 Nervous system helps coordinate body functions to maintain homeostasis  Enables body to respond to changing conditions  Nerve cells are called neurons- specialized to react to physical and chemical changes in surroundings  Nerve impulses are electrochemical changes that transmit info between neurons

 Neurons consist of: -Cell body- rounded area with nucleus -Dendrites- extensions that receive electrochemical messages -Axons- extensions that send info in nerve impulses - axon hillock- conical region of axon where it joins the cell body - Schwann cell- a glial or supporting cell that forms myelin sheaths around the axons of neurons in the PNS - myelin sheath- fatty lipoprotein protection of axon that provides electrical insulation - neurons with myelin sheath= white matter - neurons without myelin= gray matter - nodes of Ranvier- gaps between adjacent Schwann cells

 Organs can be split into two groups: - Central nervous system (CNS)- brain and spinal cord -Peripheral nervous system (PNS)- nerves that connect the CNS to other body parts  Together they provide 3 general functions: sensory, integrative, motor

 Sensory neurons: -Carry nerve impulses from PNS to CNS -Either receptor ends at tips of dendrites or dendrites closely associated with receptor cells in skin or sensory organs -Changes inside or outside body stimulate receptors; trigger sensory nerve impulses - impulses travel along axons, leading to one of CNS organs; other neurons process the impulses - internal and external environment ex) light, sound intensity, temperature, oxygen levels For example, this man’s sensory neurons allow him to feel the heat of the stove

 Interneurons - lie in the brain and spinal cord - link other neurons - transmit impulses from one part of brain/spinal cord to another; direct incoming sensory impulses  Motor Neurons - carry nerve impulses out of CNS to effectors - effectors- responsive structures that include muscles that contract and glands that secrete - motor impulses stimulate the effectors This man touched the hot stove. His sensory neurons sent an impulse to the CNS, which sent a responding impulse to his motor neurons and effectors, in this case his muscles pulling his hand back.

 Resting potential - it is the potential difference (or difference in electrical charge) between region inside membrane and region outside membrane in neurons not transmitting signals - depends on the ionic gradients; net movement of K+ and Na+ ions from higher to lower concentration - in a resting cell membrane K+ ions diffuse out of the cell more rapidly than Na+ ions can diffuse in ; outside gains “+” surplus and inside gains “-” surplus - meanwhile, Na+/K+ pumps actively transport ions in opposite directions to maintain this concentration gradient

 Potential changes: - nerve cells are excitable; respond to changes in environment - changes affect resting potential in a particular region of nerve cell membrane - depolarization- membrane’s resting potential decreases (inside less negative than outside); opening of gated Na+ channels and K+ channels remain closed - Changes in resting potential are graded: change in potential is directly proportional to the intensity of the stimulations -threshold potential- summation of depolarizations to a certain membrane voltage, after which an action potential occurs…

 Action Potential: - rapid sequence of depolarization and repolarization - at threshold potential, channels in membrane open and Na+ ions begin to diffuse freely inward and membrane loses its negative charge; becomes depolarized - then, the Na+ channels close and channels open that allow K+ ions to pass out, and inside becomes negatively charged again; it is repolarized and membrane returns to resting potential - this process forms the nerve impulse that is propagated along the axon Action potential animation

 Nerve impulse- movement of action potentials along a nerve axon  Local current: when action potential occurs in one region of nerve cell membrane, causes bioelectric current to flow to adjacent portions;  stimulates to threshold level and trigger another action potential= chain reaction!  Wave of action potentials moves down the axon

 An unmyelinated axon conducts an impulse over its entire surface  Myelinated axon is different b/c myelin insulates and prevents almost all ion flow through the membrane it encloses  Therefore it is not continuous; nodes interrupt the sheath and action potentials occur at these nodes; so impulse appears to jump from node to node  Termed saltatory and is much faster than unmyelinated  Speed of nerve impulse conduction is proportional to diameter of axon; greater diameter=faster impulse

 Nerve impulse conduction is an “all-or-none-response”  If neuron responds at all, it responds completely  A greater intensity of stimulation does not produce a stronger impulse, but rather more impulses per second  Refractory period- short time following a nerve impulse when a threshold stimulus will not trigger another impulse on an axon; limits the frequency of impulses  Calcium’s role:  - too little Ca + = spasms  - too much Ca+ = neuron is hard to fire, impulse hard to trigger

 How the impulse passes from neuron to neuron  Synapse- junction between any two communicating neurons; not in direct physical contact (axon to dendrite, or axon to cell body) - electrical synapse- gap junctions that allow current to flow directly from cell to cell; synchronize activity of neurons for certain rapid, stereotypical behaviors - chemical synapse- majority are this; release of chemical neurotransmitters  Synaptic cleft- gap between neurons; communication along a nerve pathway must cross these gaps  Presynaptic neuron- the sender; the neuron carrying the impulse into the synapse  Postsynaptic neuron- the receiver; receives the impulse  Synaptic transmission- process of crossing the synaptic gap  Neurotransmitters- biochemicals that carry out synaptic transmission; it diffuses across the synaptic cleft and reacts with specific receptors on the postsynaptic neuron membrane

Synaptic knob- extension from axon Synaptic vesicle- membranous sac that releases neurotransmitters postsynaptic

 When an action potential reaches a synaptic knob (terminal), it depolarizes the membrane, opening voltage-gated calcium channels.  Calcium ions (Ca 2+ ) then diffuse into the knob, and the increase in ion concentration causes some of the synaptic vesicles to fuse with the membrane  The vesicles then release their neurotransmitters, which diffuse across the synaptic cleft; subsequent affect on postsynaptic neuron may be:  Direct synaptic transmission- neurotransmitters bind to ligand- gated ion channels in membrane of postsynaptic cell; result is change in membrane potential  Indirect synaptic transmission- neurotransmitter binds to a receptor that is not part of an ion channel; activates a signal transduction pathway; slower but effects last longer

 Excitatory neurotransmitters- increase postsynaptic membrane permeability to sodium ions which will bring it closer to the threshold and may trigger nerve impulses  Inhibitory neurotransmitters- decrease the potential that threshold will be reached; less chance that nerve impulse will occur  Synaptic knobs of 1000 or more neurons could potentially interact with one postsynaptic neuron; some knobs will release excitatory, some inhibitory; effect on the neuron depends on the ratio of each  synapse animation synapse animation

Type/NameFunctional ClassSecretion Sites AcetylcholineExcitatory to vertebrate skeletal muscles; either at other sites CNS; PNS; vertebrate neuromuscular junction Biogenic Amines: Dopamine Serotonin Generally excitatory Released in brain and affects sleep, mood, attention, learning Amino Acids GABA Glutamate Inhibitory excitatory At most inhibitory synapses in brain CNS Neuropeptides Substance P Met-enkephalin (an endorphin) Excitatory Generally inhibitory CNS; PNS; mediates our perception of pain Decreases pain perception