nervous system physiology - A + P 1

what are nervous system reflex pathways

they are long distance pathways that:

• receive input about a change

• integrate the information

• use other parts of the nervous system and/or endocrine system to react appropriately

what is the general nervous system reflex pathway structure

> stimulus

> receptor

> afferent pathway

> integrating center

> efferent pathway

> target (effector) tissue

> response

what is homeostasis

it is when an internal environment is actively maintained constant by the function of cells, tissues, and organs that are organized in nervous system reflex pathways (negative feedback systems)

nervous system reflex pathways - sensory receptors function

they continually monitor conditions in the internal and external environment

nervous system reflex pathways - afferent (sensory) neurons function

receptors send information along afferent neurons to the brain/spinal cord

nervous system reflex pathways - integrating center

• the brain/spinal cord integrates the incoming information

• considered the integrating center homeostasis, movement, and many other body functions

nervous system reflex pathways - efferent (motor) neurons and target cells function

brain/spinal cord sends output signals directing an appropriate response (if any) through efferent (motor) neurons to the target cells of the body

what is the entire nervous system reflex pathway

1. stimulus

2. sensor or receptor

3. afferent pathway

4. integrating center

5. efferent pathway

6. target or effector

7. response

8. negative feedback loop

what is a special quality of the CNS demonstrated during thinking and dreaming

the CNS is able to initiate activity without sensory input and does not need to create any measurable output

what does axoplasm (in an axon) contain and what does it lack

• contains cytoskeleton fibers (microfilaments, intermediate filaments, microtubules)

• lacks ribosomes, endoplasmic reticulum, Golgi apparatus

where are neurocrines needed by the axon synthesized and how do they travel to the axon

• neurocrines (proteins) needed by the axon or axon terminal must be synthesized in the soma

• neurocrines are packaged into vesicles and moved down the axon via axonal transport

what is slow axonal transport

• moves material via axoplasmic flow from the soma to the axon terminal

• used for substances that are not consumed rapidly by the cell like enzymes and cytoskeleton proteins

what is fast axonal transport

• moves neurocrine-filled vesicles and other materials at rates of up to 400 millimeters per day

• stationary microtubules are used as tracks

• transported particles walk along the tracks with the aid of attached foot-like proteins

• these motor proteins bind and unbind to the microtubules with the help of ATP

what is anterograde fast axonal transport

• fast axonal transport that goes forward

• moves material from the soma to the axon terminal

what is retrograde fast axonal transport

• fast axonal transport that goes backward

• moves material from the axon terminal back to the soma for recycling

what is the functional unit of the nervous system and what is its function

• neurons are the functional unit

• carries out the communication process

what are the universal properties of neurons

• excitability (irritability)

• conductivity

• secretion

what is the excitability of neurons

• ability to respond to environmental changes (stimuli) by generating and propagating electrical signals when ions move across the cell membrane

• highly developed in neurons

• unique property of nerve and muscle cells

what is the conductivity of neurons

ability to response to stimuli by producing electrical signals that reach other cells at distant locations

what is the secretion of neurons

when an electrical signal reaches the end of a nerve cell, the neuron secretes chemicals called neurocrines that cross the gap and stimulate the next cell

neuron excitability - why do do ions move across the cell membrane

they move across the cell membrane due to the uneven distribution of electrical charge between the intracellular and extracellular compartments

neuron excitability - resting membrane potential difference definition

• defined as uneven distribution of electrical charge between the intracellular and extracellular compartments

• called electrical and chemical disequilibrium in the body

• cell membrane separates electrical charges in the body

what uses simple diffusion to move across plasma membranes

• oxygen

• carbon dioxide

• water

• lipids

what uses facilitated diffusion channel proteins to move across plasma membranes

• ions

• water

what uses facilitated diffusion carrier proteins tome across plasma membranes

• ions

• glucose

• amino acids

what uses active transport carrier proteins to move across plasma membranes

• large polar molecules

• proteins

what is the electro-chemical gradient

• combination of electrical and concentration gradients

• for example, the neuromuscular junction

how is the neuromuscular junction an example of an electro-chemical gradient

• sodium influx exceeds potassium efflux and results in a positive charge inside the muscle fiber

• when sodium enters the skeletal muscle fiber, it moves down its concentration gradient and down its electrical gradient

