• Proteins, lipis, or even gases secreted by cells that prompt an effect in neighboring or distant cells
• molecules that serve bodily communication
• all share the same travel itinerary-they are released from a secreting cell, trabel through a fluid to a target cell, and affect the activity of the target cell by binding to a specific receptor. 
• also known as ligands

• result in disease

• Decoders of chemical signals
• proteins that change the activity of the cell when bound by the chemical signal
• chemical signals exert an effect only on cells that contain the correct __________

• travel through the bloodstream
• act on distant cells
• Ex. Testosterone- travels through blood to muscle cells, where it binds to receptors and stimulates the growth of muscle cells
• hormones released by nerve cells=neurohormones

• Chemical signlas released by body cells
• act on nearby cells
• reach target by diffusion through extracellular fluid
• ex. Hay fever- release of histamines from white blood cells in response to pollens. Histamines signal nearby blood vessels to enlarge (red eyes) or nearby epithelial cells to secrete mucus (runny nose)

Neurotransmitters

• class of paracrine factors
• released by neurons 
• travel very short distance across a narrow intercellular space to another neuron or an effector cell to induce an electrical signal
• ex. acetylcholine travels from a nerve to a muscle cell to induce an electrical signal that stimulates contraction

• bind to intracellular receptors and stimulate the synthesis of specific proteins
• slow process
• most common= steroid hormones
• ex. testosterone- stimulates production of proteins that stimulate sperm production and muscle development

1. crosses cell membrane by simple diffusion
2. binds to a receptor in the cytosol; the receptor changes shape
3. enters the nucleus through nuclear pore
4. bings to regulatory region of a particular gene
5. transcribed in mRNA
6. translated into a protein which exerts its effect within the cell or by traveling to other cells

• cannot cross lipid layer
• must bind with protein receptors in the cell membrane which span the full thickness of the membrane
• the ligand binding site is external and exposed to the extracellular fluis
• internal portion is exposed to the cytosol
• very rapid

• open to allow ions to cross the membrane
• binding of the ligand opens or closes the channel
• receptors are large proteins that contain two parts- one that binds the ligand, and one that constitutes an ion channel that spans the cell membrane from outside to inside
• ions cause a cellular response
• frequently convert a chemical signal into an electrical signal

Enzyme-linked receptors

• binding cite is exposed to the extracellular fluid
• enzyme portion is exposed to cytosol
• ligand binding activates the intracelluar enzyme, which activates another enzyme, and another, and so on. 
• the activated enzyme induces a functional change in the cell

• class of membrane receptors that utilize second messengers to propogate intracellularly an extracellular signal
• ligand binds with GPCR which activates G protein
• G protein regulates the production of specific second messenger (usually cAMP)

1. glucagon travels through bloodstream to liver
2. glucagon bings to its receptor
3. GPCR activates G protein
4. G protein prompts production of 2nd messenger
5. cAMP activated enzymes
6. enzyme increases glucose production by the liver

• the signal is amplified because one hormone (signal) can produce many second messengers
• response turns off easily

• mimics effect 
• exogenous agonists bind in the absense of the endogenous ligand
• ex. asthma- albuterol causes air ways to widen

• block the binding site, preventing endogenous ligand from binding
• can also bind to a related receptor and decrease the effect of the endogenous ligand
• THEY DO NOT TURN OFF THE RECEPTOR
• prevent receptor from turning on or limit its effect
• ex. breast cancer- tamoxifen- prevents estrogen actions

• binds to brain cells and induces dreamy euphoria
• also binds to other cells that supress breathing
• reduces number of receptors
• addicts become drug tolerant (cuz of the reduced receptor number)
• in order to receive same effect, must take doses that depress breathing
• too much drug= respirations become incapable of sustaining life
• also, when relapsing, they think a big dose is needed, but the recovered receptors are numerous and sensitive, causing them to take too much and dream into respiratory paralysis and death

