CNS in (unconscious) action: reflex arc Simple connection of neurons that results in a reflex action in response to a stimulus Very rapid Depend on only 3 neurons Not under voluntary control, happens before your brain “processed” what happened Usually response to hot/sharp objects 1

Neurons

Neuron nerve cell, basic structural and functional unit nucleus cell body axon conducts electrochemical signals to regulate body processes can be bundled into nerves 4

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surrounded by glial cells supporting cells (nourish, defend) often coated with an insulating layer: myelin sheath made of Schwann cells protective speeds signal transmission 8

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Neuron Function Sensory input collects and transmits input to CNS Integration integrate CNS and PNS Motor output - transmit information from CNS to target, ex. muscles, glands 11

Nerves are Electrical! membrane potential electrical charge separation across a cell membrane (potential energy) resting membrane potential results from difference in ion distribution inside and outside of cell (-70mV) measured in an unstimulated, polarized neuron (no impulse) 12

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Forces Behind Resting Potential 1. Selective Permeability -some molecules pass through membrane more freely than others, ex. ion channels for K+ 2. Sodium-Potassium Pump -transports 3 Na+ out of, 2 K+ into cell, using ATP - Excess positive charge outside cell 15

Concentration Gradient Electrical Gradient Result: Concentration Gradient Electrical Gradient 16

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The Action Potential how nerves send on their message change in charge that happens with nerve depolarization 21

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Molecular Basis of Action Potential An action potential is triggered when the threshold is reached. Voltage-gated sodium channels open (-50 mV) : influx of sodium into the cell Sodium channels close (+40 mV) and potassium channels open: K+ exits the cell Potassium channels close and resting potential is restored. 23

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http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf 26

refractory period membrane cannot go through another action potential Time needed for repolarization

the signal gets passed down the axon, in a single direction it initiates a threshold at the junction with the next cell 28

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Myelin In myelinated neurons action potentials only happen at the nodes of Ranvier Exposed areas on neuron with many voltage-gated sodium channels, able to depolarize and pass on signal Action potential “jumps” from node to node, causing voltage-gated Na+ channels to open 30

Figure 2.20  Saltatory conduction in a myelinated axon An action potential at the node triggers flow of current to the next node, where the membrane regenerates the action potential. 31

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Results in nerve impulse Passing on of signal down the neuron Much faster in myelinated neurons (120 m/s) than in non-myelinated (0.5 m/s) 33

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At the molecular level, sensory receptors located on the cell membrane of sensory neurons are responsible for the conversion of stimuli into electrical impulses. The type of receptor employed by a given sensory neuron determines the type of stimulus it will be sensitive to. For example, neurons containing mechanoreceptors are sensitive to tactile stimuli, while olfactory receptors make a cell sensitive to odors.[

The Synapse The synapse is the junction between neurons Synapses are found between sensory receptors and sensory neurons and between motor neurons and muscle cells The transmitting cell is called the presynaptic cell and the receiving cell is the postsynaptic cell There are two main types of synapses 1) electrical synapses 2) chemical synapses 36

a) Electrical Synapse Pre and post synaptic cells are connected by gap junctions that allow action potentials to pass directly from one cell to another 37

b) Chemical Synapse 38

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Transmission across a synapse The electrical signal is converted into a chemical signal, and then back to an electrical signal in the post cell Within the cytoplasm of the pre cell there are numerous sacs filled with neurotransmitters, the messenger molecules that will be released into the synaptic cleft The neurotransmitters will cause a depolarization of the post-synaptic neuron (action potential) Neurotrasmitters in cleft are degraded by enzymes or taken up by neurons for re-use 40

Chemical Synapse 41

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Neurotransmitters 1) Dopamine and serotonin (amines) Technically the only two things you enjoy! Mood regulators, linked to sensations of pleasure Dopamine has been linked to schizophrenia and Parkinson’s disease. Not enough serotonin is linked to depression Serotonin and LSD (hallucinogen) have similar structures 43

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Endorphins (peptides, endogenous morphines) Natural painkillers, affect emotions Released during exercise: “runner’s high” Similar to opiates, morphine and heroin bind to the same receptors 45

Norepinephrine Complements the actions of ephinephrine, readies he body to respond to danger/stress 46

OK, but why do some things feel kind of good and others REALLY good? 47

How then is stimulus intensity coded? The All-or-None Law The size, amplitude, and velocity of an action potential are independent of the intensity of the stimulus that initiated it. How then is stimulus intensity coded? 48

Summation The figure below shows a typical neural pathway When neuron A and Neuron B fire together an action potential is triggered in neuron D Note that neuron C is inhibitory 49

Temporal and Spatial Summation 50

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Peripheral Nervous System 52

controls voluntary movement of skeletal muscles myelinated divided into two parts Somatic System controls voluntary movement of skeletal muscles myelinated 53

2. Autonomic System controls involuntary glandular secretions and functions of smooth and cardiac muscle divided into: 54

i. sympathetic nervous system activates “fight or flight” response norepinephrine (neurotransmitter) works with epinephrine (hormone) to activate stress response blood pressure increases (vasoconstriction), heart beats faster, digestion slows down, etc. released by modern stressors: anxiety 55

ii. parasympathetic nervous system involuntary control in opposition to the sympathetic nervous system activated when the body is calm and at rest (ex. meditation) the “break” to sympathetic NS’s “gas pedal” 56

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HOMEOSTASIS!!!! 58