Ascending sensory pathways

The generally accepted view is that the stimulatory drive for the ARAS comes from collaterals of the classical ascending sensory pathways. Indeed, this is another way in which sensory stimuli can affect our state of arousal (Fig. 22.5). The diffuse activating... 

These ascending sensory pathways cross from one side of the CNS to the other so that sensory input from the left side of the body is transmitted to the somatosensory cortex of the right cerebral hemisphere and visa versa. Therefore, damage to this region of cortex in a given hemisphere results in... 

Figure 22.5 Pathways involved in cortico-thalamic synchrony and EEG arousal. The ascending reticular activating system (ARAS) extends from the cephalic medulla through the pons and midbrain to the thalamus (see Moruzzi and Mayoun 1949). It is activated by impulses in collaterals of the spinothalamic sensory pathway running to specific thalamic nuclei (SpThNc) and in turn activates much of the cortex, partly through the non-specific thalamic nuclei (NspThNc), which also receive inputs from SpThNc and also via the nucleus basalis (NcB). Its stimulation is followed by EEG arousal. It is probable that reciprocal links between cortical areas and the thalamus, particularly NspThN, lead to slow-wave (8 Hz) cortical EEG synchrony and, in the absence of appropriate sensory input and ARAS activity, a sleep state...

Stimulation of a nociceptor in the periphery of the body elicits action potentials in the first-order neuron, which transmits the signal to the second-order neuron in the dorsal horn of the spinal cord. From the spinal cord, the signal is transmitted to several regions of the brain. The most prominent ascending nociceptive pathway is the spinothalamic tract. Axons of the second-order sensory neurons project to the contralateral (opposite) side of the spinal cord and ascend in the white matter, terminating in the thalamus (see Figure 8.1). The thalamus contributes to the basic sensation or awareness of pain only it cannot determine the source of the painful stimulus. 

Mechanism of Action An opioid agonist that binds to opioid receptors in the CNS, reducing stimuli from sensory nerve endings and inhibiting ascending pain pathways. Therapeutic Effect Alters pain reception and increases the pain threshold. Pharmacokinetics ... 

Sensory systems Ears Topical papaverine for the treatment of vasospasm in neurosurgery was associated with transient disturbance for neurophysiological function of the ascending auditory pathway [149 ]. 

The spinal cord is the most anatomically inferior portion of the CNS and its functions are at the lowest level of sophistication (see Table 6.1). As mentioned earlier, the spinal cord receives sensory input from the periphery of the body and contains the cell bodies of motor neurons responsible for voluntary and involuntary movements. Once again, the involuntary and neurologically simple reflexes are processed entirely at the level of the spinal cord. Voluntary, deliberate movements are initiated and controlled by thought processes in the cerebrum. The second important function of the spinal cord is to transmit nerve impulses to and from the brain. Ascending pathways carry sensory input to higher levels of the CNS and descending pathways carry impulses from the brain to motor neurons in the spinal cord. 

The autonomic nervous system is by definition that part of the nervous system that innervates smooth muscle, cardiac muscle and glands. It is thus a motor system. Perception arising from the viscera involves pathways similar to those arising from the body surface and skeletal muscle. Thus there are visceral afferent fibres that pass from the viscera to the central nervous system. Such impulses then ascend the spinal cord to the thalamus and are thence relayed to the post-central gyrus of the brain (or sensory cortex). Visceral reflex arcs use visceral afferent fibres to convey impulses to the cord, but the efferent limb of such a visceral reflex is the autonomic nervous system. Although visceral reflexes are under higher central control, it is usually impossible to bring them under the control of the will. 

Caudal to the thalamus are the midUne structures of the brainstem the midbrain, pons, and medulla oblongata. Dorsal to the pons, but inferior to the tentorium cere-belli, is the cerebellum which is the prominent structure in the inferior cranial fossa. The brainstem as a whole is concerned with somatosensory information from the neck and head as well as the specialized senses of taste, audition and balance. It acts as a conduit for ascending and descending pathways of motor and sensory information between the cortical regions of the brain and the body. In addition, the brainstem is responsible for mediating levels of consciousness and arousal. 

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