Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, and so on) and could not control motor movements.
Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion. The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a ‘fight-or-flight’ event). Most postganglionic neurons release acetylcholine onto target organs, although some release nitric oxide. The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs. Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord, as shown in Figure 16.27. While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous system allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). The strength and speed of the sympathetic response helps an organism avoid danger, and scientists have found evidence that it may also increase LTP-allowing the animal to remember the dangerous situation and avoid it in the future.
Somatic nervous system effector organs skin#
The physiological effects of this norepinephrine release include dilating the trachea and bronchi (making it easier for the animal to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain (so the animal can think and run). This is both because one preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal gland also releases norepinephrine (and the closely related hormone epinephrine) into the blood stream. As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the sympathetic nervous system are quite pervasive. Postganglionic neurons then release norepinephrine onto target organs. The acetylcholine activates the postganglionic neurons. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal cord). Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord, as illustrated in Figure 16.27. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system. Signaling to the target tissue usually involves two synapses: a preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the target organ, as illustrated in Figure 16.26. The autonomic nervous system controls these organs largely without conscious control it can continuously monitor the conditions of these different systems and implement changes as needed. It controls the lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system serves as the relay between the CNS and the internal organs.
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