Kaplan MCAT - Biology Chapter 4

Neuron definition

what is true about the shape of a neuron

a specialized cell capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals.

*each neuron has a shape that matches its function

1.Where is the nucleus located and what is this also called

2. what is located within the nucleus

The nucleus is located in the cell body which is also called the soma

ER and ribosomes are located there

Soma

neuron cell body

Dendrites

The portion of a neuron that receives stimuli from other cells and conveys them toward the cell body

Axon Hillock what happens there? and what is it

Transition point between the cell body (soma) and the axon of a neuron; the site of action potential initiation**

Action Potentials

Is when an abrupt change in the membrane potential of a nerve or muscle caused by changes in membrane ionic permeability; results in conduction of an impulse in nerves or contraction in muscles

or the transmission of electrical impulses down the axon.

Axon

The long fiber of a neuron; it conducts impulses away from the cell body toward the synapse

Myelin

what most mammalian nerve fibers are insulated by

a fatty membrane

myelin sheath

The white, lipid-containing material surrounding the axons of the many neurons in the central and peripheral nervous systems.

-Maintains the electric signal within one neuron and increases the speed of conduction in the axon.

Oligodendrocytes

Memory device

Myelin Producing Cells in the Central Nervous System

Oligodendrocyte is oligarchy which is all about ME

My from “myelin” as in me me me so think centering yourself equals the central nervous system

Schwann Cells

Myelin Producing cells in the Peripheral nervous system

Nodes of Ranvier

What is their role?

Points on a myelinated axon that are not covered by myelin

They are necessary for rapid conduction

Nerve Terminal and what is it also called and what is its role

End of the axon from which neurotransmitter molecules are released; also called a "synaptic bouton"

Synaptic Bouton (Knob)

nerve terminal

also called axon terminal— not to be confused with axon hillock which is at the start of the cell. Remember, terminal means END like terminal cancer

Neurotransmitters

The chemicals that transmit information from one neuron to another.

Synaptic Cleft

Space between the neurons into which the terminal portion of an axon releases neurotransmitters which bind to the dendrites of the postsynaptic neuron

Synapse

together, the nerve terminal, synaptic cleft, and postsynaptic membrane are known as a synapse.

What is a nerve? (memory devices) and the three types

A bundle of neurons in the PNS

• you’re getting on my nerve— think PMSing

1)sensory

2)motor

3)mixed

Sensory Nerve

Nerves that carry sensory information (afferent)

Motor Nerve

Nerves that carry motor information

(efferent)

Mixed Nerve

Nerves that carry both motor and sensory information

Tracts and differentiate them from nerves

AXONS bundled together in the CNS

*unlike nerves, tracts only carry ONE type of information. Nerves carry multiple information sensory, motor, or both.

Multiple NEURONS bundle together to form a_______.

nerve in the PNS

Axons bundled together form_________.

tracts in the CNS

Cell bodies of neurons of the same type within a NERVE cluster together in_____________.

ganglia in the PNS.

Cell bodies of the individual neurons within a TRACT cluster together in _______________.

nuclei in the CNS.

Axon bundles in CNS:

Multiple neuron bundles PNS:

Tracts

nerves

Nuclei

The cell bodies of neurons in the SAME TRACT in CNS

Glial Cells also called and what is their function

Neuroglia

cells in the nervous system outside of neurons that play a role in structure and support

Neuroglia cells

(Glial Cells) play supportive and structural roles for neurons; responsible for functions such as holding neurons in place, supplying neurons with oxygen and nutrients, insulating neurons from other neurons, destroying pathogens, and removing dead neurons

Astrocytes

type of glial cell

nourish neurons and form the blood-brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue

Ependymal Cells

type of glial cells

line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber

Microglia

are phagocytic glial cells that ingest and breakdown waste products and pathogens in the CNS

Mylenating glial cells in CNS and PNS?

In CNS=oligodendrocytes

IN PNS=schwann cells

*both produce myelin around axons

Resting Membrane Potential and what is it for neurons

is the net electric potential difference that exists across the cell membrane, created by movement of charged molecules across that membrane.

-for neurons this potential is about -70mV with the inside of neuron being (-) and outside being (+).

What are the two most important ions involved in generating maintaining the resting potential?

potassium (K+) and sodium (Na+)

what is true about the concentration of potassium inside and outside the cell

it is higher inside the cell

what does this concentration difference cause

causes potassium to move outside the cell

what enables the movement of potassium outside the cell

potassium leak channels

what happens as potassium leaks out of the cell

causes a build of negative charge inside the cell and positive charge outside

therefore, some potassium will begin to move back into the cell

eventually, there will be one potassium coming into the cell for every one potassium that is pushed out the cell

this means, no net movement

what is it called when there is no net movement of potassium

potassium equilibrium

what is the potential difference that represents potassium equillibrium

equilibrium potential of potassium

what is true about the concentration of sodium

it is higher outside the cell

in what direction does sodium move and how is this movement enabled

it moves from outside to inside

its movement is facilitated by sodium leak channels

what is it called when there is no net movement and what potential difference represents this state

sodium equillibrium

equillibrium potential of sodium

what is true about the movement of sodium and potassium

it occurs at the same time

describe the tug of war between sodium and potassium

Potassium’s movement pulls the cell potential toward –90mv

sodium’s movement pulls the cell potential the opposite way, toward +60 mV.

a balance of these two effects is reached at around –70 mV for the average nerve cell,

This balance, this net effect of sodium and potassium’s equilibrium potentials, is the resting membrane potential.

