1/16/2023 0 Comments Define somatic nervous system![]() ![]() Lateral view of the metencephalon and a spinal cord section with ventral and dorsal root fibers, and dorsal root ganglia. The cerebral cortex consists of various types of cortices (such as the olfactory bulbs, Figure 1.2B) as well as closely related subcortical structures such as the caudate nucleus, putamen, globus, amygdala and the hippocampal formation (Figure 1.2C). The telencephalon includes the cerebral cortex (cortex is the outer layer of the brain), which represents the highest level of neuronal organization and function (Figures 1.2A and 1.2B). The mesencephalon, metencephalon, and the myelencephalon comprise the brain stem. These three vesicles further differentiate into five subdivisions: telencephalon, diencephalon, mesencephalon, metencephalon, and the myelencephalon (Figure 1.1B). The brain and the spinal cord arise in early development from the neural tube, which expands in the front of the embryo to form the three primary brain divisions: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain) (Figure 1.1A). The adult spinal cord is approximately 40 to 50 cm long and occupies about 150 cc. It occupies a volume of about 1400 cc - approximately 2% of the total body weight, and receives 20% of the blood, oxygen, and calories supplied to the body. The adult human brain weighs between 1,200 to 1,500 g and contains about one trillion cells. ![]() It also controls clapping.Lateral view of human embryo at the beginning of the 3rd (A) and 5th (B) week of gestation. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex "knee jerk" activated by tapping the patellar tendon, contribute to ordinary behavior. Reflex circuits vary in complexity-the simplest spinal reflexes are mediated by a three-element chain, beginning with sensory neurons which activate interneurons in the spinal cord, which then activate motor neurons. ![]() ![]() Reflex arcsĪ reflex arc is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. For vertebrates, however, the response of a muscle fiber to a neurotransmitter (always acetylcholine (ACh)) can only be excitatory. In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. These signals then proceed to the neuromuscular junctions of skeletal muscles.įrom there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors ( Nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers. In turn, alpha motor neurons relay the stimuli received down their axons via the ventral root of the spinal cord. Upper motor neurons release a neurotransmitter, acetylcholine, from their axon terminal knobs, which are received by nicotinic receptors of the alpha motor neurons. These stimuli are conveyed from upper motor neurons through the ventral horn of the spinal cord, and across synapses to be received by the sensory receptors of alpha motor neurons (large lower motor neurons) of the brainstem and spinal cord. Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the corticospinal tract, via axons to control skeletal (voluntary) muscles. The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons ( upper motor neurons) within the precentral gyrus (which approximates the primary motor cortex). The somatic nervous system controls all voluntary muscular systems within the body, with the exception of reflex arcs.
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