons that end in nuclei of cranial nerves. However, many descending fibers that influence cranial nerves end near, but not within, the nuclei. The less precise term "corticobulbar" is therefore preferred.

3.  In the thalamus. The VPL and VPM nuclei project to the first somatosensory area of the cerebral cortex. These thalamic nuclei also receive input from the same cortical areas.
(This isn't the only thalamocortical projection that is reciprocated; they all are.)

Recommended reading

You should read the appropriate pieces in the textbook for Neuroscience 9500:  Kandel, Schwarz & Jessell: Principles of Neural Science, 4th ed. [UWO Sciences Library: WL102.P9547 2000]. Neuroanatomical tracing methods are covered in more than one chapter.  Somatic sensory pathways receive rather brief treatment, but that's adequate for this course.

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If you want to learn more than you need to know for the exam, I will be happy to recommend additional readings. My office is Rm 453 in the Medical Sciences Building at UWO. Do drop in. I can always  (= almost always) take a break to advise students about Neuroanatomy or Histochemistry. For a wide range of free instructional resources and research-related items follow the links from my personal UWO web page at http://publish.uwo.ca/~jkiernan or look me up with Google. Notes for my current neuroanatomical courses can be found by entering neuroanatomy uwo in a Google search box.    

John A. Kiernan
Dept of Anatomy & Cell Biology
University of Western Ontario
London, Canada.

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Last updated 3rd November 2008

    1. Contralateral loss of pain and temperature sensation below the head.
    2. Ipsilateral loss of pain and temperature sensation in the head (face, mouth, pharynx etc).
    3. Discriminative touch and proprioception are not impaired. This lesion cause other effects too (Wallenberg's syndrome).

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