A&P - Neuro 1

what is acetylcholinesterase

1 of 4 neurotransmitters (structure differs substantially from the others)

synthesized from acetate and choline and stored in synaptic vesicles

function of acetylcholinesterase

used in PNS to stimulate skeletal muscle, used in CNS to increase arousal

effect on target cell depends on receptor present (nicotinic or muscarinic)

mechanism for clearing acetylcholine from a synapse

action potential triggers its release into synaptic cleft - some briefly attaches to postsynaptic receptor - then cleared from cleft by being broken down to acetate and choline by acetylcholinesterase

acetate and choline then recycled by presynaptic cell

chronologic order of events in synaptic transmission

1. action potential arrives at axon terminal

2. voltage gated calcium channels open

3. neurotransmitter vesicles fuse w/ presynaptic membrane

4. neurotransmitter released into and diffuse across synaptic cleft

5. neurotransmitter binds to receptors on postsynaptic membrane

6. neurotransmitter diffuses across cleft

7. neurotransmitter bind to postsynaptic receptor

8. postsynaptic potential generated

effect of voltage gated calcium channel opening and neurotransmitter release at the synaptic knob

depolarization of membrane potential, neurotransmitters released into synaptic knob

what occurs when voltage gated K+ channels open in conductive segment

K+ flows out of neuron down concentration gradient causing repolarization of membrane potential (becomes more negative)

impact on resting membrane potential if there are no Na+ leak channels

if K+ only is leaked, RMP would be where K+ concentration and electrical gradients at equilibrium (-90mV)

relative concentration locations of potassium and sodium

higher concentration of K+ in cytosol vs interstitial fluid

higher concentration of Na+ in interstitial fluid vs cytosol

how repolarization and depolarization relate to Na+/K+ channel states (steps in depol and repol)

steps in depolarization:

1. reaching threshold - Na+ enters from adjacent region and voltage gated Na+ channels open

2. Na+ enters axon causing membrane to have positive potential

2b. inactivation state - Na+ channels close, becoming temporarily inactive (unable to open)

- steps repeat in adjacent regions and impulse moves toward synaptic knob -

steps in repolarization:

3. reaching threshold slowly opens K+ channels and K+ diffuses out, causing negative membrane potential

4. hyperpolarization - K+ channels stay open for a longer time so K+ exit makes cell more negative than RMP

5. reestablishing RMP - K+ channels eventually close and RMP reestablished by leak channels and Na+/K+ pumps

- steps 3-5 repeat in adjacent regions as impulse moves toward synaptic knob -

order and definition of absolute and relative refractory periods

refractory period (in general): period of time after start of action potential when difficult or impossible to fire another action potential

- absolute refractory: ab 1 ms, no stimulus can initiate another action potential, Na+ channels are open then inactivated (ensures propogation goes toward synaptic knob and doesn't reverse direction)

- relative refractory: just after absolute, another action potential is possible (Na+ channels have reset) but minimum stimulus strength is now greater (some K+ channels are still open so cell is slightly hyperpolarized and further from threshold)

all or none law of action potentials

- if threshold reached, axon potential generated and propogated down axon w/o any loss in intensity

- if threshold not reached (subthreshold), voltage gated channels stay closed - no action potential

- axon shows same intensity of response to values greater than threshold

think of a gun: sufficient pressure on trigger makes it fire, insufficient not fired; firing is the same no matter how hard you squeeze the trigger

factors allowing for the fastest nerve impulse propogation

conduction speed depends on axon diamater and myelination; thicker fibers faster than thin (less resistance) and myelinated faster than unmyelinated

function of myelin and which cells produce it in CNS/PNS

myelin: several layers of membrane of glial cells; high lipid content gives glossy white appearance and insulates axon (responsible for appearance of white matter in brain and spinal cord)

glia are neuromyelocytes in PNS and oligodendrocytes are neuromyelocytes in CNS

soma/ cell body of neuron

(aka perikaryon) spherical w/ triangular cone shaped axon hillock, plasma membrane enclosed cytoplasm, contains mucus and chromatophilic substance (nissl bodies) made of ribosomes (both free and bound)

dendrites of neuron

short unmyelinated processes branching of cell body of neuron; receive input and transfer it to cell body

axon of neuron

- long processes emanating from axon hillock (triangular region) of cell body of neuron

