Autonomic Nervous System & Endocrine System: Mini-Textbook Study Guide

Autonomic Nervous System (ANS)

Somatic vs Sympathetic vs Parasympathetic Neurons

The nervous system controls bodily functions through different types of neurons. The somatic and autonomic (sympathetic and parasympathetic) divisions differ in their structure and neurotransmitter usage.

• Somatic Nervous System: Involves a single neuron from the CNS to the skeletal muscle. Neurotransmitter: Acetylcholine (ACh).

• Sympathetic Nervous System: Uses two neurons: a short preganglionic neuron (releases ACh) and a long postganglionic neuron (releases epinephrine/norepinephrine).

• Parasympathetic Nervous System: Also uses two neurons: a long preganglionic neuron (releases ACh) and a short postganglionic neuron (releases ACh).

Example: Somatic neurons directly stimulate skeletal muscle contraction, while sympathetic neurons prepare the body for 'fight or flight' responses.

Organ Responses: Sympathetic vs Parasympathetic

The ANS regulates organ function through opposing actions:

• Sympathetic: Increases heart rate, dilates pupils, inhibits digestion, relaxes airways.

• Parasympathetic: Decreases heart rate, constricts pupils, stimulates digestion, constricts airways.

Example: During stress, the sympathetic system increases heart rate; during rest, the parasympathetic system slows it down.

Anatomical Differences: Sympathetic vs Parasympathetic

The two divisions originate from different regions of the CNS:

• Parasympathetic: Craniosacral division (brainstem and sacral spinal cord).

• Sympathetic: Thoracolumbar division (thoracic and lumbar spinal cord).

Example: Parasympathetic fibers exit via cranial nerves; sympathetic fibers exit via thoracic/lumbar nerves.

Sympathetic Trunk: White Rami vs Gray Rami

The sympathetic trunk is a chain of ganglia alongside the vertebral column. Rami communicantes connect spinal nerves to the trunk:

• White rami: Carry preganglionic fibers (myelinated) into the trunk.

• Gray rami: Carry postganglionic fibers (unmyelinated) out to spinal nerves.

Acetylcholine Receptors (Cholinergic)

Cholinergic receptors respond to acetylcholine:

• Nicotinic: Found on skeletal muscle and postganglionic cell bodies of both sympathetic and parasympathetic systems.

• Muscarinic: Found on target organs innervated by the parasympathetic system.

Adrenergic Receptors

Adrenergic receptors respond to norepinephrine and epinephrine:

Receptor	Main Location	Effect
Beta 1	Heart	Increases heart rate and force
Beta 2	Lungs	Bronchodilation
Beta 3	Adipose tissue	Lipolysis
Alpha 1	Blood vessel smooth muscle	Vasoconstriction
Alpha 2	Various	Modulates neurotransmitter release

Endocrine System

Exocrine vs Endocrine Glands

Glands are classified by how they release their products:

• Exocrine glands: Secrete products into ducts (e.g., sweat, salivary glands).

• Endocrine glands: Release hormones directly into the bloodstream (e.g., pituitary, thyroid).

Lipid-Soluble Hormone Transport

Lipid-soluble hormones (e.g., sex hormones, aldosterone, cortisol) require carrier proteins for transport in blood:

• Carrier proteins: Albumin and others bind hormones for transport.

• Hormones enter cells directly, bind to intracellular receptors, and act as transcription factors in the nucleus.

• Effects are slower (hours to days).

Protein Hormones

Protein hormones (e.g., insulin) are water-soluble:

• Do not require carriers.

• Bind to membrane-bound receptors.

• Effects are rapid (seconds to minutes).

Up-Regulation & Down-Regulation of Receptors

Cells adjust their sensitivity to hormones:

• Up-regulation: Increase in receptor number, enhancing response.

• Down-regulation: Decrease in receptor number, reducing response.

Anterior vs Posterior Pituitary Hormone Release

The pituitary gland has two distinct parts:

• Anterior pituitary: Releases hormones in response to hypothalamic releasing hormones.

