Neuro-endocrine connection - the hypothalamus and pituitary gland Flashcards
How does the hypothalamus connect to the pituitary gland?
Hypothalamus to Anterior pituitary:
- Hypothalamic neurons release chemicals (hormones) into blood portal connection to anterior pituitary gland cells
Hypothalamus to Posterior pituitary:
- Hypothalamic neurons have long axons that extend into and terminate (axon terminals) in the posterior pituitary gland
Describe the posterior pituitary gland
The hypothalamus is connected to the posterior pituitary by neurons:
- cell bodies in the hypothalamus
- axons terminate in the posterior pituitary gland
Posterior pituitary hormones:
- are made in the hypothalamic neuron cell bodies
- travel down the axon to be stored in the axon terminals until required
- are released into the main bloodstream when an action potential depolarises the axon terminal
What are the posterior pituitary hormones and their functions?
Oxytocin and Anti-Diuretic (ADH)
Oxytocin:
- Water soluble
- Stimulates milk release during breastfeeding an infant
- Stimulates the contraction of uterine muscles during childbirth
(both examples of positive feedback)
Anti-Diuretic (ADH):
- Water soluble
- Stimulates the kidney to reabsorb water when a person is dehydrated
- One of the hormones secreted during excersise and stress response
Describe the Anterior pituitary gland
The hypothalamus is connected to the anterior pituitary gland by a portal bloodstream:
- Hypothalamic neurons secrete ‘releasing’ or ‘inhibiting’ hormones
- …that travel via a blood portal system to the anterior pituitary
- and bind to membrane receptors on anterior pituitary cells
- causing the anterior pituitary cells to release another hormone
What are the anterior pituitary hormones?
Growth hormone (GH), Thyroid stimulating hormone (TSH) & Adrenocorticotropin hormone (ACTH)
Growth hormone (GH):
- Water soluble
- stimulates the liver, skeletal muscle and adipose (fat) for fuel metabolism
- stimulates the liver to release IGF-1 for growth
Thyroid stimulating hormone (TSH):
- Water soluble
- Stimulates the thyroid gland to release thyroid hormones to increase basal metabolic rate
Adrenocorticotropin hormone (ACTH):
- Water soluble
- Stimulates the adrenal gland (cortex) to release cortisol in a daily pattern and during the stress response
Hormones from what glands/organs are involved in cell metabolism?
Hypothalamus, Pituitary gland and Thyroid gland
What is the general pattern of hormone release from the hypothalamus?
- Stimulus
- Hypothalamus: releases a hormone
- Anterior pituitary gland: releases pituitary hormone
- Target endocrine organ: releases 3rd hormone
- Target effectors receive 3rd hormone and have the desired effect
This is a negative feedback loop: the 3rd hormone will negatively feedback to reduce release of anterior pituitary and hypothalamic hormones
Note: there are exceptions to this general pattern
Describe the hypothalamic-pituitary-thyroid axis
- Stimulus: exercise and cold stress
- Hypothalamus: secretes thyrotropin-releasing hormone (TRH)
- Which stimulates Anterior pituitary gland: secretes thyroid-stimulating hormone (TSH)
- Which stimulates Thyroid gland: secretes thyroid hormones (T3 & T4)
- target of thyroid hormones: all body cells.
Effects: increase basal metabolic rate, stimulation of growth in foetus and during early childhood, increased mental alertness
This is a negative feedback loop: Thyroid hormones (T3 & T4) negatively feedback to reduce release of TSH and TRH
Process stops when excersise stops or cold stress removed
Describe the thyroid gland location and hormones
Location:
- Wraps around the trachea, just below the ‘Adam’s Apple’ (thyroid cartilage)
- On the anterior and lateral surfaces of the trachea
Hormones:
- Thyroid hormones (T3 & T4) - made by cells that line the follicles, to increase metabolic activity, growth, alertness
- Calcitonin - minor hormone involved in Ca2+ homeostasis
Describe the synthesis, storage and structure of thyroid hormones (T3 & T4)
- The structure and function units of the thyroid gland are the thyroid follicles
- Thyroglobin (TGB) is a protein made in thyroid follicles and contains tyrosine (Y)
- Iodine (I) enters cells lining the thyroid follicle and reacts with tyrosine (Y) in thyroglobin (TGB)
- T3 - tyrosine with three iodine
- T4 - tyrosine with 4 iodine - Thyroid hormones (T3 & T4) detach from iodised TGB as they are needed
- Thyroid hormones (T3 & T4) travel in blood bound to a carrier protein
Note: iodine is a necessary part of our diet as it is required for the production of thyroid hormone
Describe target cells activation by thyroid hormones (T3 & T4)
- Made in advance and stored until required (this is different to steroids - other lipid solible hormones)
- Travels bound to a carrier protein
- Detaches from the carrier protein and enters the target cell
- Thyroid hormones bind to the receptor in the nucleus
- Specific genes are activated to transcribe messenger RNA (mRNA)
- specific proteins are synthesised that lead to increased basal metabolic rate (BMR)
- Response time: 45 minutes - days
What is basal metabolic rate (BMR)?
Basal metabolic rate is the energy the body requires for its most basic (basal) functions to sustain life.
- Your BMR is influenced by your sex, age, body type and size, food intake, activity/excersise level and environmental temperature
What are the effects of thyroid hormones on basal metabolic rate (BMR)?
Thyroid hormones increase BMR by increasing:
- thermogenesis (body heat production)
- oxygen and ATP consumption
- fat and protein breakdown
Thyroid hormones also have a complex interaction with various organs to ensure enough glucose is available for metabolic processes, without disturbing plasma glucose homeostasis (ie. glucose in = glucose out)
Hormones from what glands/organs are involved in growth and cell metabolism?
Hypothalamus and Pituitary gland
Describe the patterns of growth hormone secretion
Daily secretion patterns:
- Growth hormones (GH) plasma concentration fluctuates over a day
- GH concentrations are highest during sleep
Lifetime secretion pattern:
- Growth hormone (GH) plasma concentration is higher in children than adults
- GH concentrations are highest during puberty and decline with age