- cytoplasm termed axoplasm

- membrane termed axolemma

- spills into branches called axon collaterals

- tips of extensions are synaptic knobs which house synaptic vesicles containing neurotransmitter

- may be insulated by myelin sheath formed by glial cells (neurofibril nodes are the gaps btwn regions of myelin)

cytoskeleton of neuron

composed of microfilaments (actin), intermediate filaments (neurofilaments) which aggregate to form bundles called neurofibrils which provide tensile strength, and microtubules

multipolar neurons

many dendrites, one axon (mc type)

bipolar neurons

one dendrite, one axon (limited number, ex: retina)

unipolar neurons

(psedounipolar) one process extends from cell body, splits into 2 processes : peripheral process (splits into several receptive dendrites) and central process (leads to synaptic knobs in CNS)

mixed nerve

contain both sensory and motor (mc nerve)

autonomic motor division

- (visceral motor) sends involuntary commands to heart, smooth muscle, and glands

- has sympathetic and parasympathetc divisions

somatic motor division

detects stimuli we consciously perceive

somatic sensory division

detects stimuli we consciously perceive

visceral sensory division

detects stimuli we do not typically perceive

major glial cells

- astrocytes

- oligodendrocytes

- ependymal cells

- microglia

- satellite cells

- neurolemmocytes

glial cells

(neuroglia) nonexcitable support cells found in CNS and PNS, approximately same number of glial cells as neurons; accounts for about half the volume of the nervous system

astrocytes

- (star shaped) most abundant glial cell in CNS; have processes that end in vascular feet

- helps form BBB by wrapping perivascular feet around brain capillaries (controls which substances have access to brain)

- regulate tissue fluid composition (chemical environment around neurons, ex: K+ concentration forms structural support for nearby neurons, assists neuronal development, and alters synaptic cleft - add/eliminate/influence)

- occupy the space of dying neurons

ependymal cells

line cavities in brain and spinal cord; part of choroid plexus (which produces spinal fluid)

microglia

small phagocytic cells of immune system that wander CNS and replicate during infxn; remove synapses that are no longer needed

oligodendrocytes

large cells w/ slender extensions; extensions wrap around axon, forming myelin sheath

satellite cells

arranged around neuronal cell bodies in a ganglion; electrically insulate and regulate exchange of nutrients and wastes

neurelemmocytes

(schwann cells) elongated flat cells that ensheath PNS axons w/ myelin; allows for faster action potential propogation

how do glial cells differ from neurons

glial cells: nonexcitable supportive/regulatory roles - homeostasis, form myelin, schwann cells, structual support (support, protect, and nourish neurons)

neurons: excitable, transmit electrical and chemical signals - mood, muscle movement, sensory perception

what do all glial have in common

non excitable, supportive role, maintain homeostasis, do not form synapses for signal transmission (unlike neurons)

types of neuronal pools/circuits

converging, diverging, parallel-after-discharge, and reverberating

converging neuronal circuit

input converges at a single postsynaptic neuron (ex: multiple sensory input synapses on neurons in salivary nucleus - eg sight, sound, smell of cooking leads to single output of salivation)

diverging neuronal circuit

spreads info from one presynaptic neuron to several postsynaptic (ex: neurons in brain that control walking send commands to several different muscles for proper balance, posture, and motion)

reverberating neuronal circuit

uses feedback to produce repeated cyclical activity; once started, stays active until an inhibitory stimulus or synaptic fatigue (ex: circuits that keep regular breathing during sleep)

parallel after-discharge neuronal circuit

- input transmitted simultaneously along several paths to a postsynaptic cell

- since paths vary in number of synapses, signal arrives at postsynaptic cell at various times (believed to be involved in higher order thinking)

which division of the nervous system shows greater regenerative capacity

PNS

conditions for successful axon regeneration

- after traumatic injury, axon regeneration possible if neuron cell body is intact and enough neurilemma remains