The resting potential is closer to potassium’s equilibrium potential because the cell is more permeable to potassium.

Neither ion is ever able to establish its own equilibrium, so both ions continue leaking across the cell membrane.

what is necessary given the continual leaking of sodium and potassium in and out of the cell and what accomplishes this

a movement of sodium and potassium back against their gradients

Na+/K+ATPase

Na+/K+ ATPase function and memory device

Sodium-Potassium Pump.

Pumps out 3 sodium for every 2 potassium pumped in. Maintains gradient resting membrane potential.

memory devices: 3 out, 2 in because the membrane potential is negative, it makes sense that it pumps out more positive particles than it takes in

A greater concentration of Na+ is found outside neuron and a greater concentration of K+ found inside a neuron at the resting potential

-ATP required for each transport since both are moved against the gradient.

think PumpKin (K in and Na out— reverse of their gradients)

mnemonic of the direction of ion movement by Na/K ATPase

pumpKin

- K=potassium into the cell

-therefore, sodium out of the cell

Sodium and Potassium concentration inside and outside the cell

outside the cell (net positive): sodium>>potassium

inside the cell (net negative charge):

potassium>> sodium

potassium leak channels

facilitate the outward movement of potassium, which allows the slow leak of potassium out of the cell. ( leaks K+ out)

What types of input can neurons receive and what does this cause

-excitatory input causes depolarization

-inhibitory input causes hyperpolarization

Depolarization

Raising the membrane potential (Vm) from its resting potential; more positive and likely to fire an action potential

Hyperpolarization

Lowering the membrane potential from its resting potential--more negative on inside

Threshold

The lowest magnitude of stimulus strength that will induce a response-- -55 to -40 mV

Summation and the two types

the additive effect of multiple signals.

A small excitatory signal may not be sufficient to bring the axon hillock to threshold.

a postsynaptic neuron may receive information from several different presynaptic neurons, some of which are excitatory and some of which are inhibitory. The additive effect of multiple signals is known as summation

two types of summations:

-spatial

-temporal.

Temporal Summation

In temporal summation, multiple signals are integrated over a small period of time. A number of small excitatory signals firing at nearly the same moment could bring a postsynaptic cell to threshold, enabling an action potential.

Spatial Summation

the additive effects are based on the number and location of the incoming signals.

A large number of inhibitory signals firing directly on the soma will cause more profound hyperpolarization of the axon hillock than the depolarization caused by a few excitatory signals firing on the dendrites of a neuron.

What happens when the threshold potential is reached and what does this rely on

voltage gated sodium channels open

sodium rushes inside

A gradient consisting of a difference in electrical potential as well as chemical concentration

relies on the electrochemical gradient because the inside of the cell is more negative than the outside

Na⁺ wants to go into the cell because the cell is more negative inside (electrical gradient) and has a lower concentration of Na⁺ inside (chemical gradient).

What happens first to sodium channels as membrane potential becomes more positive

Inactivated Sodium Channels

When Vm approached +35mV (peak of AP), potassium channels open.

What happens next to sodium channels as membrane potential becomes more positive

Deinactivated Sodium Channels

When Vm is brought back near -70mV after sodium channels have been inactivated

three states of sodium channels

open closed inactive

Closed Sodium Channels

Sodium Channels before the cell reaches threshold and after inactivation has been reversed

Open Sodium Channels

Sodium Channels from threshold -70mV to approximately +35mV

Inactive Sodium Channels

Sodium Channels from approximately +35mV to the resting potential -70mV aka the repolarization/refractory period

The key concept of Action Potential Process

Action potential relies on both electrical and chemical gradients. The neuron starts at the resting potential around -70mV. At the resting potential, potassium is high inside the cell and sodium is high outside the cell. Once the cell reaches the threshold, sodium channels open and sodium floods the cell, making it more positive inside (depolarization). Then, sodium channels are inactivated and the potassium channels open. This allows potassium to flow out of the cell, bringing the potential into the negative range (repolarization, and then actually overshooting the resting potential (hyperpolarization). Na+/ K+ ATPase then work to restore the resting potential.

