It operates independently of voluntary control, although certain events, such as stress, fear, sexual excitement, and alterations in the sleep-wake cycle, change the level of autonomic activity. 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. The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. The autonomic nervous system is the part of the peripheral nervous system that regulates the basic visceral processes needed for the maintenance of normal bodily functions. The basic unit of the nervous system is the neuron. The nervous system allows organisms to sense, organize, and react to information in the environment. 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. The nervous system controls bodily function by gathering sensory input, integrating that information internally, and communicating proper motor output. The next simplest reflex arc is a three-element chain, beginning with sensory neurons, which activate interneurons inside of the spinal cord, which then activate motor neurons. The singular example of a monosynaptic reflex is the patellar reflex. Reflex circuits vary in complexity-the simplest spinal reflexes are mediated by a two-element chain, of which in the human body there is only one, also called a monosynaptic reflex (there is only one synapse between the two neurones taking part in the arc: sensory and motor). Stimuli from the precentral gyrus are transmitted from upper motor neurons, down the corticospinal tract, to lower motor neurons ( alpha motor neurons) in the brainstem and ventral horn of the spinal cord: upper motor neurons release a neurotransmitter called glutamate from their axon terminal knobs, which is received by glutamate receptors on the lower motor neurons: from 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.Ī reflex arc is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output.
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). Efferent neurons (motor or descending) send neural impulses from the CNS to the peripheral tissues, instructing them how to function. The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary reflex arcs.
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