A&P Ch. 12: Membrane Potential and the Action Potential

action potential notes

1. not local- travels long distances 2. only causes depolarization then repolarization then hyperpolarization then back to resting potential 3. voltage-gated channels 4. does not become weaker as it travels 5. duration is always the same no matter how intense the stimulus

five main membrane processes in neural activites

1. resting potential 2. graded potential 3. action potential- an electrical impulse 4. synaptic activity 5. information processing

two major efferent systems

Somatic Nervous System (SNS) Autonomic (visceral) Nervous System (ANS)

neural responses to injuries

Wallerian degeneration (occurs in the PNS)- axon distal to injury site degenerates; macrophages migrate into area and remove cellular debris (dead cells, etc)

other neurotransmitters

at least 50 neurotransmitter other then ACh; major categories of neurotransmitters: biogenic amines, amino acids- glycine (inhibitory), glutamate and aspartate (excitatory), neuropeptides, dissolved gasses- NO, CO

*step 1*: depolarization to threshold

graded potential reaches axon hillock to depolarize it; it then passes to the initial segment of the axon; if threshold is reached then step 2 occurs

interneurons

most are located in brain, spinal cord, and autonomic ganglia; approximately 20 billion in body; found between sensory and motor neurons; are responsible for: distribution of sensory information; coordination of motor activity; are involved in higher functions: memory, planning, learning

effectors

respond to efferent signals coming from CNS; cells and organs are effectors; has two components: Somatic Nervous System (SNS) and Autonomic Nervous System (ANS)

electrical gradient

separate charges of positive and negative ions; result in potential difference

postsynaptic cell

target cell that receives message

resistance

the amount of current a membrane restricts

information

"information" travels within the nervous system; as propagated electrical signals (action potentials); the most important information (vision, balance, motor commands); is carried by large-diameter, myelinated axons type A (SMS); less urgent info, carries by Type B (email) or Type C fibers (snail mail)

control interstitial environment

(fluid); K+, Na+, CO2 nutrients, recycling nuerotransmitters, enhancel supress communication between neurons

neuroglia

(glial cells); cells that support and protect neurons; half the volume of the nervous system; many types of neuroglia in CNS and PNS

excitatory neurotransitters

(influx of Na+, more positive inside cell and depolarization) causes depolarization of postsynaptic membranes; promotes action potentials in the postsynaptic cell

maintain blood-brain barrier

(isolates CNS); solutes in blood fluid do not have free access to interstitial fluid of CNS; astrocytes regulate flow of material from capillary secrete chemicals that alter permeability of capillary endothelial cells in CNS

cytoskeleton (the cell body)

*neurofilaments* and *neurotubules* similar to intermediate filaments and miccrotubules; *neurofibrils*- bundles of neurofilaments that provide support for dendrites and axon

four steps in the generation of action potentials

*step 1*: depolarization to threshold *step 2*: activation of Na+ channels *step 3*: inactivation of Na+ channels and activation of K+ channels *step 4*: return to normal permeability

steps in propagation

*step 3*: first segment enters refractory period *step 4*: local current depolarizes next segment

two methods of propagating action potentials

1. *continuous propagation* (unmyelinated axons) 2. *saltatory propagation* (myelinated axons)

three functional classifications of neurons

1. *sensory neurons* (afferent neurons of PNS) 2. *motor neurons* (efferent neurons of PNS) 3. *interneurons* (association neurons)

three groups of axons

1. *type A fibers* 2. *type B fibers* 3. *type C fibers* ordered from fastest to slowest these groups are classified by: diameter, myelination, speed of action potential

events at a cholinergic synapse

1. action potential arrives, depolarizes synaptic terminal of the presynaptic cell 2. calcium ions enter synaptic terminal through voltage-gated calcium channels (these open because of action potential reaching the axon terminal), trigger exocytosis of ACh 3. ACh binds to receptors on postsynaptic cell depolarizes postsynaptic membrane 4. ACh removed by AchE; AChE (acetylcholinestererase)breaks into acetate and choline

structural classification of neurons

1. anaxonic neurons 2. bipolar neurons 3. unipolar (pseudounipolar) neurons 4. multipolar neurons

three classes of gated channels

1. chemically gated-channel 2. voltage-gated channels 3. mechanically gated channels

three states of gated channels

1. closed, but capable of opening 2. open (*activated*) 3. closed, not capable of opening (*inactivated*); only certain gated channels can have this third state