• Posterior pituitary: Stores and releases hormones produced by the hypothalamus (oxytocin, ADH).

Posterior Pituitary Hormones

• Oxytocin: Released in response to neural stimuli (e.g., childbirth, breastfeeding). Targets uterus (contraction) and breast (milk ejection).

• ADH (Antidiuretic Hormone): Released in response to increased blood osmolarity. Targets kidneys to promote water reabsorption.

Anterior Pituitary Hormones

Hormone	Stimulus	Releasing Hormone	Target Tissues	Actions
Growth Hormone (GH)	Low blood GH, hypoglycemia	GHRH	Liver, muscle, bone	Stimulates growth, protein synthesis
ACTH	Stress, low cortisol	CRH	Adrenal cortex	Stimulates cortisol release
Prolactin	Pregnancy, breastfeeding	PRH	Breast	Stimulates milk production
FSH & LH	Puberty, menstrual cycle	GnRH	Ovaries/testes	Stimulate gamete production, sex hormone release
TSH	Low thyroid hormone	TRH	Thyroid gland	Stimulates thyroid hormone release

Example: GH deficiency leads to pituitary dwarfism; excess causes gigantism or acromegaly.

ACTH: Excess causes Cushing's syndrome; deficiency causes Addison's disease.

Prolactin vs Oxytocin: Prolactin stimulates milk production; oxytocin stimulates milk ejection.

TSH: Excess causes hyperthyroidism; deficiency causes hypothyroidism.

Thyroid Gland: Follicular vs Parafollicular Cells

• Follicular cells: Produce thyroid hormones (T3, T4) for metabolism regulation.

• Parafollicular cells: Produce calcitonin for calcium regulation.

Blood Calcium Regulation

Three hormones regulate blood calcium:

Hormone	Source	Action on Bone	Action on Kidneys	Action on Gut
Parathyroid Hormone (PTH)	Parathyroid gland	Increases bone resorption	Increases Ca2+ reabsorption	Increases Ca2+ absorption (via calcitriol)
Calcitriol	Kidneys (from vitamin D)	Promotes bone resorption	Increases Ca2+ reabsorption	Increases Ca2+ absorption
Calcitonin	Thyroid (parafollicular cells)	Inhibits bone resorption	Decreases Ca2+ reabsorption	No significant effect

Relationship: Calcitriol increases calcium absorption in the gut.

Adrenal Gland Anatomy & Hormones

The adrenal gland consists of a cortex (three layers) and medulla:

• Cortex: Three layers (GFR: Glomerulosa, Fasciculata, Reticularis)

• Glomerulosa: Produces aldosterone (salt regulation)

• Fasciculata: Produces cortisol (sugar regulation)

• Reticularis: Produces sex hormones (testosterone, estrogen)

• Medulla: Produces epinephrine/norepinephrine

Pancreas: Insulin & Glucagon

The pancreas regulates blood glucose via two hormones:

• Insulin (Beta cells): Released in response to high blood glucose. Promotes glucose uptake by cells, lowers blood glucose.

• Glucagon (Alpha cells): Released in response to low blood glucose. Promotes glucose release from liver, raises blood glucose.

Metabolic Terms

• Glucagon: Hormone that raises blood glucose.

• Glycogen: Storage form of glucose in liver/muscle.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

• Glycogenesis: Formation of glycogen from glucose.

• Lipolysis: Breakdown of fats.

• Lipogenesis: Formation of fats.

• Anabolism: Building up molecules.

• Catabolism: Breaking down molecules.

Diabetes Types: Prevalence & Etiology

Type	Prevalence	Etiology
Type 1 Diabetes	Less common	Autoimmune destruction of beta cells; insulin deficiency
Type 2 Diabetes	Most common	Insulin resistance; often associated with obesity
Gestational Diabetes	Occurs during pregnancy	Hormonal changes cause insulin resistance

Example: Type 1 requires insulin therapy; Type 2 may be managed with diet, exercise, and medications.

Additional info: Some explanations and examples were expanded for academic completeness.