CNS in (unconscious) action: CNS in (unconscious) action: reflex reflex arcarc Simple connection of neurons that results Simple connection of neurons that results

in a reflex action in response to a in a reflex action in response to a stimulusstimulus Very rapidVery rapid Depend on only 3 neuronsDepend on only 3 neurons Not under voluntary control, happens before Not under voluntary control, happens before

your brain “processed” what happenedyour brain “processed” what happened Usually response to hot/sharp objectsUsually response to hot/sharp objects

NeuronsNeurons

NeuronNeuron nerve nerve cellcell, basic structural and functional unit, basic structural and functional unit

nucleusnucleus cell bodycell body axonaxon

conducts conducts electrochemicalelectrochemical signals to regulate signals to regulate body processes body processes

can be can be bundledbundled into into nervesnerves

surrounded by surrounded by glial cellsglial cells supporting cells (nourish, defend)supporting cells (nourish, defend)

often coated with an insulating layer: often coated with an insulating layer: myelin sheathmyelin sheath made of Schwann cellsmade of Schwann cells protectiveprotective speeds signal transmissionspeeds signal transmission

Neuron FunctionNeuron Function

1.1. SensorySensory input input- collects and transmits input to collects and transmits input to CNSCNS

2.2. IntegrationIntegration- integrate integrate CNSCNS and and PNSPNS

3.3. MotorMotor output output- transmit information from CNS to target, - transmit information from CNS to target,

ex. muscles, glandsex. muscles, glands

Nerves are Electrical!Nerves are Electrical!

membrane potentialmembrane potential- electrical charge separation across a cell electrical charge separation across a cell

membrane (potential energy)membrane (potential energy)

restingresting membrane potential membrane potential- results from difference in ion distribution results from difference in ion distribution

inside and outside of cell (-70mV)inside and outside of cell (-70mV)- measured in an unstimulated, measured in an unstimulated, polarizedpolarized

neuron (no impulse)neuron (no impulse)

Forces Behind Resting PotentialForces Behind Resting Potential

1. 1. Selective PermeabilitySelective Permeability-some molecules pass through membrane more -some molecules pass through membrane more freely than others, ex. ion channels for K+freely than others, ex. ion channels for K+

2. 2. Sodium-PotassiumSodium-Potassium Pump Pump-transports 3 Na-transports 3 Na++ out of, 2 K out of, 2 K++ into cell, using ATP into cell, using ATP

- Excess positive charge outside cell- Excess positive charge outside cell

Result:Result:Concentration GradientConcentration GradientElectrical GradientElectrical Gradient

The Action PotentialThe Action Potential how nerves send on their messagehow nerves send on their message change in charge that happens with change in charge that happens with

nerve nerve depolarizationdepolarization

Molecular Basis of Action Molecular Basis of Action PotentialPotential

1.1. An action potential is triggered when the An action potential is triggered when the thresholdthreshold is reached. is reached.

2.2. Voltage-gated sodiumVoltage-gated sodium channels open (-50 channels open (-50 mV) : influx of sodium into the cellmV) : influx of sodium into the cell

3.3. Sodium channels Sodium channels closeclose (+40 mV) and (+40 mV) and potassium channels openpotassium channels open: K+ exits the : K+ exits the cellcell

4.4. Potassium channels close and Potassium channels close and resting resting potentialpotential is restored. is restored.

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

refractory periodrefractory period membrane cannot go through another membrane cannot go through another

action potentialaction potential Time needed for Time needed for repolarizationrepolarization

the signal gets passed down the the signal gets passed down the axonaxon, in a single direction, in a single direction

it initiates a threshold at the junction it initiates a threshold at the junction with the next cellwith the next cell

MyelinMyelin

In myelinated neurons action In myelinated neurons action potentials only happen at the potentials only happen at the nodes nodes of Ranvierof Ranvier Exposed areas on neuron with many Exposed areas on neuron with many

voltage-gated sodium channels, able to voltage-gated sodium channels, able to depolarize and pass on signaldepolarize and pass on signal

Action potential “jumps” from node to Action potential “jumps” from node to node, causing voltage-gated Na+ node, causing voltage-gated Na+ channels to openchannels to open

Figure 2.20  Saltatory conduction in a myelinated Figure 2.20  Saltatory conduction in a myelinated axonaxon

An action potential at the node triggers flow of current to An action potential at the node triggers flow of current to the next the next

node, where the membrane regenerates the action node, where the membrane regenerates the action potential.potential.

