Hypothalamus and Pituitary
What three longitudinal regions can the hypothalamus be divided into?
-anterior region -tuberal region -posterior region THESE ARE ALL LOCATED IN A LARGER AREA CALLED THE MEDIAL REGION
The hypothalamus can also be divided medio-laterally. What are the three regions we talk about in this orientation?
-periventricular zone -medial region (contains the anterior, tuberal and posterior smaller regions within it) --> all the action is happening here!! -lateral region The medial and lateral region are separated by the fornix.
The two main sources of afferents to the hypothalamus - in more detail (from outline) can mostly ignore but might be helpful
1. *Brainstem and Spinal cord: convey visceral and somatic sensory information* a. sensory info from reticular formation (including the raphe nuclei [sleep/wake cycle, modulate CNS activity]), periaqueductal gray matter via dorsal longitudinal fasciculus (DLF) and medial forebrain bundle (MFB) b. collaterals from spinothalamic tract (pain & temp) c. locus coeruleus (regulates activity level of CNS neurons) and ventral tegmental area (motivation, normal cognitive function) via MFB 2. *Forebrain areas (particularly limbic system components) - convey information that allows hypothalamus to mediate autonomic and somatic aspects of affective (emotional) states* a. spetal nuclei (pleasure and reward center) via medial forebrain bundle b. hippocampus (memory) via fornix c. amygdale (emotion) via ventral amygdalofugal pathway and stria terminalis d. orbital cortex of frontal lobe (olfactory info), other cortical areas via MFB Other afferents to the hypothalamus include those from a subpopulation of retinal ganglion cells to the suprachiasmatic nucleus, the circadian "clock"
What are some of these external zeitgebers/factors that can reset neuronal circadian rhythm?
1. Light exposure via retinal input (retinal hypothalamic pathway) which terminates in the SCN. The SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland (part of the epithalamus), which then secretes the hormone melatonin in response. Secretion of melatonin peaks at night and ebbs during the day. 2. Diurnal (daytime) locomotor activity (thalamus → SCN) 3. Sleep patterns (serotonergic raphe system → SCN)
What are the functions of the hypothalamus?
1. monitor and control the internal environment (maintain *homeostasis* of water balance body temperature, body weight) 2. integrate activities of the *autonomic nervous system* to provide coordinated, meaningful behavior to particular stimulus 3. coordinate *drive related behaviors*, including feeding, sexual, and parental behaviors; it also regulates the *circadian rhythm* of sleep/wake cycles, body temperature, *hormonal secretion from hypophysis (pituitary)*, and physical activity
What is the set point for the human body?
98.6 F or a little lower
Boundaries of the hypothalamus What is the shape of the hypothalamus?
>Medial boundary = 3rd ventricle >Lateral boundary = indistinct >Superior boundary = thalamus (the bottom part of the thalamus is called the hypothalamic sulcus) >Inferior boundary: the optic chiasm, the optic tracts, and posterior edge of mammillary bodies >Anterior boundary = optic chiasm through the lamina terminalis to the anterior commissure >Posterior boundary = red nucleus of the mesencephalon; posterior commissure The hypothalamus is wedge shaped.
Fornix
A fiber tract that extends from the hippocampus to the mammillary body, it divides the hypothalamus longitudinally The fornix is the major output and input tract between the hippocampus and hypothalamus - its function involves the memory.
What ADH and oxytocin responsible for?
ADH-involved in regulation of water balance (see above) Oxytocin-involved in milk let-down, uterine contractions during parturition, and fosters social interactions (increases trust between individuals) The releasing system of these hormones can both be affected by your emotional state and vice versa! ex: oxytocin is needed for trust levels to build between mother and child
Cool sensitive neurons - review
Are disinhibited and stimulate the PVN--> sympathetic mechanisms to increase body temperature
Dorsal Longitudinal Fasciculus (DLF) Pathway
Begins in the medial regions of the hypothalamus and passes through periaqueductal gray of the midbrain and the floor of the fourth ventricle. It terminates on cells of the reticular formation, dorsal motor nucleus of CNX, superior and inferior salvitory nuclei of the brainstem with some fibers reaching intermediolateral and sacral autonomic nuclei of the spinal cord.
what are releasing factors?
