Human Physiology

Lesson 36 : General Endocrine System

Introduction

Body responds to external stimuli through two mechanisms /systems that are Nervous system and endocrine system. In case of nervous system, information (stimulus) is gathered, transported, integrated and disseminate through nerve impulses. Where as, in endocrine system, the information is transmitted from sense organ or effector cell via a chemical messenger. These chemical messengers are traced in body fluids.

Interesting features are:

• Diabetes is one of the most common hormonal problems. Too much glucose sugar is present in the blood, creating many health problems. The pancreas does not make the hormone insulin properly, or the insulin doesn’t act properly.
• If the pituitary gland makes too much growth hormone, the body gets bigger too fast.
• If the pituitary gland makes too little growth hormone, the body grows too slowly.
• Hormonal conditions such as these can be treated by medical drugs, given as pills or injections.

The chemical transmission can occur through four methods -

1. Paracrine transmission - messenger diffuses through interstitial fluid and acts on nearby cells so called paracrine transmission. Sometimes messenger affects on cell itself, so also called autocrine.

1. Neurocrine transmission - messenger travels short distance from neuron to neuron or neuron to target cell. The release of messenger from neuron takes place as a result of stimulus. Neurocrine transmission is by neurotransmitters or neuro-secretions.
2. Endocrine transmission-Here the messenger is secreted into blood travels some distance and the acts on target organ /cell to being about the desired change, the messenger is called hormone.
3. Exocrine transmission - the messenger is secreted outside the body (e.g. secretion in GIT lumen) and exerts effect on cells lining the GIT distally.

Some chemicals act both ways e.g. epinephrine. When released in blood from adrenal medulla, it is a hormone but in nerve terminals, it is a neurotransmitter.

Nervous system and endocrine system are not distinct as it seems, the functions of two systems overlap and also integrate to control the body functions. e.g. Hypothalamus controls the endocrine secretions of pituitary by nervous control, however the circulatory levels of various hormones controls the hypothalamic neurons, thus stimulates pituitary. Location of various endocrine glands is shown in Fig 36a.