O Blood Flow Arterial Pressure and Cardiac Function

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1. Presympathetic Vasomotor Neurons in the Brain Stem and the Hypothalamus

When the cervical or upper thoracic spinal cord is transected, the arterial blood pressure decreases to very low levels because the rostral ventrolateral medulla oblongata contains presympathetic vasomotor neurons whose activity tonically excites the spinal preganglionic sympathetic neurons so that regional vasomotor tone and arterial pressure are maintained. The activity of the medullary neurons is maintained either by intrinsic pacemaker activity or by interactions between different neuronal groups in a manner which currently is not understood. The presympathetic vasomotor neurons in the rostral medulla include the C1 catecholamine neurons (with the requisite enzymes to synthesize adrenaline being present in several species, including rats, cats, and humans) and an approximately equal number of noncatecholamine bulbospinal neurons. More medially placed bulbosp-inal neurons in the raphe and parapyramidal region have a role in regulating the changes in skin blood flow associated with body temperature regulation and with

Raphe Nucleus Skin Bloodflow

Figure 4 Schematic sagittal section through a rabbit brain showing the distribution of neurons (presympathetic motoneurons) with descending axons that innervate sympathetic preganglionic motoneurons in the intermediolateral columns of the thoracic and upper lumbar spinal cord. (modified with permission from Blessing, 1997).

Figure 4 Schematic sagittal section through a rabbit brain showing the distribution of neurons (presympathetic motoneurons) with descending axons that innervate sympathetic preganglionic motoneurons in the intermediolateral columns of the thoracic and upper lumbar spinal cord. (modified with permission from Blessing, 1997).

cutaneous vasoconstriction in response to frightening or painful events. Other regions of the brain stem and the hypothalamus also contain presympathetic motoneurons (Fig. 4), but these cells have not been functionally characterized.

2. Inhibitory Cardiovascular Neurons in the Caudal Ventrolateral Medulla

The caudal ventrolateral medulla contains a group of GABAergic neurons with short axons projecting rostrally to innervate presympathetic vasomotor neurons in the rostral medulla. Discharge of the caudal vasomotor neurons inhibits the excitatory rostral bulbospinal neurons, thereby lowering arterial pressure. Since the caudal depressor neurons are tonically active, interference with their function removes the inhibitory control so that arterial blood pressure rises. It is possible that malfunction of the caudal vasodepressor neurons underlies some forms of hypertension.

3. Cardiovascular Secondary Afferent Neurons in the Nucleus of the Tractus Solitarius

Cranial nerves IX (glossopharyngeal) and X (vagus) include afferent neurons (cell bodies in nodose and petrosal ganglia) with distal processes specialized for the detection of arterial pressure, cardiac atrial pressure, arterial oxygen and content and blood acidity, and other chemoreceptors present in the lung and the heart. The central processes of these neurons project into the medulla oblongata as the tractus solitarius, synapsing principally in the caudal one-half of the nucleus of the tractus solitarius in the dorsal region of the medulla oblongata.

4. CNS Pathway Mediating the Baroreceptor-Vasomotor Reflex

When arterial pressure rises, the baroreceptor afferent neurons increase their discharge rate, thereby increasing the discharge of secondary afferent neurons in the nucleus of the tractus solitarius. These cells project to subclasses of the inhibitory vasomotor cells in the caudal ventrolateral medulla so that activation of the nucleus tractus solitarius cells increases the discharge of the inhibitory vasomotor neurons in the caudal ventrolateral medulla, reducing the activity of the sympathoexcitatory bulbospinal in the rostral medulla neurons and thereby lowering arterial pressure.

P. Breathing

When we breath in, the lungs inflate because contraction of the diaphragm and other respiratory chest muscles means that the pressure around the lungs becomes less than atmospheric pressure. The inspira-tory respiratory muscles are innervated by somatic motor nerves (including the phrenic nerve) whose cell bodies are located in the C3 and C4 segments of the cervical spinal cord. These spinal motoneurons have no spontaneous activity. Transection of the upper cervical cord isolates them from descending excitatory control so that breathing ceases and survival means artificial respiration via a tracheal tube (positive pressure to the lung) or with an iron lung (negative pressure around the lung). Stimulating the phrenic nerve via an implanted pacemaker may also be possible.

The neurons that project from the brain to the cervical spinal cord to regulate spontaneous inspiration are located in the medulla oblongata, in the

Rat Medulla Oblongata
Figure 5 Schematic sagittal section of the rat brain stem summarizing the neuroanatomical and functional organization of the brain stem neural circuitry responsible for breathing (modified with permission from Blessing, 1997).

so-called rostral ventral respiratory group (rVRG). In turn, activity of rVRG neurons depends on net excitation from other medullary neurons either from the combined activity of a network of respiratory neurons or from the pacemaker activity of a particular group of respiratory neurons, probably the pre-Botzinger cells located in the ventrolateral medulla, rostral to the rVRG cells (Fig. 5).

During expiration, activity in rVRG neurons ceases so that the diaphragm and other inspiratory chest wall muscles no longer contract. Since lung tissue is elastic, the lungs deflate by themselves so the first part of expiration is passive. The final part of normal expiration, as well as lung deflation during forced expiration, depends on active contraction of the expiratory muscles in the chest and abdominal regions. These muscles are also innervated by spinal motoneurons activated by descending inputs from special expiratory motoneurons in the medulla oblongata, the Botzinger group, and the caudal ventral respiratory group, as shown in Fig. 5.

Other groups of respiratory neurons in the pons and the midbrain help to coordinate the activity of the medullary network. The cerebral hemispheres must contain the respiratory neurons which are responsible for voluntary breathing and for the voluntary modulation of breathing that occurs during activities such as speaking, singing, clearing one's throat, or blowing out a candle. The descending axons of these cells synapse either on unknown interneurons in the medulla ob-longata or perhaps directly on rVRG neurons.

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