CRH, TRH, GnRH, GHRH
What is the function of the paraventricular region?
Connects to both anterior and posterior nuclei ANS information --> paraventricular region --> contacts both the anterior and posterior nuclei depending on if parasympathetic or sympathetic action is needed
Function of the DLF What information does the DLF carry TO and FROM the hypothalamus?
Conveys *visceral pain, somatic pain & temperature, taste sensation TO hypothalamus* (from solitary and spinothalamic n to the paraventricular area of the hypothalamus) Conveys *parasympathetic info* FROM hypothalamus* to intermediolateral horn spinal cord and craniosacral parasympathetic neurons (all the way down to the pelvis and to CNs 3, 7, 9, and 10) Lower functioning
What nuclei and neurons are responsible for heating mechanisms?
Cool sensitive neurons in the *posterior region of the hypothalamus* --> MAKE THINGS WARMER (as a function of being disinhibited by the warm sensitive neurons); THIS IS A SYMPATHETIC FUNCTION (stimulates PVN --> mostly sympathetic mechanisms activated to raise body temperature) Results in decreased sweating, cutaneous vasoconstriction, shivering.
Name some hypothalamic syndromes
Diabetes insipidus: (low ADH) --> tons of urination Endocrine imbalance: (hypoadrenalism, hypothyroidism, abnormalities of menstrual cycle) Hyperthermia or hypothermia Severe obesity or wasting Disturbances of sleep: (preoptic lesions --> insomnia; post. region lesions --> hypersomnia) Emotional disorders: (ventromedial n. lesions --> violent, aggressive behavior)
The medial region of the hypothalamus can be further divided into the anterior, tuberal and posterior regions. What nuclei are found in the tuberal portion?
Dorsomedial nucleus - promotes feeding Ventromedial nucleus - inhibits feeding Arcuate (infundibular) nucleus - inhibits prolactin release, produces hypothalamic releasing factors
The hypothalamus is connected to the hippocampus via the ________________. The hypothalamus is connected to the amygdala via the _______________ and __________________.
Fornix (bundle of afferent and efferent fibers that run from the hippocampal formation to the mammillary bodies of the hypothalamus. This pathway is involved in short-term memory formation) The *ventral amygdalofugal pathway and the stria terminalis* (which contain both efferent and afferent pathways)
Connections between the hypothalamus and the limbic system What is the general role of the limbic system? What are the components of the limbic system and their specific roles?
Homeostatic mechanisms cause drives & *emotional reactions* that motivate behaviors and emotions; these can then cause autonomic responses The limbic system = bridge between hypothalamus and cortical areas (premotor, supplementary motor and association) that *instigates behaviors and helps you make decisions.* The limbic system (primarily via the amygdala) also contributes an emotional component to behaviors. amygdala - emotion (ventral amygdalofugal pathway; stria terminals); reciprocal pathway to *anterior* hypothalamus hippocampus - memory (fornix to mammillary bodies of the hypothalamus); reciprocal pathway
How are set points related to the limbic system?
Hypothalamic connections between the hypothalamus and the limbic system&cerebral cortex affect BEHAVIORS that are appropriate to regain the set point (if you feel cold outside in the snow --> you go inside)
How is body temperature regulated? (an example of of autonomic control exerted by the hypothalamus)
Hypothalamus contains INTRINSIC SENSORY NEURONS that respond directly to physical stimuli like BLOOD TEMPERATURE; spinothalamic tracts transmit skin temperature that contact these intrinsic sensory neurons of the hypothalamus In other words: The medial and lateral preoptic nuclei contain neurons that are directly temperature sensitive (warm- neurons increase their firing rate when blood temperature rises; cold-sensitive neurons increase firing rate when blood temperature falls). If one type is firing --> the other type gets inhibited. These nuclei also receive input about skin temperature from the spinothalamic tract.