how neurotransmitters and neuromodulators work

1. direct effects on membrane channels; for example, ACh, glycine, aspartate 2. indirect effects via G proteins (second messenger systems involving cAMP); forexample, E, NE, dopamine, histamine, GABA 3. indirect effects via intracellular enzymes; for example, lipid-soluble gases (NO, CO)

four types of neuroglia in the CNS

1. ependymal cells 2. microglia 3. oligodendrocytes 4. astrocytes

two types of postsynaptic potentials

1. excitatory postsynaptic potential (EPSP) 2. inhibitory postsynaptic potential (IPSP)

three types of sensory recpetors

1. interoceptors 2. exteroceptors 3. proprinoceptors

graded potential notes

1. local- travel short distances 2. depolarize or hyperpolarize 3. chemially-gated channels 4. becomes weaker further it travels 5. entry of ion duration variable- causes variable size graded potenial graded potential can lead to the production of action potential neurotransmitter attaches onto the postsynaptic and lets sodium in, goes to the axon hillock, which then can create an action potential

functions of astrocytes

1. maintain blood-brain barrier 2. create three-dimensional framework fro CNS 3. Repair damaged neural tissue 4. guide neuron development and interconnections 5. control interstitial environment (fluid)

whether depolarizing or hyperpolarizing, share four basic characteristics

1. the membrane potential is most changed at the site of stimulation, and the effect decreases with distance (the "ripple" effect) 2. the effect spreads passively, due to local currents; the spread is in all direction, not just in one direction (creating ripples in a pool of water- ripples travels in all directions) 3. the graded change in membrane potential may involve either depolarization or hyperpolarization; the properties and distribution of the membrane channels involved determine the nature of the change; for example, in a resting membrane, the opening of sodium channels causes depolarization, whereas the opening of potassium channels causes hperpolarization; the change in membrane potential reflects whether positive charges enter or leave the cell 4. the stronger the stimulus, the greater the change in the membrane potential and the larger the area affected; depends on the channel (chemically-gated channel, stronger the stimulus= more chemicals binding to receptor which causes the channels to open longer)

the response of a postsynaptic neuron to the activation of a presynaptic neuron can be altered by

1. the presence of neuromodulators or other chemicals that cause facilitation or inhibition at the synapse 2. activity under way at other synapses affecting the postsynaptic cell 3. modification of the rate of neurotransmitter release through presynaptic facilitation or presynaptic inhibition

anatomical divisions of the nervous system

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

2. indirect effects

G proteins; work through second messengers; enzyme comples that binds GTP; link between neurotransmitter (first messenger) and second messenger; activate enzymes adenylyl cyclase; which produces second messenger cyclic-AMP (cAMP) which can then open membrane channels, activate intracellular enzymes, or both (vending machine)

*step 4*: return to normal permeability

K+ channels begin to close; when membrane reaches normal resting potential (-70 mV); Na+ channel (activation gate) is now closed and inactivation gate is open- important; K+ channels finish closing; membrane is hyperpolarized to -90 mV (K+ channels are slow to close, so little extra K+ leaves cell causing inside to be more negative than at rest) ; membrane potential returns to resting level (due to Na+ ion movement through passive channels); action potential is over

Local current in segment 2 travels to segment 3 and also to segment 1. Segment 3 depolarizes to threshold and AP is formed.

Local current in segment 2 travels to segment 3 and also to segment 1. Segment 3 depolarizes to threshold and AP is formed.

without ATP

Na+ and K+ concentration gradient would disappear (very slowly, after tens of thousands of action potentials) and also ions leaking through the passive channels; neurons stop functioning (without the ion gradient, an action potential cannot be generated)

schwann cells form path for new growth after injury

Schwann cells do not degenerate; proliferate and form solid cellular cord which follows path of original axon; Wrap new axon in myelin once the axon has grown back across the damage site

serotonin

a CNS neurotransmitter; affects attention and emotional states; decreased serotonin production may be responsible for some cases of severe chronic depression, treated with selective serotonin reuptake inhibitors (SSRI's)-prozac, Paxil, Zoloft

dopamine

a CNS neurotransmitter; may be excitatory or inhibitory (for movement control); damage to neurons that produce dopamine involved in Parkinson's disease and cocaine use; Cocaine inhibits dopamine removal from synapse increases postsynaptic effect and feeding of "high"