• selective serotonin reuptake inhibitor
• antidepressant
• make you fire more
• serotonin gives you EPSP (stimulatory)
• it enhances effect of serotonin-making it also EPSP because it blocks the reuptake of it, making the serotonin bind to the receptors more
• depressed=serotonin deprived

• both graded potentials
• EPSP=depolarizing= more likely to fire
• IPSP= hyperpolarizing= less likely to fire
• happen on the postsynaptic cell

Action potentials

• resting potential=-70mv
• Threshold potential= -55mv
• Graded potential= sodium channels opening (EPSP)- voltage closed, timing open
• At threshold potential, action potential start, voltage gated sodium channels open, sodium goes,  in to the cell= depolarization to 30mv (past zero), timing still open
• Potassium channels open and potassium starts to leave the cell (voltage gated), higher you go up the curve- the more potassium that leaves
• Sodium channels close because of a timing mechanism, timing closes, voltage still opens
• hyperpolarization= potassium leaves the cell
• threshold- potassium channels start to close
• overshoots= because potassium closes sluggishly (makes retriggering harder to fire again, limits how fast you can make it fire)

Cells divide

form desmosomes

fill with keratin

die

cells divide 

form desmosomes

fill with keratin

die

• dendritic= fight infections
• melanocytes= produce melanin that protects skin from burning
• keratinocytes= all the other ones (stratum basale, stratum spinosum, stratum granulosum, stratum corneum)

• Fibroblasts= make collagen
• White blood cells= fight infection

• Adipocyte= insulation, protection, nutrition and energy to an extent

regular skin cells

make keratin (waterproof layer)

Components of a joint

• bone
• cartilage- cushion, shock absorber
• synovial fluid
• ligaments (extracapsular, 
• bursae (sacs, contain synovial fluid)

• endochondral= cartilage
• entromembranous= membrane turning into bone, seen in flat bones in the skull
• babies fontanel, the bones are not closed yet
• chondrocytes divide, swell die, osteoclasts break up classification, osteoblasts fill the holes with new bone

• Ca2+
• sensor, processor= parathyroid
• low- parathyroid gland releases PTH: increases calcium levels, decrease osteoblasts, increases o'clasts (more bone break down, less bone formation), stimulates uptake of vitamin D, excretion of Vitamin D decreases
• limits release of calcium
• OKAY= balance between calcium uptake and excretion

If calcium levels are too high

• Calcitonin
• increases o'blasts
• decreases calcium
• stimulates done deposition
• antagonist hormone

• epiphyseal plate= cartilage
• chondroctyes= dividing and secreting cartilage which pushes the epiphysis away, and deposits cartilage
• chondrocytes swell, calcify, and die 
• osteoclasts eat up the dead chondrocytes and leave holes
• osteoblasts come and fill that hole with bone

Physical Stress

(excercise)

• bones get bigger
• increases o'blasts
• bones become stronger

• layers= lamellae
• in between layers= holes called lacunae
• osteocyte inside the lacunae, they stay alive because of the canalaliculi (which contain gap junctions) that connect them to other osteocytes, which allow nutrients to pass from osteocyte to osteocyte

• no central canal
• has blood vessels and marrow for nutrients

• fibroblasts- create less collagen, skin becomes weaker
• white blood cells- cant fight infection as well
• sweat glands- hard time maintaining correct temp.
• sebaceous glands- skin becomes dry and not hydrated as well causing it to crack and break more easy

• loss of adipose= skin is not as insulated=temp control, less cushion= weaker

• oil secretion
• keeps skin soft= protection

• only in armpit and genitals
• scent gland

• secrete water in the form of sweat

• desmosomes die and peel off

1. AP arrives at axon
2. AP triggers exocytosis; neurotransmitter is released
3. neurotransmitter binds to receptor
4. receptor induces signal which travels away from synapse
5. GABA is transported back into the axon