Good picture for referencing the location of the various regions of the hypothalamus
In total there are 6 parts on each side of the hypothalamus, plus the paraventricular zone.
Warm sensitive neurons - review
Intrinsic membrane receptors that are sensitive to changes in brain and blood temperature above 98.6 it is stimulated by temps greater than 98.6, and their increased discharged INHIBITS cool sensitive neurons, stimulates the PVN--> mostly parasympathetic to reduce body temperature
What kind of lesions are necessary for symptoms to appear in hypothalamic syndromes?
Lesions must be bilateral for symptoms to appear They are usually due to tumors: like astrocytoma, langerhans cell histiocytosis (LCH), etc
What are the DLF and MFB?
MFB and DLF are parallel spinal tracts running the length of the spinal cord all the way to hypothalamus. One is for emotional states (MFB), and one is for visceral pain/temp and other visceral sensory sensations (DLF)
The medial region of the hypothalamus can be further divided into the anterior, tuberal and posterior regions. What nuclei are found in the posterior portion?
Mammillary body - receives input from the hippocampus (memory and emotion); projects to thalamus Posterior nucleus - thermoregulation/sympathetic control
The medial region of the hypothalamus can be further divided into the anterior, tuberal and posterior regions. What nuclei are found in the anterior portion?
Medial preoptic nucleus - hormone release (ADH --> increase water intake) Supraoptic nucleus - water balance; increase water intake Paraventricular nucleus (PVN) - ANS control Anterior nucleus - thermoregulation/ parasympathetic control Suprachiasmatic nucleus - sleep/wake
Details about the SCN What things does the SCN influence to control circadian rhythm?
Minimal activity at night, cycle repeats every 24-25 hours the neuronal rhythm can be reset by external zeitgebers/factors The SCN uses these factors/inputs to modulate its pacemaker activity, which *in turn influences the sleep/wake cycle, diurnal temperature variation, and the diurnal variations in the secretory activity of the pituitary gland (hypophysis).*
More concise statement of the efferents from the hypothalamus
Most efferents are reciprocal to afferent pathways To Parasympathetic brainstem nuclei: DLF, MFB--> periaqueductal gray --> parasympathetic nuclei (CN 3, 7, 9. 10) To preganglionic sympathetic neurons of the spinal cord (T1 to L2): DLF, MFB --> reticular formation --> reticulospinal tract --> intermediolateral column
A dysregulation of any of the functions of the hypothalamus could potentially result in what?
Physical symptoms such as panic attacks, IBS, sleeplessness/insomnia, etc
The pituitary gland What are the alternative names for: The pituitary gland The anterior lobe of the pituitary gland The posterior lobe of the pituitary gland
Pituitary gland = the hypophysis Anterior lobe = the adenohypophysis (connected by blood to the hypothalamus) Posterior lobe = the neurohypophysis (connected by neuronal connections/fibers to the hypothalamus)
Function of the Medial Forebrain Bundle (MFB) What type of information does the MFB carry? Where do its efferents terminate?
Prefrontal cortex/amygdala pathway/higher functioning stuff Conveys reward information
The paraventricular nucleus receives sensory info from the __________ and motor info from the ________.
Receives both sensory and motor info from the DLF! Also receives sensory info from the MFB (and motor info too, but we didnt talk about this much in class)
The lateral region of the hypothalamus contains which important bundle?
The MEDIAL forebrain bundle
Medial Forebrain Bundle (MFB) Pathway
The MFB extends from the septal area and basal forebrain, through the lateral region of the hypothalamus to the brainstem Info in the MFB passes in two direction and fibers both leave and enter it as it traverses the hypothalamus Hypothalamic efferents terminate in the septal area, reticular formation (in brainstem), autonomic nuclei of the brainstem (dorsal motor nucleus, CN X, salivatory nuclei), and cortex (where planning, judgement, and fantasies are conjured)
What part of the hypothalamus controls the parasympathetic system?