facilitation

a neuron becomes facilitated; as EPSPs accumulate; raising membrane potential closer to threshold; until a small stimulus can trigger action potential

inhibition

a neuron that receives many IPSPs: is inhibited from producing an action potential; because the stimulation needed to reach threshold is increased (the cell is hyperpolarized)

depolarization

a shift in membrane potential toward 0 mV; movement of Na+ through channel; produces local current; depolarizes nearby plasma membrane (graded potential); change in potential is proportional to stimulus (drop a small pebble, create small ripples in pond, drop a big rock, create bigger ripples in the pond, that travel farther than the small ripples)

synaptic delay

a synaptic delay of .2-.5 msec occurs between: arrival of action potential at synaptic terminal and effect on postsynaptic membrane;Synaptic delay occurs because neurotransmitter has to be released from axon terminal, then diffuse across synaptic cleft, bind onto postsynaptic receptor, and causes the receptor to open fewer synapses means faster response; reflexes may involve only one sypnase

presynaptic inhibition

action of an axoaxonic synapse at a synaptic terminal that decreases the neurotransmitter released by presynaptic membrane; released of GABA prevents opening of voltage-gated Ca2+ channels at axon terminal

presynaptic faciliatation

action of an axoaxonic synapse at a synaptic terminal that increases the neurotransmitter released by presynaptic membrane (ex. Serotonin affects axon terminal, opposite to GABA effects)

saltatory propagation

action potential along myelinated axon; faster and uses less energy then continuous propagation; myelin insulated axon (internode), prevents continuous propagation; local current "jumps" from node to node; depolarization occurs only at nodes, areas of axon covered by myelin do not regenerate the action potential

cycle repeats

action potential travels in one direction (1m/sec); why does it (the action potential) travel in only one direction?

continuous propagation

action potentials along an unmyelinated axon; affects one segment of axon at a time; steps in proagation: step 1: action potential in segment 1; step 2: depolarizes second segment to threshold

many drugs

affect nervous system by stimulating receptors that respond to neurotransmitters; can have complex effects on perception, motor control, and emotional states

functional divisions of the PNS

afferent division efferent division

neurons perform

all communication, information processing, and control functions of the nervous system

ion movements and electrical signals

all plasma (cell) membranes produce electrical signals by ion movements; membrane potential is particularly important to neurons; the membrane potential is caused by a difference of charge between the inside and outside of the cell

satellite cells

also called amphicytes; surround ganglia; regulate environment around neuron; (similar in function to astrocytes in the CNS)

graded potentials

also called local potentials; changes in membrane potential- cannot spread far from site of stimulation (like throwing a pebble into a pond, the ripples that form only a short distance away before they disappear); includes any stimulus that opens a gated channel and produces a graded potential

schwann cells

also called neurilemma cells; form myelin sheath (neurilemma) around peripheral axons; one schwann cell sheaths one segment of axon; many schwann cells are needed to sheath an entire axon; myelination not complete, nodes of Ranvier located between internodes

norepinephrine (NE)

also known as noradrenaline; released at adrenergic synapses; excitatory and depolarizing effect; widely distributed in brain and portions of ANS

neuromodulators can

alter either the rate of neurotransmitter release or the response of a postsynaptic neuron to specific neurotransmitters

action potenial

an electrical impulse; produced by graded potential (if it is strong enough); propagates along surface of axon to the synapse

cholinergic synapses

any synapse that releases ACh; located at: 1. all neuromuscular junctions with skeletal muscle fibers 2. many synapses in CNS 3. all neuron-to-neuron synapses in PNS 4. all neuromuscular and neurogalndular junctions of ANS parasympathetic division

afferent division

approaches CNS; carries sensory information; form PNS sensory receptors to CNS

motor neurons

approximately 1/2 million in body; carry instructions from CNS to peripheral effectors; via efferent fibers (axons)

functions of sensory neurons

approximately 10 million; monitor internal environment (visceral sensory neurons); monitor effects of external environment (somatic sensory neurons)

passive channels (leak channels)

are always open; permeability changes with conditions

neurotransmitters

are chemical messangers; are released at presynaptic membrane (mostly by exocytosis); affect receptors of postsynaptic membrane; are broken down by enzymes; enzymes found in the synaptic cleft; are reassembled at axon terminal; can be reabsorbed by presynaptic cell to be reassembled and reused

action potentials (nerve impulses)