Results in Results in nerve impulsenerve impulse Passing on of signal down the neuronPassing on of signal down the neuron Much faster in myelinated neurons (120 Much faster in myelinated neurons (120

m/s) than in non-myelinated (0.5 m/s)m/s) than in non-myelinated (0.5 m/s)

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

The SynapseThe Synapse The synapse is the junction between neuronsThe synapse is the junction between neurons

Synapses are found between sensory receptors Synapses are found between sensory receptors and sensory neurons and between motor neurons and sensory neurons and between motor neurons and muscle cellsand muscle cells

The transmitting cell is called the presynaptic cell The transmitting cell is called the presynaptic cell and the receiving cell is the postsynaptic celland the receiving cell is the postsynaptic cell

There are two main types of synapsesThere are two main types of synapses

1) electrical synapses1) electrical synapses

2) chemical synapses2) chemical synapses

a) Electrical Synapsea) Electrical Synapse Pre and post synaptic cells are connected by gap Pre and post synaptic cells are connected by gap

junctions that allow action potentials to pass junctions that allow action potentials to pass directly from one cell to anotherdirectly from one cell to another

b) Chemical Synapseb) Chemical Synapse

Transmission across a synapseTransmission across a synapse

The electrical signal is converted into a chemical The electrical signal is converted into a chemical signal, and then back to an electrical signal in the signal, and then back to an electrical signal in the post cellpost cell

Within the cytoplasm of the pre cell there are numerous Within the cytoplasm of the pre cell there are numerous sacs filled with neurotransmitters, the messenger sacs filled with neurotransmitters, the messenger molecules that will be released into the synaptic cleftmolecules that will be released into the synaptic cleft

The neurotransmitters will cause a depolarization of the The neurotransmitters will cause a depolarization of the post-synaptic neuron (action potential)post-synaptic neuron (action potential)

Neurotrasmitters in cleft are degraded by enzymes or Neurotrasmitters in cleft are degraded by enzymes or taken up by neurons for re-usetaken up by neurons for re-use

Chemical SynapseChemical Synapse

NeurotransmittersNeurotransmitters

1) Dopamine and serotonin (amines)1) Dopamine and serotonin (amines)

Technically the only two things you enjoy!Technically the only two things you enjoy!

Mood regulators, linked to sensations of pleasureMood regulators, linked to sensations of pleasure

Dopamine has been linked to schizophrenia and Dopamine has been linked to schizophrenia and Parkinson’s disease.Parkinson’s disease.

Not enough serotonin is linked to depressionNot enough serotonin is linked to depression

Serotonin and LSD (hallucinogen) have similar structuresSerotonin and LSD (hallucinogen) have similar structures

2)2) Endorphins (peptides, endogenous morphines)Endorphins (peptides, endogenous morphines)

Natural painkillers, affect emotionsNatural painkillers, affect emotions

Released during exercise: “runner’s high”Released during exercise: “runner’s high”

Similar to opiates, morphine and heroin bind Similar to opiates, morphine and heroin bind to the same receptorsto the same receptors

3)3) NorepinephrineNorepinephrine Complements the actions of Complements the actions of

ephinephrine, readies he body to ephinephrine, readies he body to respond to danger/stressrespond to danger/stress

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

The All-or-None LawThe All-or-None Law

The size, amplitude, and velocity of an action The size, amplitude, and velocity of an action potential are independent of the intensity of potential are independent of the intensity of the stimulus that initiated it.the stimulus that initiated it.

How then is stimulus intensity coded?How then is stimulus intensity coded?

SummationSummation The figure below shows a typical neural pathwayThe figure below shows a typical neural pathway When neuron A and Neuron B fire together an action When neuron A and Neuron B fire together an action

potential is triggered in neuron Dpotential is triggered in neuron D Note that neuron C is inhibitoryNote that neuron C is inhibitory

Temporal and Spatial Temporal and Spatial SummationSummation

Peripheral Nervous Peripheral Nervous SystemSystem

divided into two partsdivided into two parts

1.1. Somatic SystemSomatic System controls voluntary movement of controls voluntary movement of

skeletal musclesskeletal muscles myelinatedmyelinated

2. Autonomic System2. Autonomic System

controls involuntary glandular controls involuntary glandular secretions and functions of smooth secretions and functions of smooth and cardiac muscleand cardiac muscle

divided into:divided into:

i. sympathetic nervous i. sympathetic nervous systemsystem

activates “fight or flight” responseactivates “fight or flight” response

norepinephrine (neurotransmitter) works norepinephrine (neurotransmitter) works with epinephrine (hormone) to activate with epinephrine (hormone) to activate stress responsestress response blood pressure increases (vasoconstriction), blood pressure increases (vasoconstriction),

heart beats faster, digestion slows down, etc.heart beats faster, digestion slows down, etc.

released by modern stressors: anxietyreleased by modern stressors: anxiety

ii. parasympathetic nervous ii. parasympathetic nervous systemsystem

involuntary control in opposition to the involuntary control in opposition to the sympathetic nervous systemsympathetic nervous system

activated when the body is calm and activated when the body is calm and at rest (ex. meditation)at rest (ex. meditation)

the “break” to sympathetic NS’s “gas the “break” to sympathetic NS’s “gas pedal”pedal”

HOMEOSTASIS!!!!HOMEOSTASIS!!!!