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