are transmitted from presynaptic neuron; to postsynaptic neuron (or other postsynaptic cell) across a synapse

the synapse

area where a neuron communicates with another cell; involves 2 cells (they do not make physical contact); presynaptic cell and postsynaptic cell

internodes

areas of axon covered by myelin; insulates axon from outside environment

*step 3*: inactivation of Na+ channels and activation of K+ channels

at +30 mV; inactivation gates close (Na+ channel inactivation); note: Na+ channel has 2 gates, so the inner inactivation gate closes at this time while the outer activation gate is still open; K+ channels open, net flow of K+ out of cell; repolarization begins

net effects

at axon hillock determines if action potential is produced; therefore, a change in transmembrane potential that determines whether or not action potentials are generated is the simplest form of information processing

effects of graded potentials

at cell dendrites or cell bodies; trigger specific cell functions; for example, exocytosis of glandular secretions from gland cell; at motor end plate; release ACh (acetycholine- a neurotransmitter) into synaptic cleft; causes motor end plate (postsynaptic membrane of skeletal muscle end) to depolarize and generate an action potential

spinal nerves

attach to spinal cord

multiple sclerosis (MS)

autoimmune disorder- caused by inflammation of nerves triggered by immune cells (immune cells probably triggered by virus or defective gene); demyelination disease affecting CNS axons; vision, speech, balance, motor coordination affected; treated with corticosteroids and interferon (anti-inflammatory agents) to slow disease progression

guillain-barre syndrome

autoimmune disorder- triggered by a virus; demyelination disease affecting peripheral nerves; sensations of weakness or tingling of legs that spread to arms; progression of disease leads to paralysis (diaphragm also affected); mostly fully recover, but some continue to have residual weakness

neurons may

be facilitated or inhibited by extracellular chemicals other than neurotransmitters or neuromodulators

the resting potential

because the plasma membrane is highly permeable to potassium ions: the resting potential of approximately -70 mV is fairly close to -90 mV, the equilibrium potential for K+; the electrochemical gradient for sodium ions is very low; Na+ has only a small effect on the normal resting potential, making it just slightly less negative than the equilibrium potential for K+; the sodium-potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it brings into the cell; it serves to stabilize the resting potential when the ratio of Na+ entry to K+ loss through passive channels is 3:2; at the normal resting potential, these passive and active mechanisms are in balance; the resting potential varies widely with the type of cell; a typical neuron has a resting potential of approximately -70 mV

membrane potential exists across plasma membrane

because: cytosol and extracellular fluid have different chemical/ionic balance (inside of cell has negatively charged proteins that are not usually found outside of the cell); the plasma membrane is selectively permeable

organs of the nervous system

brain and spinal cord; sensory receptors of sense organs (eyes, ears, etc.); nerves (axons of neurons) connect nervous system with other systems

collaterals

branches of a signal axon

postsynaptic neuron

can receive a lot of information from a lot of cells

graded potential

caused by stimulus; temporary, localized change in resting potential; can make more positive or more negative

inhibitory neurotransmitters

causes hyperpolarization of postsynaptic membranes; suppress action potentials in the postsynaptic cell

neurons

cells that send and receive signals; the basic functional units of the nervous system

ependymal cells

cells with highly branched processes; form epithelium called ependyma; lacks basement membrane line central canal of spinal cord and ventricles of brain; secrete cerebrospinal (CSF)- fills passageways of brain and spinal cord; have cilia or microvilli that circulate CSF; monitor CSF; contain stem cells for repair; may divide and differentiate into more neurons, unknown regulatory mechanism

membrane potential

changes with plasma membrane permeability; in response to chemical or physical stimuli

passive forces acting across the plasma membrane

chemical gradients electrical gradients

heavy metal poisoning

chronic exposure to heavy metals can damage neuroglia and lead to demyelination which causes axons to deteriorate irreversibly (Arsenic poisoning)

the extra ceullar fluid (ECF) and intraceullar fluid (cytosol) differ greatly in ionic composition

concentration gradient of ions (Na+, K+); more Na+ outside, less inside; more K+ inside, less outside

chemical gradients

concentration gradients (chemical gradient) of ions (Na+, K+)

cranial nerves

connect to brain

Central Nervous System (CNS)

consists of the spinal cord and brain; contains neural tissue, connective tissue, and blood vessels

