DIRECT INTERCELLULAR COMMUNICATION Small molecules and ions
(a) Gap junctions
(b) Transient direct linkup of cells’ surface markers
INDIRECT INTERCELLULAR COMMUNICATION VIA EXTRACELLULAR CHEMICAL MESSENGERS Secreting cell
Local target cell
Paracrine
(c) Paracrine secretion
Local target cell Electrical signal
Secreting cell (neuron) Neurotransmitter (d) Neurotransmitter secretion
Blood Secreting cell (endocrine cell)
Neurohormone Blood Electrical signal
Hormone Distant target cell Nontarget cell (no receptors) (e) Hormonal secretion
Secreting cell (neuron) Nontarget cell (no receptors)
Distant target cell
(f) Neurohormone secretion Fig. 4-18, p. 92
Chemical Messengers • Four types of chemical messengers 1. Paracrines • Local chemical messengers • Exert effect only on neighboring cells in immediate environment of secretion site
2. Neurotransmitters • Short‐range chemical messengers • Diffuse across narrow space to act locally on adjoining target cell (another neuron, a muscle, or a gland)
Chemical Messengers 3. Hormones • Long‐range messengers • Secreted into blood by endocrine glands in response to appropriate signal • Exert effect on target cells some distance away from release site
4. Neurohormones • Hormones released into blood by neurosecretory neurons • Distributed through blood to distant target cells
Chemical Messengers • Extracellular chemical messengers bring about cell responses primarily by signal transduction – Process by which incoming signals are conveyed to target cell’s interior
• Binding of extracellular messenger (first messenger) to matching receptor brings about desired intracellular response by either – Opening or closing channels – Activating second‐messenger systems • Activated by first messenger • Relays message to intracellular proteins that carry out dictated response
Second Messengers • cAMP (cyclic AMP) • Ca++ • Others – Tyrosine kinase – JAK Kinase – G proteins
• All activate multiple enzyme pathways that produce intracellular specific intracellular effects: – Secrete, contract, synthesize – All enzymes remain “on” until modulated to “off” by some controlling process • Results in multiple intracellular “effector reactions” • Intracellular cascades
Hormones • Endocrinology – Study of homeostatic activities accomplished by hormones
• Two distinct groups of hormones based on their solubility properties – Hydrophilic hormones • Highly water soluble • Low lipid solubility
– Lipophilic hormones • High lipid solubility • Poorly soluble in water
Table 4-3, p. 95
Fig. 4-19, p. 95
Testosterone, a masculinizing hormone
Estradiol, a feminizing hormone
Fig. 4-19, p. 95
Hormones • Hormones generally produce their effect by altering intracellular proteins • Hydrophilic hormones alter preexisting proteins via second‐messenger systems – cyclic adenosine monophosphate (cAMP) – Ca2+
• By stimulating genes, lipophilic hormones promote synthesis of new proteins
Extracellular (first) messenger (Activates) G protein intermediary
ECF Plasma Plasma membrane membrane
Adenylyl cyclase (effector protein) G-protein (Activates) coupled receptor Second messenger 1 Binding of 2 Adenylyl extracellular cyclase converts Messenger ATP to cAMP to receptor 3 cAMP activates Activates a protein kinase A. G protein, the α subunit of which Inactive Active shuttles to protein and activates protein kinase A kinase A adenylyl cyclase. 4 Protein kinase A phosphorylates inactive designated protein, activating it. Inactive designated protein
Key Phosphate
(changes shape and function)
5 Active designated protein brings about desired response.
ICF
Active designated protein
Cellular response
Fig. 4-20, p. 97
Blood vessel Plasma protein carrier Steroid hormone
ECF
Plasma Plasma membrane membrane Cytoplasm
Cellular response
1 Free lipophilic hormone
9 New protein brings about desired response.
diffuses though plasma membrane
Steroid hormone receptor
New protein
Portion that binds hormone
8 New protein is released from ribosome and processed into final folded form.
Portion that binds to DNA
2 Hormone binds with intracellular receptor specific for it.
7 Ribosomes “read” mRNA to synthesize new proteins.
DNA-binding site (active)
6 New mRNA leaves nucleus.
3 Hormone receptor complex binds with DNA’s hormone response element. mRNA
4 Binding activates gene.
5 Activated gene transcribes mRNA.
DNA
Nucleus
Hormone Gene response element
Fig. 4-21, p. 100
Comparison of the Nervous and Endocrine Systems • The nervous system transmits electrical impulses to skeletal muscles and the exocrine glands. • It is “wired”, sending electrical signals through distinct, highly organized pathways. These pathways have interconnected parts. • The endocrine system secretes hormones (chemical messengers) into the circulating blood to distant sites in the body. • These glands are not connected. They are scattered throughout the body • Each neuron has a close anatomic relationship to its target cells. It has a narrow range of influence.
Comparison of the Nervous and Endocrine Systems • A neuron releases a specific neurotransmitter to a specific target cell • The target cells have specific receptors that bind to the neurotransmitter secreted by a neuron. • Although that neuron can potentially signal other cells, it is limited to the target cells in close proximity to that neuron • A group of endocrine cells secretes a specific hormone into the blood. Although the hormone is circulated throughout the body, only specific target cells have receptors for a specific hormone.
Comparison of the Nervous and Endocrine Systems • A hormone cannot influence all body cells. It influences the target cells with receptor cells that bind to that hormone. • The nervous system coordinates rapid, precise responses • Its signal is an action potential. The duration of this signal is brief • The target cells are skeletal muscles and glands • The endocrine system controls activities of longer duration • This system requires a flow of blood to send a message • The effect of a hormone lasts longer
Comparison of the Nervous and Endocrine Systems • The nervous and endocrine systems are interconnected functionally. • Often they influence the same body process, such as the rate of heartbeat. • Neuroendocrinology is the study of the relationships between these two systems
Table 4-4, p. 101