The anterior part
What type of sensory information does the DLF carry to the hypothalamus? And from what regions does this information originate?
The paraventricular nucleus of the hypothalamus receives *VISCERAL SENSORY info about blood pressure, pO2, etc)* via DLF from *solitary nucleus and tract* The hypothalamus receives *VISCERAL PAIN AND TEMP* info via the DLF from the *spinothalamic tracts and spinal trigeminal nucleus.*
What two mechanisms does the hypothalamus use to control the pituitary?
The hypothalamus controls hormone release from the two different parts of the hypophysis by two different mechanisms: 1. a neural projection to the neurohypophysis and 2. a vascular link to be adenohypophysis. Two of the pituitary hormones (ADH and oxytocin) are produced by neurons in the hypothalamus. The reminder of the pituitary hormones are derived from the cells of the adenohypophysis, but their secretion is controlled by hormones called *releasing and inhibiting factors* that are produced by hypothalamic neurons. The releasing factors enter the hypothalamohypophyseal portal system to reach the cells of the adenohypophysis, and affect their hormone secretion.
Hypothalamus pathways - what different systems are in contact with the hypothalamus?
The hypothalamus has RECIPROCAL afferent/input (sensory) and efferent/output (motor) connections to: 1. the autonomic nervous system 2. the limbic system and The hypothalamus sends outputs to: 1. The pituitary gland (via neurosecretory pathways) 2. The CN nuclei (tells them what to do) The hypothalamus receives inputs from: 1. auditory and visual centers.
What is a set point of the body?
The hypothalamus maintains a desired value of a particular physiological variable similar to a thermostat The afferent limb of the system brings information about the current value of the physiologic variable - if it differs too much from the set point, the efferent limb prompts other areas of the CNS to take corrective action
What are the main sources of efferents from the hypothalamus to other regions? can mostly ignore but may be helpful
The hypothalamus projects to the following regions: 1. Parasympathetic nuclei of the brainstem via dorsal longitudinal fasciculus (DLF). DLF projects to periaqueductal gray, which in turn projects to parasympathetic nuclei of brainstem. The medial forebrain bundle (MFB) also projects to brainstem. 2. Preganglionic sympathetic cell bodies from T1-L2 via DLF. DLF enters reticular formation which in turn projects to spinal cord via reticulospinal tract NOTE: The parasympathetic system is controlled by the anterior portion of the hypothalamus 3. Limbic components via stria terminalis, fornix, MFB 4. Thalamus (anterior nucleus) via mammillothalamic tract 5. Reticular formation vis mammillotegmental tract (branches off from mammillothalamic tract)
What type of sensory information does the MFB carry to the hypothalamus? And from what regions does this information originate?
The paraventricular nuclei of the hypothalamus receives sensory info via MFB from the limbic system: amygdala (fear emotion) and septic nuclei (reward and reinforcement); gives an emotional input to ANS control (example: if you're scared, your heart beats faster; and this is all coordinated by the hypothalamus)
What type of motor information does the DLF carry FROM the hypothalamus? What structures is it going to contact?
The paraventricular nucleus of the hypothalamus sends motor signals via DLF to -dorsal motor x -nucleus ambiguus -cardiovascular and respiratory centers in medullary reticular formation -intermediolateral columns (spinal cord preganglionic CNS) Basically sends motor info to all the autonomic sources
Hypothalamus and its connections to the pituitary gland
The pituitary gland releases hormones (in response to commands from the hypothalamus) into the systemic circulation that control the release of hormones from other endocrine organs (e.g. adrenal gland [ACTH], thyroid gland [TSH], reproductive organs [FSH, LH] as well as hormones that control growth [GH], lactation [prolactin], uterine contractions [oxytocin] and osmotic water balance [ADH].
What part of the hypothalamus controls the sympathetic system?
The posterior part
What is the main source of sensory input/afferent neural connections to the hypothalamus?