Autonomic Nervous System (ANS)

controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretion; made of up the sympathetic division and parasympathetic division; visceral motor neurons innervate all other peripheral effectors (involuntary); smooth muscle, cardiac muscle, glands, adipose tissue

Somatic Nervous System (SNS)

controls voluntary and involuntary (reflexes) skeletal muscle contractions; includes all somatic motor neurons that innervate skeletal muscles (voluntary)

axoplasm

cytoplasm of axon; contains neurofibrils, neurotubules, enzymes, orangelles

functions of the PNS

deliver sensory information to the CNS; carry motor commands from the CNS to peripheral tissues and systems; nerves (also called peripheral nerves); bundels of axons with connective tissues and blood vessels; carry sensory information and motor commands in PNS; cranial nerves and spinal nerves

nissl bodies

dense areas of rough endoplasmic reticulum and ribosomes; make neural tissue appear gray (gray matter)

the effect of a neurotransmitter on a postsynaptic membrane

depends on the receptor, not on the neurotransmitter; for example acetylcholine (ACh) usually promotes action potentials in the postsynaptic cell but inhibits cardiac neuromuscular junctions

step 1: action potential in segment 1

depolarizes membrane to +30 mV; local current is produced, travels in all directions

receptors

detect changes or respond to stimuli; stimuli can be internal or external; receptors can be found on neurons and specialized cells

electrical synapses

direct physical contact between cells; are locked together at gap junctions (connexons); allow ions to pass between cells; produce continuation local current and action potential propagation; are found in areas of brain, eye ciliary ganglia (PNS), cardiac muscle cells

diptheria

disease casued by bacterial toxin; damages Schwann cells and destroys PNS myelin sheath; can lead to fatal paralysis; rare disease since vaccine available

3. membrane permeability varies by ion

due to presence of specific passive (leak) channels that are opened all the time

characteristics of neuromodulators

effects are long term, slow to appear; responses involve multiple steps, intermediary compounds; affects presynaptic membrane, postsynaptic membrane, or both; released alone or with a neurotransmitter

two types of synapses

electrical synapses chemical synapses

four classes of opioids

endorphins enkephalins endomorphins dynorphins- most potentat pain-relieving effect of the opiods and even more powerful than morphine

two classes of neurotransmitters

excitatory neurotransmitters inhibitory neurotransmitters

efferent division

exits CNS; carries motor commands; from CNS to PNS muscles and glands

exterorecptors

external senses (touch, temperature, pressure); distance senses (sight, smell, hearing)

the venom of a king cobra contains an alpha neurotoxin that irreversibly binds into cholinergic receptors and inactivates them. how would this venom affects a typical person?

feeling of weakness and numbness

telodendria

fine extensions of distal axon

dendritic spines

fine processes; 80-90 percent of neuron surface area; receive information from other neurons

the electrochemical gradient

for a particular ion (Na+, K+) is the sum of chemcial and electrical forces acting on the ion across a plasma membrane; a form of potential energy

anaxonic neurons

found in brain and sense organs (axons and dendrites difficult to distinguish)

unipolar (pseuodunipolar) neurons

found in sensory neurons of PNS, have fused dendrite and axon with the cell body on one side

bipolar neurons

found in special sensory organs (sight, smell hearing), have 1 dendrite and 1 axon

rate of generation of action potentials

frequency of action potentials depends on the degree of depolarization above threshold; holding membrane above threshold level: has the same effect as a second, larger stimulus; reduced relative refractory period; if the initial segment of the axon is no longer int the absolute refractory period and the membrane is still depolarized above the threshold, an action potential will be generated

1. excitatory postsynaptic potential (EPSP)

graded depolarization of postsynaptic membrane

2. inhibitory postsynaptic potential (IPSP)

graded hyperpolarization of postsynaptic membrane

postsynaptic potentials

graded potentials developed in a postsynaptic cell; in response to neurotransmitters

parasympathetic division

has a relaxing effect

sympathetic division

has a stimulating effect; fight or flight response

all-or-none potential

if a stimulus exceeds threshold amount; action potential is triggered; action potential is the same no matter how large the stimulus; if the axon hillock does not reach the threshold voltage, an action potential will not be generated

information is relayed in the form of action potentials

in general, the degree of sensory stimulation or the strength of the motor response is proportional to the frequency of action potentials

membrane potential rises or falls

in response to temporary changes in membrane permeability; resulting from opening or closing specific membrane channels; the channels only allow their specific ion to enter/leave; (wheather they enter or leave depends on the concentraiton gradient- they move from [higher] to [lower]; passive channels are always open; gated channels allow the ions to go in or out; more K+ channels= more negative inside; more Na+ channels= more positive inside

multipolar neurons

include all skeletal muscle motor neurons, have many dendrites but 1 axon; common in the CNS

the nervous system

includes all neural tissue in the body; neural tissue contains two kinds of cells neurons and neurogila

