Sensory inputs

Because of its motor, i.e., activating effect on vascular smooth muscle and its inhibitory effect on intestinal smooth muscle, the sympathetic nervous system has been cast into the role of the component of the nervous system that executes control of visceral function in times of physical emergency for the organism. The phrase fight or flight has been often used to describe the circumstances in which the adrenergic transmitters of the sympathetic system are dominant over the cholinergic parasympathetic system. This concept is perhaps oversimplified but it has the utility of a first approximation of how the two components of the ANS interact in the periphery. Sensory inputs which lead to increased blood pressure, for example, activate the sympathetic pathways. 

The effect of LSD on perception is sometimes referred to as illusiogen-ic, because, rather than creating a perception of a nonexisting stimulus, LSD produces a distortion of sensory input from the environment (Kulberg 1986). Visual symptoms are most frequently experienced for example, vision may be blurred, and the perception of distance and depth may be changed. Objects in the surroundings may be perceived as unusually intense in color, shape, and/or size. With the eyes closed, geometric and kaleidoscopic patterns are perceived. Synesthesia, by which a sensory stimulus of one modality is transformed into a perception from another sense, is a type of a perceptual distortion typically experienced under the influence of LSD for example, smells... 

Lesions in conjunction with concentration studies can also be useful. Section of dorsal roots and degeneration of afferent fibres produces a reduction in glutamate and substance P which can then be associated with sensory inputs. Temporary reduction of the blood supply to the cord causes preferential destruction of interneurons and a greater loss of asparate and glycine, compared with other amino acids and so links... 

Neurotransmitter receptors have evolved as one of the key components in the ability of the central nervous system to coordinate the behaviour of the whole animal, to process and respond to sensory input, and to adapt to change in the environment. These same receptors are therefore ideal targets for drug action because of their central role in the activity of the nervous system. A rational approach to the development of new therapeutic strategies involving the action of drugs at receptors in the nervous system is based on knowledge of receptor structure, distribution and function. 

Opioids act in the brain and within the dorsal horn of the spinal cord, where their actions are better understood. The actions of opioids important for analgesia and their side-effects involve pre- and postsynaptic effects (1) reduced transmitter release from nerve terminals so that neurons are less excited by excitatory transmitters, and (2) direct inhibitions of neuronal firing so that the information flow from the neuron is reduced but also inhibitions of inhibitory neurons leading to disinhibition. This dual action of opioids can result in a total block of sensory inputs as they arrive in the spinal cord (Fig. 21.5). Thus any new drug would have to equal this dual action in controlling both transmitter release and neuronal firing. 

Collaterals from neurons of neighbouring specific thalamo-cortical relay nuclei. Because these neurons are themselves activated by sensory inputs transmitted along the spinothalamic tract, this provides one way in which sensory stimuli can influence cortical activity generally, as well as specifically. 

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...

Lateral Olfactory To hypothalamus Tract (integration with other sensory input)... 

A description of the patient s actions before admission to the hospital often provides valuable clues to the diagnosis. Since PCP seems to diminish the user s ability to integrate sensory input into meaningful behavior (Pender 1971), symptoms and findings suggesting this problem should be sought. A demonstration of sensory deprivation was provided by a patient who was found head down in a dumpster filled with garbage. He was nude and the weather was cold, but he didn t seem to be uncomfortable. He also... 

Motion sickness is caused by stimulation of the vestibular system. This area contains many histaminic (Hj) and muscarinic cholinergic receptors. The higher brain (i.e., cerebral cortex) is affected by sensory input such as sights, smells, or emotions that can lead to vomiting. This area is involved in anticipatory nausea and vomiting associated with chemotherapy. 

Can have bladder overactivity and Ul without urgency, if sensory input from the lower urinary tract is absent. 

In the vertebrate CNS monoamines have been associated with a number of physiological functions (reviewed in Kandel et al., 1991). Serotonin has functions associated with mood, pain, sleep, learning, and memory. Dopamine has functions associated with schizophrenia, Parkinson s disease, and cocaine addiction. In vertebrates, dopamine is further metabolized into two additional neurotransmitters, norepinephrine and epinephrine. Norepinephrine increases the excitability of cells in response to sudden sensory input such as fear. Epinephrine has been identified in specific neurons of the brain, but the function of these cells is unknown. In addition, AADC has also been found in a class of neurons that do not have any of the four neurotransmitters discussed above (Jaeger et al., 1983). These neurons may use one of the trace amines, tyramine, tryptamine, or phenylethylamine, as a neurotransmitter. 

The brain receives, processes, and stores sensory input generates thoughts and determines the reactions that the body should perform in... 

Figure 6.1 Types of neurons. Afferent neurons, which transmit impulses toward the CNS and efferent neurons, which transmit impulses away from the CNS, lie predominantly in the peripheral nervous system. Intemeurons, which process sensory input and coordinate motor responses, lie entirely within the central nervous system.

Brainstem Receives sensory input and initiates motor output... 

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. 

Highest level of processing, integration, and interpretation of diverse sensory input for planning purposeful action... 

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... 

The spinocerebellum influences muscle tone and coordinates skilled voluntary movements. It receives sensory input from interneurons in the spinal cord transmitting somatic information, in particular from muscle and joint proprioceptors providing data regarding body movements and positions that are actually taking place. It also receives input from the cortical motor areas providing information regarding intended or desired movement. The spinocerebellum then compares these inputs. If the actual status of a body part differs from the intended status, the spinocerebellum transmits impulses back to the motor areas of the brain to make appropriate adjustments in activation of the associated skeletal muscles. 

Processes sensory input from the skin, joints, and muscles of the trunk and limbs and initiates reflex responses to this input... 

All ascending tracts cross to the opposite side of the CNS. For example, sensory input entering the left side of the spinal cord ultimately terminates on the right side of the cerebral cortex. These tracts may cross — at the level of entry into the spinal cord a few segments above the level of entry or within the medulla of the brainstem. The locations of specific ascending tracts are illustrated in Figure 7.2 and a summary of their functions is found in Table 7.1. 

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

Sensations interpreted as pain, including burning, aching, stinging, and soreness, are the most distinctive forms of sensory input to the central nervous system. Pain serves an important protective function because it causes awareness of actual or potential tissue damage. Furthermore, it stimulates an individual to react to remove or withdraw from the source of the pain. Unlike other forms of sensory input, such as vision, hearing, and smell, pain... 

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