Endocrine System

Endocrine System

• The endocrine system consists of glands widely separated from each other with no physical connections
• Endocrine glands are groups of secretory cells surrounded by an extensive network of capillaries that facilitates diffusion of hormones (chemical messengers) from the secretory cells into the bloodstream.
• They are commonly referred to as ductless glands because hormones diffuse directly into the bloodstream.
• Hormones are then carried in the bloodstream to target tissues and organs

• Homeostasis of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system.
• The autonomic nervous system is concerned with rapid changes, while endocrine control is mainly involved in slower and more precise adjustments.
• Hypothalamus is classified as a part of the brain rather than an endocrine gland, it controls the pituitary gland and has an indirect effect on many others.

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Overview of Hormone action

• When a hormone arrives at its target cell, it binds to a specific receptor, where it acts as a switch influencing chemical or metabolic reactions inside the cell.
• A hormone is released in response to a specific stimulus and usually its action reverses or negates the stimulus through a negative feedback mechanism. e.g. insulin and glucagon.
• The effect of a positive feedback mechanism is amplification of the stimulus and increasing release of the hormone until a particular process is complete and the stimulus ceases, e.g. release of oxytocin during labou

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Pituitary gland and hypothalamus

• The pituitary gland and the hypothalamus act as a unit, regulating the activity of most of the other endocrine glands.
• The pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus, to which it is attached by a stalk.
• It is the size of a pea, weighs about 500 mg
• The anterior pituitary (adenohypophysis) is an upgrowth of glandular epithelium from the pharynx.

• the posterior pituitary (neurohypophysis) is a downgrowth of nervous tissue from the brain.
• There is a network of nerve fibres between the hypothalamus and the posterior pituitary
• Between these lobes is a thin strip of tissue called the intermediate lobe and its function in humans is not known

Blood supply

Arterial blood

This is supplied by branches from the internal carotid artery

Venous drainage

This comes from both lobes, containing hormones, and leaves the gland in short veins that enter the venous sinuses between the layers of dura mater.

The influence of the hypothalamus on the pituitary gland

• Influence of hypothalamus is different in the anterior and posterior lobes of the pituitary gland.

The anterior pituitary

• This is supplied indirectly with arterial blood that has already passed through a capillary bed in the hypothalamus.
• This network of blood vessels forms part of the pituitary portal system, which transports blood from the hypothalamus to the anterior pituitary where it enters thin-walled sinusoids that are in close contact with the secretory cells.

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• As well as providing oxygen and nutrients, this blood transports releasing and inhibiting hormones secreted by the hypothalamus.
• These hormones influence secretion and release of other hormones formed in the anterior pituitary

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The posterior pituitary

• This is formed from nervous tissue and consists of nerve cells surrounded by supporting cells called pituicytes.
• These neurones have their cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus and their axons form a bundle known as the hypothalamohypophyseal tract
• Posterior pituitary hormones are synthesised in the nerve cell bodies, transported along the axons and stored in vesicles within the axon terminals in the posterior pituitary
• Their release by exocytosis is triggered by nerve impulses from the hypothalamus.

Anterior pituitary

• Some of the hormones secreted by the anterior lobe stimulate or inhibit secretion by other endocrine glands (target glands) while others have a direct effect on target tissues.
• he release of an anterior pituitary hormone follows stimulation of the gland by a specific releasing hormone produced by the hypothalamus and carried to the gland through the pituitary portal system of blood vessels
• The whole system is controlled by a negative feedback mechanism

Anterior Pituitary Hormones

Growth hormone (GH)

• Most abundant
• It stimulates growth and division of most body cells but especially those in the bones and skeletal muscles.
• Its release is stimulated by growth hormone releasing hormone (GHRH) and suppressed by growth hormone release inhibiting hormone (GHRIH), also known as somatostatin, both of which are secreted by the hypothalamus

Thyroid stimulating hormone (TSH)

• This hormone is synthesised by the anterior pituitary and its release is stimulated by thyrotrophin releasing hormone (TRH) from the hypothalamus.
• This hormone is synthesised by the anterior pituitary and its release is stimulated by thyrotrophin releasing hormone (TRH) from the hypothalamus.
• It stimulates growth and activity of the thyroid gland, which secretes the hormones thyroxine (T4) and tri-iodothyronine (T3).

Adrenocorticotrophic hormone (ACTH, corticotrophin)

• Corticotrophin releasing hormone (CRH) from the hypothalamus promotes the synthesis and release of ACTH by the anterior pituitary

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Prolactin

• This hormone is secreted during pregnancy to prepare the breasts for lactation (milk production) after childbirth. The blood level of prolactin is stimulated by prolactin releasing hormone (PRH) released from the hypothalamus and it is lowered by prolactin inhibiting hormone (PIH, dopamine) and by an increased blood level of prolactin.

Gonadotrophins

Gonadotrophins

• Just before puberty two gonadotrophins (sex hormones) are secreted in gradually increasing amounts by the anterior pituitary in response to luteinising hormone releasing hormone (LHRH), also known as gonadotrophin releasing hormone (GnRH).
• Both in males and females
• follicle stimulating hormone (FSH)
• luteinising hormone (LH).

In both sexes

• FSH stimulates production of gametes (ova or spermatozoa) by the gonads

In females

• LH and FSH are involved in secretion of the hormones oestrogen and progesterone during the menstrual cycle

In males

• LH, also called interstitial cell stimulating hormone (ICSH) stimulates the interstitial cells of the testes to secrete the hormone testosterone

Posterior pituitary

• Oxytocin and antidiuretic hormone (ADH or vasopressin) are the hormones synthesised in nerve cell bodies in the hypothalamus and then stored in the axon terminals within the posterior pituitary gland
• These hormones act directly on non-endocrine tissue and their release from synaptic vesicles by exocytosis is stimulated by nerve impulses from the hypothalamus.

Posterior Pituitary

Oxytocin

• Oxytocin stimulates two target tissues during and after childbirth (parturition): uterine smooth muscle and the muscle cells of the lactating breast.
• Example of positive feedback

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Antidiuretic hormone (ADH, vasopressin)

• The main effect of antidiuretic hormone is to reduce urine output (diuresis is the production of a large volume of urine).

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