Four main tissue types
Muscle, nervous, epithelial, connective
Intracellular fluid
The fluid inside cells
Extracellular fluid
Fluid present in the blood and spaces surrounding the cell
Plasma
Fluid in the blood (20-25% of all ECF) in which various blood cells are suspended
Interstitial fluid
Fluid which lies around and between cells (75-80% of all ECF)
Homeostasis
A state of dynamic constancy
Steady state
A system in which a particular variable (temperature) is not changing, but in which energy (heat) must be added to maintain a stable, homeostatic condition
Equilibrium
A particular variable is not changing, and no input of energy is required to maintain the constancy
Set point
Ex) the steady-state temperature is known as the set point of the thermoregulatory system
Set points can be physiologically reset to a new value (ex. fever)
Negative feedback loops
A system in which an increase or decrease in the variable being regulated brings about responses that tend to move the variable in the direction opposite to the direction of the original change (very important homeostatic mechanism)
Positive feedback loop
Mechanism accelerates a process; has no obvious means of stopping; to as common as negative feedback
Ex) clotting cascade
Leak channels
Ungated ion channels; always open; allow ions to flow down their concentration gradient (unregulated diffusion)
Cell membrane permeability to different ions
In neurons, the K+ permeability may be as much as 100 times greater than that for Na+ and Cl-, so neuronal resting membrane potentials are typically fairly close to the equilibrium potential for K+
Resting membrane potential
The energy stored in the charge separation across the plasma membrane; separation of charge between IC and EC; determined by the nature of the electrochemical gradient
this separation creates an electrical potential/gradient
Chemical gradient
The differences in concentration created by a membrane potential
Excitability
The ability of a neuron or muscle cell to change their membrane potential by allowing charge to flow across the plasma membrane
What stimulates chemically (ligand) gated channels?
Often stimulated by chemical messengers or phosphorylation
Ex) post-synaptic receptors that are stimulated by the NTs released into the synapse y the pre-synaptic neuron
What determines the magnitude of the resting membrane potential?
- The differences in specific ion concentration in IC and EC fluids
- The differences in membrane permeabilities to the different ions (which reflect the number of open ion channels (leak or open gated channels) for the different ions in the plasma membrane
How are mechanically gated channels stimulated?
By the stretch or deformation of the cell membrane
Ex) sensory receptors in skin; tendon reflexes
How are voltage gated channels stimulated?
They are stimulated by a change in membrane potential
Ex) the voltage gated Na+ channels that are involved in propagating action potentials in neurons
Graded potentials
Small changes in the membrane potential (don’t reach threshold) that are confined to a relatively small region of the plasma membrane; multiple graded potentials can be summed; the change in membrane potential dissipates over time/distance
Afferent neurons
Sensory; transmit information into the CNS from receptors at their peripheral endings
Efferent neurons
Motor; transmit information out of the CNS to effector cells
Interneurons
Neurons that lie entirely within the CNS and form circuits with other interneurons or connect afferent and efferent neurons
