Instinctive Action

The hypothalamus plays a central role in the control of instinctive behaviors, such as those involved in locomotion, orientation, and reproduction, and in neuroendocrine function. Pools of neuroendocrine neurons in the periventricular parts of the hypothalamus secrete hormones into the vascular system. Their terminals in the neurohypophysis (posterior pituitary) release oxytocin and antidiuretic hormone (vasopres-sin). Although it might seem counterintuitive to consider such secretions as motor in function, they serve as a mechanism through which the CNS controls other parts of the body, just as alpha motor neurons do.

Different cells in the periventricular hypothalamus serve a less direct endrocrine control role. These neuroendocrine cells secrete higher order control hormones, often termed releasing factors, into the portal blood supply of the pituitary. The adenohypo-physis (anterior pituitary) responds to these hormones, such as corticotropin-releasing factor, which increase or decrease the secretion of pituitary hormones such as adrenocorticotropin. These hormones play crucial roles in regulating growth and in homeostatic and reproductive functions.

The hypothalamus also affects the body through its control of the autonomic nervous system (ANS). The hypothalamus sends descending projections to the autonomic motor nuclei in the spinal cord and brain stem. There, neurons send outputs to the peripheral ganglia of the two components of the ANS, the sympathetic and parasympathetic systems. Sympathetic motor neurons are located at thoracic and lumbar levels of the spinal cord. Parasympathetic motor neurons are found in the sacral spinal cord as well as in the brain stem, from which they give rise to parts of cranial nerves V (trigeminal), VII (facial), IX (glossopharyngeal), and X (vagus). Autonomic motor neurons, like alpha motor neurons, project to the periphery and release acetylcholine. In the sympathetic nervous system, most of these cholinergic influences terminate on ganglia relatively near the CNS. These sympathetic ganglia contain neurons that release noradrenaline, which induces a generalized arousal of the "fight, flight, and fright'' variety. In the parasympathetic system, cholinergic motor neurons project to ganglia relatively far from the CNS, near their visceral targets. Thus, the ANS can address the targets of the parasympathetic system more specifically than it can for the sympathetic system. Most parasympathetic postganglionic synapses use acetyl-choline as a transmitter, although many neuropep-tides, such as vasoactive intestinal peptide, are released along with acetylcholine. The hypothalamus, through its control of the ANS, exerts an important influence over such motor functions as vasoconstriction, respiration, and heart rate.

However, the hypothalamus does not confine its motor functions to the ANS. Parts of the hypothalamus play an important role in ingestive behaviors (such as eating and drinking), defensive and agonistic behaviors (such as flight from danger and aggression), arousal and orientation, and social behaviors, including sexual and other behaviors involved in reproduction (e.g., rearing of progeny and other means by which genes are passed to future generations). Its motor roles thus include initiating complex action patterns, which include epigenetically expressed but genetically encoded motor programs often termed species-specific or species-typical behaviors. For example, aggressive displays such as snarling at or staring down an adversary might result from fear, intermale competition, or irritation, with each state and response mediated by different, partially overlapping networks in the hypothalamus. The hypothalamus effectuates many of these behaviors via projections to the thalamus and to various brain stem structures (e.g., the periaqueductal gray).

The several systems influenced by the hypothalamus—endocrine, autonomic, and instinctive—may seem dissimilar and unrelated. However, the hypothalamus (along with components of the amygdala) coordinates these aspects into a fully coordinated behavior. Imagine combat soldiers engaged in a stressful and dangerous situation. A variety of species-typical behaviors accompany such perilous situations, including heightened states of vigilance and arousal, which require high expenditures of energy. Parts of the medial hypothalamus detect inputs that signal such situations and, through their projections to the brain stem, release and intensify arousal and vigilance behaviors. Those hypothalamic regions also influence the periventricular hypothalamus to secrete corticotropin-releasing factor, which in turn induces the release and circulation of adrenocorticotropic hormone. This hormone stimulates the adrenal cortex to produce and release larger amounts of glucocorti-coids. A parallel ANS control signal from the hypothalamus to the motor nucleus of the vagus promotes insulin secretion by the pancreas. Thus, the neuroendocrine system (through gluocorticoids) induces the liver to release more glucose at the same time as the ANS (through insulin) promotes the uptake of glucose into cells, especially muscle fibers. Together, the hypothalamus coordinates an adaptive response to a stressful state: the mobilization and utilization of stored nutrients to support high energy expenditure as well as the actions appropriate to that state.

B. Reflex Responses

The motor system controls a large number of reflexes, of which this article highlights only withdrawal and muscle-afferent reflexes.

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