• when potassium leaves the skeletal muscle fiber, it moves down its concentration gradient and up its electrical gradient

what is the movement of water when a membrane is permeable only to water and not to any solutes

• water moves by osmosis from a less concentration solute (hyposmotic) solution into a more concentrated solute (hyper osmotic) solution

• water always moves to dilute the more concentrated solute solution

why do osmotic equilibrium occur

cells typically remain in osmotic equilibrium because water can freely move across the membrane in response to solute movement

breakdown of resting membrane potential difference - what does ‘resting’ refer to

• membrane potential has reached a steady state and is not changing

• this electrical gradient is seen in all living cells, even those that appear to be without electrical activity

breakdown of resting membrane potential difference - what does ‘potential’ refer to

• electrical gradient created by active transport of ions across the cell membrane

• source of stored or potential energy, just like how concentration gradient is a form of potential energy

breakdown of resting membrane potential difference - what does ‘difference’ refer to

difference in the electrical charge inside and out of the cell

how does the trigger and resulting movement of sodium within the neuromuscular junction serve as an example of work done by an electrical gradient

• acetylcholine binds to a nicotinic receptor channel protein on the motor end plate of a skeletal muscle fiber

• binding causes this chemically gated channel to open

• sodium moves into the skeletal muscle cell and potassium moves out of the cell through this newly opened channel

• more sodium moves into the cell than potassium that moves out of the cell because sodium moves down its electrical gradient AND its chemical gradient while potassium moves down its chemical gradient but UP its electrical gradient

• the inside of the muscle cell becomes more positive

• this change in charge causes voltage gated sodium ion channels to open which allows more sodium to move into the cell

• this additional influx of sodium has a domino effect causing more voltage gated sodium ion channels to open

• this is called the propagation of an action potential

what is an example of work done by a chemical gradient

• trans epithelial transport

• movement of glucose from the small intestine lumen (or the nephron tubule men) into an epithelial cell using the sodium concentration gradient

which two factors influence resting membrane potential difference

• concentration gradients of ions across the membrane

• membrane permeability to ions

what are the concentration gradients that affect the resting membrane potential difference

• sodium, chloride, bicarbonate, and calcium are more concentrated in the extracellular fluid than in the cytosol

• potassium and protein are more concentrated in the cells than in the extracellular fluid

what characteristic of membrane permeability of ions affects the resting membrane potential difference

• resting cell membrane is much more permeable to potassium than to sodium or calcium because there are many more leak (open) potassium ion channels in cell membranes than sodium or calcium

• therefore, potassium is the major ion contributing to the resting membrane potential

what can change the resting membrane potential difference

• any change in the potassium concentration gradient or ion permeability changes the resting membrane potential difference

• normal = -70 millivolts in neurons

• normal = -90 millivolts in skeletal muscle fibers

what can predict the membrane potentials

• Nernst equation predicts membrane potential for a single ion

• the GHK equation predicts membrane potential using multiple ions

what does the ions movement across the cell membrane do

it generates and propagates electrical signals

what plays a key role in generating electrical signals in nervous and muscle tissue

an increase in sodium permeability

what happens if a membrane suddenly increases its sodium permeability

• normally, the cell membrane of neurons is only slightly permeable to sodium

• if the membrane suddenly increases its sodium permeability due to a neurocrine binding with a chemically gated sodium ion channel on the cell, sodium enters the cell moving down its electro-chemical gradient

what happens to depolarize a cell

• net movement of positive electrical charge into the cell (usually sodium) depolarizes it

• makes it more positive

• creates an electrical signal

• -55 millivolts

what happens to repolarize a cell

• net movement of positive electrical charge out of the cell (usually potassium) repolarizes the cell, usually back to its resting potential

• makes it more negative

• -70 millivolts

what happens to hyper polarize a cell

• if the cell membrane becomes more permeable to potassium (more potassium ion channels open), positive charges are lost from inside the cell and the cell becomes more negative

• -90 millivolts

what can’t we tell by observing a change in membrane potential from -70 millivolts to +30 millivolts

• change in membrane potential from -70 mV to +30 mV does NOT mean that the concentration gradients inside and outside the cell have reversed

• significant change in resting membrane potential difference occurs with the movement of very few ions