Peripheral Nervous System (PNS)

includes all neural tissue outside the CNS

hyperpolarization

increasing the negativity of the resting potential (causes cell to become more negative than the resting membrane potential); result of opening a potassium channel; opposite effect of opening a sodium channel; net movement of positive ions out, not into cell

frequency of action potentials

information received by a postsynaptic ell may be simply the frequency of action potentials received

Gamma Aminobutyric Acid (GABA)

inhibitory effect (about 20% of neurons in brain release it); functions in CNS not well understood, GABA release may reduce anxiety (some antianxiety drugs enhance GABA release)

initiating action potential

initial stimulus; a graded depolarization of axon hillock large enough (10 to 15 mV) to change resting potential (-70mV) to threshold level of voltage-gated sodium channels (-60 to -55 mV); at -60mV voltage gated sodium channels open, (quick spike up) sodium comes in, more and more positive, now inside becomes positive, sodium channels close at positive 30 mV; resting membrane potential is -70 mV; rest sodium channels has out gate that is closed and inner gate that is open, the outer gate and inner is open, then at the peak, the outer is open and the inner is closed (outer is activation gate, inner gate is inactivation gate); goes down because of voltage gate potassium channels open at positive 30 mv; outer gate is open, they are slow to close so it dips below the resting potential; sodium leaks back in through the leak channels to go back to -70 mV; the graded depolarization can be from one source or multiple sources as long as it depolarizes the axon hillock to threshold

3. indirect effects

intracellular receptors; lipid-soluble gases (NO, CO)- diffuse across plasma membrane, bond to enzymes in brain cells to alter activity inside the cell

axon diameter and propagation speed

ion movement is related to cytoplasm concentration; axon diameter affects action potential speed; the larger the diameter, the lower the resistance; lower resistance= faster movement of ions and action potential travels faster

1. direct effects

ionotropic effects- alters ion movement through the plasma membrane; open/close gated ion channels (lock and key)

sodium-potassium ATPase (exchange pump)

is powered by ATP; carries 3 Na+ out and 2 K+ in; balances passive forces of diffusion (occurs through passive channels as Na+ leak back in and K+ leak back out); maintains resting potential (-70 mV)

astrocytes

large cell bodies with many processes; largest, most numerous neurogila

steps in propagation

local current 2 travels to segment 3 and also to segment 1; segment 3 depolarizes to threshold and AP is formed; segment 1 depolarizes, but no AP formed since channels there are in refractory period

axon

long, single; sends information (electrical signals) away from the soma towards a target cell; long process that carries electrical signals (action potentials) to a target; structure is critical to function: axoplasm, axolemma, and axon hillock

information processing at the simplest level (individual neurons)

many dendrites receive neurotransmitter messages simultaneously ; some excitatory, some inhibitory

limited regeneration in CNS

many more axons likely to be involved (situation more complicated); astrocytes limit regeneration by: releasing chemicals that block regrowth of axons; Producing scar tissue (physically interfere with regrowth of axon)

ganglia

masses of neuron cell bodies; surrounded by neuroglia; found in the PNS

sodium and potassium channels

membrane permeability to Na+ and K+ determines membrane potential; they are either passive or active: passive channels (leak channels) and active channels (gated channels)

relative refractory period

membrane potential almost normal; very large stimulus can initiate action potential; larger stimulus needed to cause more Na+ entry since relative refractory period occurs when K+ is still exiting the cell during the repolarization phase; K channels open inside is becoming more and negative, the graded potentials need to be strong to activate the axon hillock

interoeptors

monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive); internal senses (taste, deep pressure, pain)

propriceptors

monitor position and movement (skeletal muscles and joints)

electrical current

movement of charges to eliminate potential difference; opposite charges attract, like charges repel

propagation (propagation of action potentials)

moves action potentials generated in axon hillock along entire length of axon towards the axon terminal

