Chapter 2 Neurophysiology
Clinical note: In vasogenic edema (typically secondary to a brain tumor), the blood vessels are poorly developed, are leaky, and lack the transport properties of a normal BBB. This abnormal vessel permeability results in accumulation of interstitial fluid in the brain. Permeability of the BBB can also be altered in infections such as bacterial meningitis; although this accounts for some of the adverse neurologic effects of infection, it also permits improved delivery of antibiotics to the CNS.
Clinical note: The composition of CSF may be altered in various disease states. Leukocytes or excess protein makes it appear cloudy; blood may make it appear red. In some diseases, the CSF has a characteristic composition. For instance, in viral meningitis, it shows increased numbers of lymphocytes, normal to slightly elevated protein concentration, normal glucose concentration, and a normal to mildly elevated “opening pressure.” In bacterial meningitis, there are increased numbers of polymorphonuclear leukocytes, an increased protein concentration, a decreased glucose concentration, and an increased opening pressure. In multiple sclerosis, the protein content, or γ-globulin content, is increased, and there is an increase in T cells.
Clinical note: In Hirschsprung disease (congenital aganglionic megacolon), the neural crest (ganglion) cells that form the myenteric plexus fail to migrate to the colon. The absence of these cells results in intestinal obstruction because of narrowing of the affected “aganglionic” segment, causing delayed passage of meconium in the neonate, abdominal distention, and vomiting. The proximal portion of the bowel is dilated (megacolon). Treatment involves resection of the narrow, aganglionic segment (samples are sent for pathologic analysis until ganglion cells are found in the bowel sections).
Clinical note: Patients with Parkinson disease have impaired dopamine production by the substantia nigra. Because dopamine cannot cross the blood-brain barrier, it must be supplied to these patients in the form of its precursor, levodopa (l-DOPA). l-DOPA crosses the blood-brain barrier and is converted into dopamine in the brain.
Pharmacology note: Sildenafil (Viagra) works by the release of nitric oxide (NO), endothelial-derived relaxing factor. This occurs in part through increasing levels of cyclic guanosine monophosphate (cGMP). Sildenafil is used for the treatment of pulmonary arterial hypertension and erectile dysfunction. Nebivolol, which is very selective for the β1 adrenergic receptor, is effective as an antihypertensive agent primarily through promoting NO release by vascular endothelial cells, which causes vasodilation and decreases peripheral vascular resistance.
Pharmacology note: Agents that mimic the actions of ACh (e.g., pilocarpine for contraction of ciliary muscle in glaucoma) are termed cholinomimetics (or parasympathomimetics). Agents that mimic the actions of epinephrine and norepinephrine (e.g., albuterol for bronchodilation in asthma) are termed sympathomimetics.
(From Crossman A, Neary D: Neuroanatomy: An Illustrated Colour Text, 3rd ed. London, Churchill Livingstone, 2006, Fig. 8-20.)
(From Weyhenmeyer J, Gallman E: Rapid Review Neuroscience. Philadelphia, Mosby, 2007, Fig. 15-2.)
(From Weyhenmeyer J, Gallman E: Rapid Review Neuroscience. Philadelphia, Mosby, 2007, Fig. 17-1.)
Clinical note: A lesion of the lateral corticospinal tract can sometimes be appreciated by the presence of Babinski sign on physical examination. In a healthy patient, stimulation of the plantar aspect of the foot normally results in downward movement of the big toe (plantar flexion). In a patient with a lesion of the pyramidal tract, however, the big toe may move upward (dorsiflexion) in response to plantar stimulation. When this occurs, Babinski sign is said to be present.
|Parameter||Upper Motor Neuron Lesion||Lower Motor Neuron Lesion|
|Cause||Lesion of cortex or corticospinal tract||Damage to lower motor neurons|
|Examples||Amyotrophic lateral sclerosis||Poliomyelitis, trauma, Guillain-Barré syndrome|
|Muscle wasting||Absent or minimal||Present|
|Hyperreflexia or hyporeflexia||Hyperreflexia, clonus||Hyporeflexia|
|Deep tendon reflexes||Hyperactive||Absent|