• to change the membrane potential by 100 mV, only one out of 100,000 potassium ions can enter or leave the cell (with no other movement)

how are electrical signals classified

• electrical signals caused by the movement of electrical charge across the membrane are classified into two types

• graded potentials

• action potentials

what are graded potentials

• variable strength signals that travel over short distances

• lose strength as they travel through the cell

• used for short distance communication

what are action potentials

• large, uniform depolarizations that can travel quickly for long distances through neurons

• do not lose strength while traveling through the cell

• ‘all or none’ phenomenon

how do graded potentials and action potentials relate to one another

if a depolarizing graded potential is strong enough when it reaches an integrating region within the cell (called trigger zone in motor neurons and interneurons), it will initiate an action potential

when and where do graded potentials occur

• they occur when ion channels open or close causing ions to enter or leave the neuron

• they occur at dendrites, the soma, or near axon terminals

what are the two types of graded potentials

• depolarization, excitatory

• hyperpolarizations, inhibitory

what is an example of a depolarizing (excitatory) graded potential

sodium moving from outside the cell to inside the cell

what is an example of a hyperpolarizing (inhibitory) graded potential

potassium moving from inside the cell to outside the cell

what cellular actions cause graded potentials

• a chemical stimulus (like acetylcholine binding to receptor channels) open sodium channels on a postsynaptic neuron membrane

• sodium ions move into the neuron

• positive charge carried by he sodium spread as a wave of depolarization through the cytoplasm (just as a stone thrown into water creates ripples that spread outward from the point of entry)

graded potentials - what is a local current flow

• wave of depolarization that move through the cell

• current flow in biological systems is defined as the net movement of positive electrical charge

graded potentials - why are they called graded

called graded because their size or amplitude is directly proportional to the strength of the triggering event

graded potentials - what is the triggering event for a graded potential

• it is the amount of neurocrine released by the presynaptic neuron into the synaptic cleft

• they reflect the strength of the stimulus that initiates them, so a small amount of neurocrine released gets a weaker graded potential than a large amount of neurocrine released

graded potentials - why do graded potentials lose strength as they move through the cytoplasm

• current leak

• cytoplasmic resistance

graded potentials - what is a current leak

some of the positive charges leak back across the membrane via open (leak) channels

graded potentials - what is cytoplasmic resistance

it provides resistance to the flow of electricity - just as water creates resistance that diminishes the ripples from the stone

graded potentials - what is the ‘trigger zone’ regarding graded potentials

• graded potentials that are strong enough eventually reach the region of the neuron known as the trigger zone

• in motor (efferent) neurons and interneurons, the trigger zone is comprised of the axon hillock and initial segment

• in sensory (afferent) neurons, the trigger zone is immediately adjacent to the receptor where the dendrites join the soma

what is the function of trigger zones in neurons

• they are the integrating center of the neuron

• they contain a high concentration of voltage gated sodium ion channels in their membrane

what must happen to make a trigger zone initiate an action potential

• resting membrane potential of the trigger zone is -70 millivolts

• to initiate an action potential, the influx of sodium into the trigger zone must change the membrane potential to -55 millivolts

if the graded potential depolarizes the trigger zone, what happens

if it depolarizes the membrane to a minimum level known as the threshold voltage (-55 mV), the voltage gated sodium channels open and an action potential is initiated

if the graded potential does not depolarize the trigger zone, what happens

• if it does not depolarize the trigger zone, it will not reach the -55 mV threshold

• no action potential is begun

• the graded potential simply dies out as it moves into the axon

graded potentials - what is an excitatory postsynaptic potential (EPSP)

• since depolarization is necessary to excite the neuron to fire an action potential, a depolarizing graded potential is known as an EPSP

• any stimulus that makes a neuron more likely to fire an action potential is considered excitatory

graded potentials - what is an inhibitory postsynaptic potential (IPSP)

• hyperpolarizing graded potential moves the membrane potential farther from the threshold value making the inside of the cell more negative so they are called IPSPs

• neuron is less likely to fire an action potential

• any stimulus that makes a cell less likely to fire an action potential is considered to be inhibitory

what characteristics of action potentials differentiate them from graded potentials

• action potentials are repeating changes in membrane potential that occur when voltage-gated ion channels open, altering membrane permeability to sodium and potassium

• mechanisms by which action potentials are generated and conducted along the axon allow them to stay constant

