Neuropsychology and Reflexes

about how many neurons are in the brain

about 86 billion, each one connecting to about 10,000 others to create a dense, interconnected network

soma (cell body)

contains nucleus

dendrites

receive input from the presynaptic neuron

axon

• sends the signal away from the soma to the next neuron

• AP is sent down this

myelin sheath

fatty material that insulates the axon for information to pass faster and efficiently

Nodes of Ranvier

• breaks in the myelin sheath

• exchange of ions along the axon can happen here

axon terminals

where the neuron connects to (with the synapse in between) to another neuron to send the AP

synapse

• gap between the first and second neuron

• AP must jump across this to pass along

multiple sclerosis (MS)

• autoimmune disease where the immune system treats myelin sheaths as foreign substances and attacks them

• tends to progress, can be incapacitating

• causes demyelination → not many AP correctly fire, signals have a harder time traveling or happening at all

• symptoms vary depending on affected regions (difficulty walking, moving, seeing etc.)

glial cells

• support neurons structurally and nutritionally

• about as many as neurons

• specific functions

  • insulating axons by forming myelin sheaths

  • providing nutrients and oxygen

  • cleaning wastes

  • possibly involved in electrical signal transmission

neuron resting potential

• about -70 mV (inside negativity charged relative to outside)

• maintained by ion concentrations

  • Na: outside, positive

  • Cl: inside, negative

ion channels

• open in response to stimulation (gating)

• allows Na to rush in and reduce the negative charge

What happens when the voltage potential reaches -55 mV

• it hits the threshold, rapid depolarization occurs

• Na influx causes internal voltage to increase to +40 mV (peak AP)

action potential

• voltage change from -70 mV to +40 mV

• driven by ion movement across the membrane

• once it peaks, neuron resets to resting potential, ready for another cycle

white matter

consists of myelinated axons that send signals over long distances

gray matter

consists of neural somas and unmyelinated regions doing information processing and local connections

first step of neuron fire

• resting potential/state (-70mV)

  • Na+ ions are outside the neuron

  • fewer K+ and Cl- are inside the neuron

  • more negative ions inside the neuron and more positive are outside

    • makes the inside more negative than the outside

second step of neuron firing

• neuron is stimulated by other neurons with neurotransmitters

  • causes Na+ ions to start slowly going inside the neuron

  • -70 mV goes up till it hits the -55 mV threshold

threshold

• -55mV

• catapults the neuron towards the AP at the peak

third step of neuron firing

• depolarization - neuron becomes more positive

  • Na+ rushes into the neuron making it more positive till it reaches +40 mV

  • action potentials is fired at +40 mV

How do we describe action potentials and why?

• “all or nothing”

• no such thing as a little excited, think of a light switch

Does a more intense stimulation (from environment stimulus) cause a more intense AP?

• no but it does cause:

  • more frequent AP

  • AP happening in more neurons

fourth step of neuron firing

• hyperpolarization/repolarization

  • K+ ions flood out because it’s too positive

    • the neuron kicks out K+ instead of Na+ because it’s faster and Na+ wants to stay inside

  • makes it more negative inside the neuron

  • process overshoots the resting state, and the cell will be very negative momentarily (hyperpolarization)

fifth step of neuron firing

• back to rest state

  • what the neuron really wants

  • Na+/K+ pumps push the K+ back into the neuron and push Na+ out

    • more Na+ outside the cell making it positive outside the cell

    • Cl- inside the cell making it more negative than outside

      • goes back to -70 mV (resting state)

Where does depolarization occur?

• at each part of the axon inside the nodes of Ranvier

• allows for the exchange of ions at breaks in the myelin sheath

How fast does an AP travel down an axon?

travels at about 50-100 m/s (not the speed of electrical current in wire)

vesicles

• located in axonal buttons in presynaptic neuron

• contains neurotransmitters

neurotransmitters

the “key”; chemicals that convey electrical signals from one neuron to the next

receptor molecules

• the “lock”

• located the dendrites of postsynaptic neuron

• when NT land on them, Na+ open gates open for Na+ to come into the neuron and start making the neuron more positive

• causes process to repeat

What determines if the ion channels opens for Na+ to come in or leave

whether a NT is excitatory or inhibitory

the sequence

1. vesicles full of NT travel down the presynaptic axon terminals

2. when they reach the cell membrane (end of the presynaptic), they burst open and attach to cell membrane releasing NT’s inside

3. NT travel across the synapse

4. key and lock: NT attach to receptors on postsynaptic with the same shape

5. ion gates in postsynaptic neuron open

6. Na+ starts entering neuron, making it more positive in hopes of creating an AP

life cycle of NT

1. synthesis

2. storage in vesicles

3. release

4. receptor interaction

5. inactivation

6. reuptake

7. degradation

How long does the rise and fall of the action potential take?

• one millisecond

• sets approximate high firing rate of 1,000 AP per second

Where does the action potential begin?

• near the junction of the soma and the axon

• local changes in the membrane trigger the next section of axon, propagating the signal along its length

Herman Von Helmholtz

• measured nerve conduction 19th century

• stimulated frog nerves at different points and timing the resulting muscle response

• found that neurons can extend over long distance

• found bundles of axons make up nerves

What happens to NT after a certain period of time?

• broken down

• reuptake

reuptake

• NT are taken back into the presynaptic neuron to be used again

serotonin reuptake inhibitors (SSRI)

• inhibits the reuptake of serotonin

• keeps the serotonin in the synapse to have more to use

• less depressed = happier (less serotonin → depression)

• “antidepressant”

excitatory postsynaptic potential (EPSP)

• more likely to fire an AP in the postsynaptic neuron

• NT can open the Na+ channels to start coming to possibly start an AP

• makes it more likely to fire bc it starts to get more positive (depolarization)

• ex: glutamate

inhibitory postsynaptic potential (IPSP)

• less likely to fire an AP in the postsynaptic neuron

• NT can open the gates of the K+ ions and have the flood out the neuron

  • can have Cl- ions come in as well

• both makes the neuron less likely to fire because it starts to get more negative (hyperpolarization)

What do neurons weigh the input of? What does this do?

• weigh the input from excitatory and inhibitory NT

• whichever there are more of wins the battle and the neuron fires or does not fire

clinical significance of EPSP and IPSP

malfunctions in inhibitory NT can lead to epilepsy, mood, and sleep disorders

Charles Sherington

• discovered the existence of the synapse

• used the dog experiment

Sherrington’s dog experiment

• scratch the dog in an area and they’ll itch

• eventually they stop scratching, meaning there’s an inhibition in the brain, Sherrington wanted to stop that

• he cut the spinal column so there was no inhibition from the brain (disinhibition)

How did Sherrington come the conclusion of the synapse

• he stimulated one are over time and it created the reaction to scratch (temporal summation)

• he stimulates three areas at just once and together they created the reaction to scratch (spatial summation)

• concluded that, therefore, there must be one area where they come together → synapse

inhibition

you stop behave

disinhibition

stops your ability to stop that behavior (ex: snipping spinal column