2. spatial summation

multiple locations; many stimuli, arrive at multiple synapses- can be EPSP's and IPSP's since from multiple synapses

1. temporal summation

multiple times; rapid, repeated stimuli at one synapse- because from only one synapse, only either EPSP's or IPSP's (not both, since the synapse causes only one or the other, not both)

myelination

myelin insulates myelinated axons; increases speed of action potentials; makes nerves appear white

type A fibers

myelinated; large diameter; high speed (120 m/sec)- approximately the length of a football field in 1 second; carry rapid information to/from CNS; for example, (to CNS) position, balance, touch, and (from CNS) motor impulses

type B fibers

myelinated; medium diameter; medium speed (18 m/sec)- a bot more than half the length of a basketball court in 1 second; carry intermediate signals; for example, sensory information (from viscera), peripheral effectors

summation of EPSPs and IPSPs

neuromodulators and hormones; can change membrane sensitivity to neurotransmitters; shifting balance between EPSPs and IPSPs

opioids

neuromodulators int he CNS; bind to the same receptors as opium or morphine; relieve pain- inhibits release of the neuropeptide substance P at synapses that relay pain sensations

types of chemical snapses

neuromuscular junction neuroglandular junction

presynaptic cell

neuron that send message

axoplasmic transport

neurotubules within the axon; transport raw materials; between cell body and axon terminal; powered by mitochondria, kinesin, and dynein

synaptic fatigue

occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli; formation of new ACh from recycled choline is not fast enough to meet demand; too many action potentials reaching the axon terminal, not enough NT produced for exocytosis synapse inactive until ACh is replenished

active channels (gated channels)

open and close in response to stimuli; at resting potential, most gated channels are closed

1. chemically gated channels

open in presence of specific chemical (Arch- acetylcholine) at a binding site; found on neuron cell body and dendrites

the resting state

opening sodium channel produces graded potential; resting membrane exposed to chemical; sodium ions enter the cell; membrane potential rises; depolarization occurs- why? (closer and closer to 0; there is no change with the outside of the cell) remember, more Na+ on outside, so when opened, Na+ flows into cell through the channel

neuromodulators

other chemicals released by synaptic terminals; similar in function to neurotransmitters

important neurotransmitters

other than acetylcholine; *norepinephrine (NE)*- biogenic amine; *dopamine*- biogenetic amine; *serotonin*- biogeneic amine; *gamma aminobutyric acid (GABA)*- amino acid

hyperpolarization is more negative= no action potential

over all negative then no action potential

depolarization is more positive= then yes action potential

overall positive then action potential can happen

neuroglia

preserve physical and biochemical structure of neural tissue

action potentials

propagated changes in membrane potential; affect an entire excitable membrane; link graded potentials at cell body with motor end plate actions

*step 2*: activation of Na+ channels

rapid depolarization; voltage gated Na+ channels open allowing net floe of Na+ ions into cytoplasm; inner membrane of that area changes from negative to positive when enough Na+ ions enter; never going to exceed +30 mV

receptors and effectors of the PNS

receptors and effectors

white matter

regions of CNS with many myelianted nerves/axons; appears white because of myelin

synaptic activity

releases neurotransmitters at presynaptic membrane; neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic cell; produces graded potentials in postsynaptic membrane

2. cells have selectively permeable membranes

remember, water soluble substance including ions cannot pass through hydrophobic region of lipid bilayer, need channel/carrier protein

2. voltage-gated channels

respond to changes in membrane potential; (a change in membrane potential causes the channels to open or close) have activation gates and inactivation gates; Na+ has two gates while K+ only has one gate (only the activation gate); Characteristic of excitable membrane; found in neural axons, skeletal muscle sarcolemma, cardiac muscle

3. mechanically gated channels

respond to membrane distortion; found in sensory receptors (touch, pressure, vibration)

information processing

response (integration of stumuli) of postsynaptic cell to the graded potentials that have been generated

step 2: depolarizes second segment to thresold

second segment develops action potential

functions of the CNS are to process and coordinate

sensory data from inside and outside body; motor commands control activities of peripheral organs (skeletal muscles); higher functions of brain: intelligence, memory, learning, emotion

dendrites

short, branched; used to receive information; highly branches, each branch contains studded processes called dendritic spines

