Large Animal Neurology. Joe Mayhew

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СКАЧАТЬ articularis cranialis

       Processus articularis caudalis

       Processus accessorius

      As well as meningeal cells, ependymal cells, neurons (somata and their axons and dendrites), and blood vessels, several other types of supporting, protective, and nutritive glial (glue) cells (neuroglia) make up a large part of the volume of the CNS. The largest of these glial cells are the astrocytes with their star‐shaped processes. These cells basically act to support the CNS, which has little cytoskeletal framework. They also act as pseudofibroblasts and can lay down collagen in response to injury to the CNS. Originally, the processes of other small glial cell types were thought to be few (oligo); these cells being named oligodendrocytes. We now know that their processes are extensive and extend to and maintain all the myelin sheaths covering CNS axons. The small microglial cells appear to be the tissue macrophages of the mononuclear phagocytic system within the CNS, responding along with migrating inflammatory cells from the circulation when there are inflammatory processes occurring in CNS tissues.

      The neuronal processes in the peripheral nervous system (PNS) with their myelin sheaths are called nerve fibers and make up the nerve roots, nerve plexuses, and peripheral nerves. Some neurons, particularly those in the autonomic nervous system and the sensory neurons, have their cell bodies in aggregations known as ganglia. Also, several networks of interwoven nerves (plexuses) occur in the PNS, the largest of which are the brachial plexus supplying the thoracic limbs and the lumbosacral plexus for the pelvic limbs.

      The cells that ensheathe PNS nerve fibers are Schwann cells. These assist in maintaining a framework for nerves, as well as producing the abutting layers of myelin that surround all the larger fibers, allowing quite rapid saltatory (leaping, jumping) conduction of electrical impulses. The PNS has a fibrous connective tissue cytoskeleton that consists of the epineurium that wraps around a whole nerve, the perineurium that surrounds a bundle or fascicle of fibers, and the endoneurium that separates the individual nerve fibers.

Functional neuroanatomy

Probably the most important structural and functional unit of the nervous system is the simple reflex pathway (Figure 1.2). A neurologic examination basically involves testing simple and complex reflex pathways and interpreting the effected reflex activity and complex responses (see Chapter 2, Neurologic Evaluation).

A simple spinal reflex pathway, depicted as a patellar tendon reflex in Figure 1.2, is composed of two neurons. Stimulation of a sensory stretch receptor in a tendon with its sensory neuronal cell body in the dorsal root ganglion stimulates an efferent motor neuron. These motor neurons are usually alpha motor neurons and are collectively referred to as final motor neurons, having their cell bodies in the ventral gray matter of the spinal cord (and in cranial nerve nuclei in the brainstem). Reflex motor responses to sensory stimuli will occur without any other afferent or efferent connections within the CNS; so long as these two (or more) neurons comprising the reflex arc, as well as the sensory nerve ending, the neuromuscular junction, and the effector muscle, all are intact. In contrast, lesions, due to any cause, that damage parts of the reflex pathway will suppress those reflexes. Indeed, if the damage is to the alpha motor neurons in the ventral gray matter of the spinal cord or the cranial nerve motor nuclei, or to their peripheral axons, there will be loss of reflexes and severe weakness and ultimate muscle atrophy, i.e., signs of a final motor neuron lesion.

Figure 1.2 Basic monosynaptic spinal reflex pathway (patellar reflex) showing the sensory neuron synapsing on the final motor neuron. In many other reflex pathways, there is at least one internuncial neuron within the CNS allowing further modulation of the reflex.

The various reflex pathways with their respective final motor neurons throughout the brainstem and spinal cord are controlled for voluntary movement by neurons in motor centers in the brain which are collectively referred to as central motor neurons. Figure 1.3 depicts generic central motor pathways. Corticospinal motor pathways with neuronal cell bodies in the cerebral cortex (A) are very important in primates but do not appear to be very important in initiating voluntary limb and body movement in domestic animals. Quite massive lesions destroying the cerebrocortical motor centers that would be devastating in higher species do not cause permanent demonstrable abnormality in the gait of large animals. After the acute effects of such large lesions in our veterinary species have resolved, there can be subtle deficits in motor functions such as jumping fences and hopping on the thoracic limb opposite the side of the lesion. In contrast, quite small lesions in the midbrain and medulla oblongata usually result in hemiparesis or tetraparesis because of damage to central motor centers and tracts such as the rubrospinal (Figure 1.3B), reticulospinal (Figure 1.3C), and vestibulospinal (Figure 1.3D) pathways. These central motor neuronal pathways tend to have a calming effect on reflexes, particularly those involved with supporting the body against gravity such as the patella reflex. Because of this, central motor pathway lesions do not suppress reflexes and can indeed result in hyperactive reflexes and responses. The major role of these central motor pathways however is to direct the various final motor neurons in (voluntary) movement such that central motor pathway lesions will result in poor or absent voluntary effort (paresis or paralysis) in the face of very active spinal (and cranial) reflexes.

Just as brain motor centers help control the final motor neurons within the reflex arcs, the sensory inputs to these arcs are relayed to brain centers to give feedback on position sense (proprioception) touch, and pain perception (nociception) (Figure 1.4). Sensory pathways travel to the thalamus, and probably to the somatosensory cerebral cortex, for the perception of pain. These pathways are multisynaptic and contain small fibers, both characteristics making them resistant to interruption. Proprioceptive pathways that are presumed to be consciously perceived (position sense at rest) travel to thalamic and thence to cerebral conscious proprioceptive centers. Other subconscious proprioceptive information (movement sense) is contained in spinocerebellar tracts that pass directly to the cerebellum.

Figure 1.3 Central motor neuronal pathways predominantly originate in the brainstem and synapse on final motor neurons to effect motor activity. Depicted here are A the cortico (rubro)spinal, B the rubrospinal, C the reticulospinal, and D the vestibulospinal neurons and tracts.

Figure 1.4 Sensory pathways convey somatic, proprioceptive, and visceral input to brain centers. Subconscious proprioceptive neurons A project to the cerebellum, whereas conscious proprioceptive neurons B and somatic and visceral afferent СКАЧАТЬ