Lecture Six

-Nervous System -Somatic Nervous System – voluntary system, skeletal muscle contraction oAutonomic Nervous System – involuntary system, maintains a state of homeostasis (steady state) -Sympathetic System = “fight-or-flight” -Prepares the body for strenuous situations (e.g. physical activity) -Parasympathetic System = “rest and digest” -Assists during the quiet phases (e.g. digestion)

• Both systems use the brain and hypothalamus • Some molecules are used as both neurotransmitters and hormones • Both systems work to regulate bodily functions • Some neurons secrete hormones (neurohormones)

oMode of transport -Endocrine secretes hormones into the blood stream -Nervous system secretes neurotransmitters which are directly released onto target cells. oSpeed of response -The endocrine system is slower compared to the nervous system oDuration of response -Hormones remain in the blood stream for min, days, weeks -Nervous system is active for as long as the action potentials are sent

• Metabolism • Control of food intake and digestion • Tissue development n Ion levels • Water balance • Heart rate and blood pressure changes • Control of blood glucose and other nutrients • Control of reproductive functions • Uterine contractions and milk release • Immune system function

• Autocrine – Secreted from an individual cell and influences the same cell it is secreted from • Hormonal Control- Produced by a wide variety of tissues and secreted into extracellular fluid and has a localised effect • Neurotransmitter – Produced by neurons and secreted into the synaptic cleft and travels a short distance to influence postsynaptic cells • Endocrine – Hormones secreted into the bloodstream, which travel some distance to target tissues

• A relatively stable concentration is maintained in the blood stream over a long time period • e.g. Lipid-soluble hormones

• Hormone concentration alters irregularly and dramatically and changes with each stimulus • e.g. Water-soluble hormones

• Hormones are secreted at regular intervals and concentrations • e.g. Lipid-soluble hormones

• Neural Activation • Hormone Activation • Hormone Control

-A neuron releases a neurotransmitter that signals the endocrine gland to release a hormone oA stimulus causes neural action potentials to release neurotransmitters into the synapse of hormone-producing cells which stimulate the release of a hormone. oIf the stimulus stops then the neural stimulation will stop = hormone release will stop. oSome neurons secrete chemical messengers into the blood, these hormones are called neurohormones (neuropeptides). These can then cause the secretion of releasing hormones, primarily from the

-A hormone (typically called a hormone-releasing factor) will stimulate an endocrine organ/gland to release a different hormone. oTropic Hormones -Stimulate the secretion of other hormones from other endocrine glands or organs. -Inhibition – some hormones prevent the secretion of other hormones

• Circulating blood-borne molecules stimulate the release of hormones • Hormones are sensitive to changes in blood levels of substances. • Inhibition – often a hormone with opposing effects is secreted to inhibit the effects of the stimulating hormone. • Hormones work together to maintain homeostasis.

-Endocrine system oEndocrine glands secrete hormones that travel in the bloodstream to target tissues oGreek = endo ‘within’ krino ‘to secrete’ oAllows cells to communicate with each other

-Increased demand for a hormone causes appropriate stimuli to induce the endocrine gland to secrete a hormone

• Removed from circulation - The endocrine gland must receive an inhibitory message

-Regulation of this stimuli and inhibition in order to maintain homeostatic levels of hormones are done by two major mechanisms; o Positive Feedback o Negative Feedback

• When a hormone: 1. Stimulates their target cell AND 2. Promotes the synthesis and secretion of the hormone = further secretion of the hormone. • Self-propagating system.

-Most common regulatory mechanism. -Hormone secretion is inhibited by itself when there are adequate hormone levels in the blood. -Self-Limiting system.

-Hormones bind to receptors (proteins) on the target cells.

• Generally, hormones can only stimulate those cells that have the matching receptor. • Some hormones can bind to many (family) receptors which have similar structures.

• Decrease sensitivity to a hormone by ‘down-regulation’ whereby the number of receptors decrease. • Increased sensitivity through ‘up-regulation’ results from the increase in the rate of receptor synthesis.

• Is a major control site of the nervous and endocrine systems. • It receives information from hormones, emotions and the central nervous system. • The hypothalamus is connected to the pituitary gland by an extension = infundibulum.

-Major releasing and inhibiting neurohormones are secreted by the hypothalamus which travel in the portal system to the anterior pituitary gland

• Also referred to as the Neurohypophysis • Extension of the hypothalamus • No blood portal system • Neuron axons from the hypothalamus travel down the infundibulum and terminate in the posterior pituitary • Secrets neurohormones that enter the circulatory system