Electrical Signals, Sensory Systems, and Movement

The Vertebrate Nervous System

Brain structure Cerebrum = involved in conscious thought and memory; cerebellum = coordinates complex motor patterns; brain stem = autonomic center for regulating heat, lungs and digestive system; hypothalamus = homeostasis!; corpus collosum = connects to hemispheres

The Neuron 1) Cell body, which contains the nucleus 2) highlight branched group of relatively short projections called dendrites; 3) a long projection called an axon, which may or may not branch; dendrites collect signals  the cell body integrates incoming signals and generates outgoing signal to axon  the axon passes electrical signals to dendrites of another cell or to an effector cell

Cell Membrane Potential Membrane Potentials K+ Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ K+ Na+ Na+ Cell Membrane Potential -70 to -80mV A difference in electrical charge between any two points creates an electrical potential; when separated across a membrane = membrane potential; membrane potentials measured in mV (millivolts); also incorporates energy stored in concenation gradient of charged ions on either side of the membrane  electrochemical gradient K+ K+ K+ K+ Na+ Na+ K+

Resting Potential Na+ / K+ - ATPase ensures that eventually the concentration of K+ is higher inside the cell the cell than outside the cell, whilc the concentration of Na+ is lower inside than outside; also established an electrical gradient  interior of membrane is more negative than exterior K+ leak channel – let K+ leak out of cell  making inside of cell more negatively charged  build-up of negative charge attracts K+

Action Potential = rapid, temporary change in mb potential Depolarization  membrane becomes less polarized (less charge difference) 2) repolarization  changes mb potential from positive back to negative 3) hyperpolarization phase in which the membrane is slightly more negative than the resting potential;for an action potential to begin the membrane must depolarize to a threshold potential  then action potential will “fire”; action potential is all or none! Once action potential “fires” it is propagated down the length of the axon  what is going on during the three phases and what causes the action potential to fire? Hyperpolarization period  refractory period

Dissecting Action Potentials Depolarization = strong inward flow of sodium ions; Polarization = strong outward flow of potassium ions Opening of Na+ channels is example of positive feedback! Controlled by voltage-gated channels  sodium channels open quickly after depolarization  potassium channels open with a delay after depolarization  continue to flip open and closed until the membrane repolarizes

Na+ voltage gated channel Exterior of Axon Na+ voltage gated channel Nat+ / K+ ATPase Pump K+ voltage gated channel 1) Action potential arrives from further up axon (this is the form of sodium ions traveling on inside of axon 2) arrival of sodium ions begins depolarization  3) in response to threshold level of depolarization, Na+ voltage gated channels open further depolarizing membrane 4) in response to depolarization, K+ voltage gated channels open helping to repolarize cell Interior of Axon

Dissecting Action Potentials Oligodendrocytes (CNS) glia (PNS) Why doesn’t action potential propagate back up the axon? Sodium channels are refractory  once they have opened and closed they are less likely to open again for a short period

Multiple Sclerosis (MS)

Where do action potentials originate? The Synapse! 1. Action potential arrives near synaptic cleft 2. voltage gated Ca2+ channels open; Ca enters presynaptic cell 3. synaptic vesicles fuse with presynaptic mb then release neurotransmitter 4. neurotransmitter acts as ligand  binds to ion channels in postsynaptic membrane, which open with neurotransmitter binds  cause change in postsynaptic cell potential 5. ion channels close when neurotransmitter is degraded

Post synaptic potentials are summed in axon hillock  if depolarizes past threshold potential  action potential will fire! Once it fires, it is propagated down the axon!

How do muscles contract? The Neuromuscular Junction 1) Action potentials trigger the release of the neurotransmitter acetylcholine (Ach) from the motor neuron into the synjaptic cleft between the motor neuron and the muscle cell 2) Ach diffueses across the synaptic cleft and binds to Ach receptors on the plasma mb of the muscle cell. Membrane depolarization occurs in response to Ach release 3) Action potentials propogate along length of muscle fiber and into interior of fiber via T tubules 4.