• an action potential measured at the distal end of an axon is identical in amplitude to the action potential that started at the trigger zone

what are general, universal characteristics of action potentials

• all action potentials are identical

• each one is a depolarization of approximately 100 millivolts amplitude, meaning the resting membrane potential changes from -70 mV to +30 mV

• the strength of a graded potential that initiates an action potential has no influence on the amplitude of the action potential

• ‘all or none’ phenomenon

• they do not diminish in strength as they travel through the neuron, meaning they can travel long distances without changing

what is the ‘all or none’ phenomenon of action potentials

• they occur as a maximal depolarization if the stimulus reaches the -55 mV threshold

• they do not occur at all if the stimulus is below threshold

what are the three phases of action potentials

• rising

• falling

• hyper polarization

action potential phases - step one

• -70 mV

• resting potential

• same resting potential as step 9

action potential phases - step two

• depolarizing stimulus (acetylcholine binds to receptor and opens channel)

• neurocrine is the stimulus

action potential phases - step three

• -55 mV

• membrane depolarizes to threshold

• voltage gated Na+ channels open and allow Na+ to enter

• this is the positive charge moving into the trigger zone and changing its charge to -55 mV

action potential phases - step four

• rapid Na+ entry depolarizes the entire cell due to a sudden temporary increase in neuron permeability to Na+

• K+ channels begin to open very slowly

• ‘Rising’ phase

action potential phases - step five

• +30 mV

• all Na+ channels close

• slower K+ channels are still opening

• no longer in the rising phase because Na+ stops entering the cell

action potential phases - step six

• K+ begins to move from inside the cell to extracellular fluid now that the channels are open

• K+ leaves the trigger zone

• ‘Falling’ phase

action potential phases - step seven

• charge goes from +30 mV all the way down to below -70 mV (approaches -90 mV)

• K+ channels remain open and additional K+ leaves the cell

• this hyper polarizes the cell

action potential phases - step eight

• voltage gated K+ channels close

• K+ leak channels are open and slowly begin re-introducing K+ back into the cell now that voltage-gated K+ channels are closed

• charge begins to slightly increase

action potential phases - step nine

• -70 mV

• cell returns to resting ion permeability (leak channels finish bringing the charge in the cell up)

• cell, therefore, returns to resting membrane potential

• same resting potential as step one, resting potential is reset

what is happening within the cell to cause the effects of step three of action potentials

as the trigger zone depolarizes to threshold (-55 mV), voltage-gated sodium ion channels open, making the membrane much more permeable to sodium

what is happening within the cell to cause the effects of step four of action potentials

since sodium is more concentrated outside the cell than within it, and since the negative membrane potential inside the neuron (-70mV) attracts positively charged sodium ions, sodium diffuses down its electro-chemical gradient into the cell

what is happening within the cell to cause the effects of steps five and six of action potentials

• sodium channels closing and potassium channels opening and letting potassium move out of the cell corresponds to an increase in potassium permeability

• voltage-gated potassium channels start to open in response to depolarization of the cell

• potassium channel gates are much slower to open so peak potassium permeability occurs later than peak sodium permeability

how many ions actually move across the axon membrane during a single action potential

• very few ions move across the axon membrane

• sodium and potassium concentrations inside and outside the cell remain essentially unchanged

• small numbers of ions that cross the membrane during an action potential do not disrupt the sodium and potassium gradients

graded potential - type of signal

input signal

graded potential - where it occurs

usually in the dendrites and cell body

graded potential - types of gated ion channels involved

mechanically, chemically, or voltage-gated channels

graded potential - ions involved

usually Na+, Cl-, Ca++

graded potential - depolarizing/hyperpolarizing

• depolarizing (such as with Na+)

• hyper polarizing (such as with Cl-)

graded potential - strength of signal

depends on stimulus and can be summed

graded potential - what initiates the signal

entry of ions through channels

graded potential - unique characteristics

• no minimum level required to initiate

• two signals coming close together in time will sum

• initial stimulus strength is indicated by frequency of a series of action potentials

action potential - type of signal

conduction signal

action potential - where it occurs

trigger zone through the axon

action potential - types of gated ion channels involved

voltage-gated channels

action potential - ions involved

Na+ and K+

action potential - depolarizing/hyperpolarizing

depolarizing

action potential - strength of signal

is always the same and cannot be summed