There are few direct (monosymaptic) connections from sensory structures to the hypothalamus, and the main source of sensory input is indirectly through the reticular formation via the Dorsal Longitudinal Fasciculus (DLF) and Medial Forebrain Bundle (MFB). These include the *limbic connections (amygdala, hippocampus)*: which convey info to the hypothalamus for mediation of autonomic and somatic aspects of *emotional states* via MFB, fornix, ventral amygdalofugal pathway, sina terminalis The hypothalamus also receives sensory info about visceral structures, taste, and pain/temp
Body weight set point and the hypothalamus How come you don't gain weight every single time you overeat?
There is a set point for body weight... slight changes in food intake trigger compensating changes in metabolic rate → no change in body weight.
True or false: both the DLF and the MFB pathways are bidirectional.
True! Key point.
What nuclei and neurons are responsible for heat loss mechanisms and inhibiting heat gain mechanisms?
Warm sensitive neurons in the preoptic nuclei of the *anterior region of the hyopothalamus*). The warm sensitive neurons have *intrinsic membrane receptors* that are sensitive to changes in blood temperature above 98.6 F. As they fire faster, animals lose heat --> MAKE THINGS COLDER; THIS IS A PARASYMPATHETIC FUNCTION (stimulates PVN --> mostly parasympathetic mechanisms activated to reduce body temperature) Results in increased sweating cutaneous vasodilation; panting in animals
hypothalamohypophyseal portal system
a specialized set of blood vessels that carry releasing factors from the hypothalamus to the anterior pituitary lobe
The amygdala contacts the ___________ part of the hypothalamus.
anterior
What is the adenohypophysis? And what is it responsible for releasing?
anterior lobe of the pituitary gland. Releases ACTH, TSH, FSH, LH, GH, prolactin The adenohypophysis is an upward growth from the roof of the mouth and contains non-neural secretory cells that produce hormones (ACTH, GH, TSH, FSH, LH, prolactin) that affect peripheral endocrine glands and organs (eg adrenal, thyroid, ovaries, testes) that secrete hormones (e.g. corticosteroids, T3, T4, estrogen, testosterone, IGF-1). Hypothalamic neurons control the secretion of these hormones through small peptide releasing hormones secreted within the median eminence, which enter the capillaries there→ portal veins → secondary capillary bed in the adenohypophysis and diffuse out of the blood to affect the secretory cells; the *hypophyseal portal system*
What part of the hypothalamus is the MAIN source of releasing/inhibitory factors?
arcuate (infundibular) nucleus in the anterior pituitary-main source of releasing/inhibitory factors
The Lateral nucleus What is its role in feeding?
feeding center also slows down metabolism lesions produce appetite loss and increased metabolism
The hypothalamus is a small part of the __________________.
diencephalon
What part of the hypothalamus releases ADH/oxyoticin?
paraventricular hypothalamic nucleus and supraoptic hypothalamic nucleus
The narrow band of tissue directly adjacent to the third ventricle is termed the ________________.
periventricular zone
What is the neurohypophysis? And what is it responsible for releasing?
posterior pituitary-ADH/oxytocin The neurohypohysis is a downward outgrowth of the median eminence. The neurons of the supraoptic nucleus and the paraventricular nucleus produce ADH and oxytocin. When action potentials travel down the axons, these hormones are released into the sinusoidal capillaries of the pars nervosa by the pituitary (hypothalamus makes these, but pituitary secretes them). These then contact the neurohypophysis.
The Ventromedial nucleus What is its role in feeding?
satiety center leptin (made by adipocytes), sensory input (CNX) from gut suppresses feeding Ghrelin (from somatic) stimulates feeding after fasting lesions produce hyperphagia, obesity, and rage
what are the inhibiting factors?
somatostatin, dopamine
What nuclei regulate circadian rhythm?
suprachiasmatic nuclei (SCN) The SCN has an intrinsic pattern of activity which is minimal at night and maximal during the day-this cycle occurs every 24-25 hours. This endogenous neuronal activity can be modified by several external factors which can re-set the "pacemaker" activity of the neurons.