- success more likely if amount of damage less extensive and distance btwn site of damage and structure it innervates is shorter

role of schwann cells/neurolemmocytes in PNS axon regrowth

steps of axon regeneration:

1. axon severed by trauma

2. proximal to the cut, axon seals off and swells

2b. distal to the cut, axon and sheath degenerate (wallerian degeneration) but neurilemma survives

3. neurilemma and endoneurium form a regeneration tube

4. axon regenerates, guided by nerve growth factors released by neurolemmocytes/schwann cells

5. axon reinnervates original effector or sensory receptor

graded potentials

small short lived changes in RMP established in receptive segment by opening chemically gated ion channels

vary in degree of change and direction of change of RMP - can be large or small, can cause depolarization (RMP becomes more positive) or hyperpolarization (RMP becomes more negative)

graded potentials in a postsynaptic neuron

postsynaptic neuron = excitatory postsynaptic potentials can be generated bc postsynaptic neuron can bind many neurotransmitters simultaneously

EPSP

excitatory post synaptic potential - post synaptic potential resuling in depolarization, depolarized by Na+ entry

IPSP

inhibitor post synaptic potential - post synaptic potential resulting in hyperpolarization, hyperpolarized by K+ entry or Cl- exit

summation of ESPS and ISPS

- occurs at axon hillock - sum may or may not reach threshold membrane potential for initiating action potential (typically ab 55 mV)

- if threshold reached, voltage gated ion channels open and action potential generated

ohm's law

current = voltage/resistance (I = v/r)

current increases w/ larger voltage and smaller resistance

- applied to neurons: current is generated when ions diffuse through channels (voltage exists across membrane d/t unequal distribution of ions)

- membrane offers resistance to ion flow which changes w/ actions of gated channels (resistance decreases when channels open)

typical resting membrane potential

-70 mV

tissue layers

3 connective tissue wrappings: epineurium, perineurium, endoneurium

epineurium

dense irregular CT enclosing entire nerve

perineurium

dense irregular CT that wraps fascicle (bundle of axons in nerve)

endoneurium

areolar CT that wraps an individual axon to separate and electrically insulate each

motor, sensory, and mixed nerves

- motor:

somatic - output to skeletal muscle

autonomic - output to cardiac muscle, smooth muscle, and glands

- sensory:

somatic - input from receptors of five senses and proprioceptors

visceral - input from receptors of internal organs eg heart and blood vessels

- mixed:

ex: Facial Nerve (CN VII) - contains both motor fibers to control facial muscles and glands, and sensory fibers responsible for taste and general sensation

neuron segments

receptive segment, initial segment, conduction segment, transmissive segment

receptive segment of neuron

binding of neurotransmitter released from presynaptic neurons; production of graded potential

initial segment of neuron

summation of graded potentials; initiation of action potential

conduction segment of neuron

propogation of action potentials

transmissive segment of neuron

release of neurotransmitter

steps after action potentials cause opening of voltage gated Ca+ channels

occuring at synaptic knob

1. Ca+ diffuses into knob (Ca+ pumps have established gradient)

2. Ca+ binds to proteins associated w/ synaptic vesicles and triggers vesicles to fuse w/ membrane

3. neurotransmitter is release via exocytosis

4. neurotransmitter binds to ligand receptors

5. only one type is released at a time, depending on frequency of action potentials that reach synaptic knob

synaptic knob

tips of axon branching extensions containing synaptic vesicles which contain neurotransmitters

interneurons

(aka association neurons)

- receive, process, and integrate info from many other neurons

- communicate btwn sensory and motor neurons

- usually multipolar, located w/in CNS, and make up 99% of our neurons

peripheral axon regeneration is possible if what remains

neuron cell body is intact and enough neurolemma remains

label image; click on image first, test yourself, then check definition

left to right:

receptive segment

initial segment

conductive segment

transmissive segment