The Neuron What do we know about Nerves? - What are they? - What do they do? - Speed?

What Is a Neuron? A cell that carries messages throughout nervous system.

Types of Neurons SensorySensory: from receptors (ie. Eyes) to CNS. MotorMotor: from CNS to effectors (ie. Muscle, organs). InterneuronInterneuron: from sensory to motor neurons Reflex –A short cut when a much quicker response is required – called a Reflex –Example: Burn finger, move it quicker than you would if you had to think about it. –Note: Brain is still notified.

Reflexes

Physics Stuff You Need to Know Voltage is the tendency of charges to move between two points. This flow can do work. Charges in bio are carried by elemental ions (Na +, K + ) or proteins & DNA. Positive ions are attracted to negative ions

The Neuron’s Membrane Neuron membrane contains: –Na-K pump - uses ATP to move 3 Na out while bringing 2 K into the cell. Keeps K concentration higher inside the cell, and Na concentration higher outside the cell –Ion channels: Specific to ion Can be open or gated (meaning it’s usually shut, but can open under specific conditions)

The Neuron at Rest Contain lots of positive & negative ions Anions are large, can’t leave cell Open channels for K, which means it can leave the cell at any time –BUT! K is attracted to anions in cell, so not all of it will leave to follow concentration gradient RESTING POTENTIALOverall there are more + outside the cell, resulting in a voltage of -70mV “RESTING POTENTIAL” –When inside becomes more positive, V decreases or becomes positive

Action Potential: The Message DEPOLARIZATIONStimulus occurs, Na + gated-channels open briefly, Na + enters cell (follows concentration gradient), V becomes positive (+40mV): “DEPOLARIZATION”. REPOLARIZATIONPositive interior closes Na + gates, and opens K+ gates, K+ moves out, lowering V: “REPOLARIZATION”. REFRACTORY PERIODThen, Sodium-Potassium pump fully restores resting potential by using energy to push 3 Na+ out and pulling 2 K+ in (per ATP): “REFRACTORY PERIOD” –No other action potential can occur during this time. 2

Action Potential: Movement Why does the action potential move along the axon? –Depolarization in one part causes the gates of neighbouring Na+ channels to open because of the spread of electric current. –The membrane on one side of action potential is still in the refractory period, so wave of depolarization can only occur in one direction.

Another Look At the Axon myelinIn CNS, some are covered by myelin (white colour) – no myelin: grey colour –Speeds up message from 2 m/s (unmyelinated axon) to 120 m/s (myelinated axon)! –Acts as insulation –myelin prevents loss of ions, so ions can skip from Node to Node (of Ranvier) causing depolarization in subsequent nodes. neurilemmaIN PNS, most are covered by myelin & neurilemma –Allows some regeneration of neurons Graft PNS neurons to CNS injuries???

All-or-None Principle Axon cannot send action potentials of different intensities –Threshold must be met (ie. There must be enough stimulus) for action potential to occur –Even if threshold is doubled, action potential doesn’t move faster, or cause more depolarization. Ie. Action potential always depolarizes membrane to 40mV. Brain can tell differences in stimulus based on the number and frequency of neurons firing.

Neuron-to-Neuron or Neuron-to-Muscle Messages Neurons never touch each other or muscles. Where 2 or more neurons meet, there is a space called a synapse. Action potential must cross this gap.

Crossing the Synapse neurotransmittersAxon terminal contains vesicles filled with “neurotransmitters” When action potential reaches terminal, vesicles fuse with membrane & release neurotransmitters into synapse. Neurotransmitters diffuse across & set up action potential in next neuron – called the post-synaptic neuron. –Neurotransmitters quickly broken down by enzymes so that refractory period can occur. Examples of neurotransmitters: Acetylcholine, Adrenaline, Noradrenaline, Dopamine, Serotonin

Types of Post-Synaptic Receptors ExcitatoryExcitatory: Messages continue in post- synaptic nerve (Receptors force Na + gates to open). InhibitoryInhibitory: Messages stops at synapse (Receptors open K + gates.) Mouse party