What is repolarization

What happens during repolarization

restoration of the resting membrane potential in neurons from

potassium floods out to make up for the increase positive charge of sodium ions flooding into the cell

the influx of potassium actually overshoots and causes excess negative charge (hyperpolarization) and induces a refractory state

What are the two Refractory Periods?

absolute and relative

Absolute Refractory Period

no amount of stimulation can cause another action potential to occur

-absolutely can't occur

Relative Refractory Period

Greater than normal stimulation can cause an action potential to occur because the membrane is starting from a potential that is more negative than its resting value

Impulse Propogation- how does it happen

Movement of an action potential down an axon, resulting in neurotransmitter release at the synaptic bouton and transmission of the impulse to the target neuron or organ.

As sodium rushes into one axon segment of the axon, it will cause depolarization in surrounding regions of the axon. This depolarization results in other regions to reach threshold, opening the Na+ channels and allowing action potential to continue in wavelike fashion. Flow in one direction because after the action potential has been fired in one area of the axon, it becomes refractory

Increased length of axon increases/decreases conduction rate?

Longer axons would decrease conduction conduction rate because it increases the resistance

Greater cross-sectional area of axon increases or decreases rate of transmission?

Increases transmission due to decreased resistance.

How does mylen speed up conduction rate?

Myelin is an extraordinarily good insulator, preventing the dissipation of the electric signal.

Saltatory Conduction

electric signal hops from node to node

mnemonic: Spanish word (saltar)

Presynaptic Neuron

neuron before the synaptic cleft

Postsynaptic Neuron

The neuron after the synaptic cleft

Effector

if a neuron signals to a gland or muscle, rather than a neuron, the postsynaptic cell is termed an effector.

mnemonic: Effector Exits neuron

How does an action potential result in neurotransmitter release?

When an action potential reaches the nerve terminal, the voltage-gated calcium channels open, allowing Calcium to flow into the cell. This sudden increase in intracellular CALCIUM triggers fusion of the membrane-bound vesicles with cell membrane at the synapse, CAUSING EXOCYTOSIS of the neurotransmitter into the synapse.

Difference between electrical and chemical transmission (key concept)

-Within a single neuron, ELECTRICITY is used to pass signals down the length of the axon.

-Between other neurons, CHEMICALS (neurotransmitters) are used to pass signals to the subsequent neuron (or gland or muscle).

Types of neurotransmitter receptors and results?

-Ligand-gated ion channels=results in either a stimulation (depolarize) or inhibitory (hyperpolarization) response

-G-protein coupled receptors (GPCR)=change levels of cAMP or influx of calcium

What are 3 ways neurotransmitter is removed from synapse?

1. broken down by enzymes (i.e. acetylcholinesterase)

2. reuptake carriers back into the pre-synaptic neuron

3. Diffusion out of synapse

Acetylcholine (ACh)

A neurotransmitter found throughout the nervous system; broken down by the enzyme acetylcholinesterase

Reuptake Carriers

Brings neurotransmitters back into presynaptic neuron. An autoreceptor will signal the presynaptic cell to stop releasing and start the reuptake process

ex. serotonin (5-HT), Dopamine, Norepinephrine

Sensory Neurons= (Afferent Neurons)

A neuron that picks up impulses from sensory receptors and transmits them toward the central nervous system (spinal cord and brain)

Motor Neurons= (Efferent Neurons)

A neuron that that transmits nervous impulses from the CNS to an effector

Mnemonic: Afferent and efferent neurons

(A)fferent neurons (A)scend in the spinal cord toward the brain; (E)fferent neurons (E)xit the spinal cord on their way to the rest of the body

Interneurons

are found between other neurons and are the most numerous of the three types. Interneurons are located predominantly in the brain and spinal cord.

Linked to reflexive behavior.

Central Nervous System (CNS) breaks down into the

The brain and spinal cord

What does the brain consist of?

white matter is deeper than grey matter in the brain. Opposite in the spine.

White Matter

consists of axons encased in myelin sheaths

- in the brain, the white matter lies deeper than the grey matter.

Grey Matter

Any region in the central nervous system that consists of unmyelinated cell bodies, dendrites, and synapses. Gray matter is on the outside of the brain.

4 parts of spinal cord and memory device

From head to toe:

1)Cervical

2)Thoracic

3)Lumba

4)Sacral

(Can They Leave? Shit!)

what protects the spinal cord

vertebral column protects spinal cord

Note important spinal structure

dorsal root ganglia

the axons of motor and sensory neurons are in the spinal cord.

The sensory neurons bring information in from the periphery and enter on the dorsal (back) side of the spinal cord.

The cell bodies of these sensory neurons are found in the dorsal root ganglia.

Motor neurons exit the spinal cord ventrally, or on the side closest to the front of the body

Peripheral Nervous System (PNS)

• what is it subdivided into

• what is its primary role

Includes all neurons outside the central nervous system, including sensory and motor neurons; it is subdivided into the somatic and autonomic nervous system.

-transmits info to and from CNS

Somatic Nervous System

Subdivision of the peripheral nervous system that governs all voluntary action.

Divisions of NERVOUS SYSTEM

Central Nervous System (CNS) and Peripheral Nervous System (PNS)