chemical synapses

signal transmitted across a gap by chemical neurotransmitters; are found in most synapses between neurons and all synapses between neurons and other cells; cells not in direct contact- there is a physical gap between the pre- and postsynaptic cells (synaptic cleft); action potential may or may not be propagated to postsynaptic cell, depending on: amount of neurotransmitter released; sensitivity or postsynaptic cell

neuropeptides

small peptide chains; neuromodulators that bind to receptors and activate cytoplasmic enzymes

oligodendrocytes

smaller cell bodies with fewer processes that wrap around axons of neurons; produces myelin and wraps axons of some CNS neurons

microglia

smallest and least numerous neuroglia with many fine-branched processes; least numerous and smallest neuroglia; migrate through neural tissue; phagocytic cells that clean up cellular debris, waste products, and pathogens; Originate from stem cells related to stem cells that produce monocytes and macrophages

what happens at the peak of the action potential curve?

sodium channels inactive, preventing sodium from flowing into the cell

absolute refractory period

sodium channels open or inactivated; no action potential possible, not even with a very strong stimulus

cell body

soma; large nucleus and nucleolus; perikaryon- cytoplasm; mitochondria- produce energy; rough endoplasmic reticulum and ribosomes- produce neurotransmitters

Nodes of Ranvier

space between internodes where axons is exposed to extracellular fluid

axolemma

specialized cell membrane; covers the axoplasm

neuroglia are essential to:

survival and function of neurons

neuroglandular junction

synapse between neuron and gland

neuromuscular junction

synapse between neuron and muscle

axoaxonic synapses

synapses between the axons of two neurons

the neurotransmitters released at a synapse may have either excitatory or inhibitory effects

the effect on the axon's initial segment reflects a summation of all of the stimuli that arrive at any moment; the frequency of generation of action potentials is an indication of the degree of sustained depolarization at the axon hillock

resting potential

the membrane potential of resting cell

the structure of neurons

the multipolar neuron; common in the CNS: cell body, dendrites, and axon

the synaptic cleft

the small gap that separates the presynaptic membrane and the postsynaptic membrane; (average 20 nm- that is 50,000 times smaller than 1mm)

which of the following controls the skeletal muscles?

the somatic nervous sytem

chemical synapse

the synaptic terminal releases a neurotransmitter that binds to the postsynaptic plasma membrane; produces temporary, localized changes in permeability or function of postsynaptic cell; changes affect cell, depending on nature and number of stimulated receptors

refractory period

the time period; from beginning of action potential to return to resting state during which membrane will not respond normally to additional stimuli; because of the Na+ channel- absolute refractory period; when Na+ channel open the inactivation gate is closed and is not voltage sensitive so nothing happens; chemically gates channels are on the cell body the allow ion the cells; voltage gated channels are at the axon hillock

axon hillock

thick section of cell body found prior to axon; attaches to initial segment of axon to soma; initial segment important to formation of an action potential

the membrane potential

three important concepts 1. the extracelluar fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition 2. cells have selectively permeable membranes 3. membrane permeability varies by ion

axon (synaptic) terminals

tips of telodendria; also known as synaptic boutons, synaptic knobs; contains mitochondria, portions of endoplasmic reticulum, and vesicles containing neurotransmitters; release of neurotransmitter signals target cell

powering the sodium-potassium exchange pump

to maintain concentration gradients of Na+ and K+ over time; requires energy (1 ATP for each 2 K+/3 Na+ exchange)

summation

to trigger an action potential: one EPSP is not enough; EPSP (and/or IPSPs) combine through summation 1. temporal summation 2. spatial summation

dynein

transports materials from axon terminal to soma; retrograde flow; Rabies virus bypasses CNS defense by infecting the CNS through retrograde flow

kinesin

transports materials from soma to axon terminal; anterograde flow

structures of sensory neurons

unipolar (mostly); cell bodies grouped in sensory gangila; processes (afferent fibers) extend from sensory receptors to CNS; a ganglion is a collection of neuron cell bodies in the PNS

gray matter

unmyelinated areas of CNS; neuronal cell bodies dendrites, and unmyelinated axons

type C fibers

unmyelinated; small diameter; slow speed (1 m/sec); carry slower information; for example, involuntary muscle, gland controls; physically impossible to have all axons myelinated; if so, then nerves would be as thick as garden hoses and spinal cord as wide as a garbage can

which type of channel is/are used to generate and propagate the action potential?

voltage gated channel

repolarization

when the stimulus is removed, membrane potential returns to normal