Unit 4- The Nervous System

Nervous system organization and structure, Action potentials --

Chapter 12

Nervous system functions ____________ function- sensory receptors detect

internal & external stimuli Sensory = afferent brain, spinal cord

increase in blood acidity a raindrop landing on your arm

____________ function- integrates sensory info analyzing & storing some info making decisions regarding appropriate responses-

interneurons ____________ function- responding to integration

decisions motor = efferent carry info out of CNS to effector

Properties of the nervous system

Shares responsibility w/ endocrine system in maintaining homeostasis.

Rapidly responds to stimuli transmits nerve impulses to adjust body

processes Responsible for perceptions, behaviors, &

memories and initiates voluntary movements

Structure of neuron, fig 12.3

______________- nerve cell consists of a cell body, dendrites, and an axon.

______________ – “little trees” neuronal process that carries nerve impulse toward cell

body _______________–usually single, long process of a

nerve cell that propagates an nerve impulse towards the __________________: toward other nerve cells, muscles fibers, or gland cells.

________________- contains nucleus surrounded by cytoplasm and typical organelles

___________________- cone shaped elevation where axon joins cell body

________________- where the impulse arises, then impulse conducts along the axon

__________________- insulation by multiple layers of protein and lipid increases speed of impulse conduction Done by Schwann cells in PNS

_____________________- a space along a myelinated nerve fiber between Schwann cells

_________________- the peripheral, nucleated cytoplasmic layer of the Schwann cell AKA - sheath of Schwann

Neurons: structural fig 12.4 ________________- several dendrites, one axon

most neurons in brain & spinal cord _______________- one main dendrite, one axon

retina of the eye, inner ear, & olfactory area of brain. ________________- sensory neurons, originate

in embryo as bipolar neurons During development dendrite and axon fuse Both branches characteristic of structure & function of

axon- long, cylindrical processes to propogate AP Dendrites from periphery, monitor sensory

stimuli (touch or stretching) Axon CNS

Neuroglia- table 12.1, fig 12.6

___________= “glue” 1/2 the volume of CNS, support cells Smaller than neurons & 5-50x more Can multiply and divide in mature NS Can fill space formerly occupied by neuron _____________- brain tumors from glial cells

Highly malignant, grow rapidly 6 types: 4 in CNS, 2 in PNS

CNS glial cells, table 12.1

__________- “star,” maintain appropriate chemical environment for impulses regulate [K+] Provide nutrients for neurons Take up excess neurotransmitters Assist w/neuron migration during brain develop Blood-brain barrier

_____________________- “few trees,”- support network around CNS neurons Myelin sheath in CNS CNS- little regrowth after injury (more later)

CNS glial cells (2)

________- protect CNS cells from disease by engulfing and invading microbes Migrate to areas of injured nerve tissue

Clear away debris of dead cells, and may kill healthy cells

__________________- line ventricles of brain and central canal of spinal cord Assist in circulation of cerebrospinal fluid

PNS glial cells, figure 12.7

_____________ - participate in regeneration of PNS axons (neurolemma aids in regeneration of PNS axons) unmyelinated axons surrounds multiple axons

with single layer of plasma membrane Not myelinated, but enclosed by the Schwann cell

myelinated axons- produces part of the myelin sheath around a single axon

______________- support neurons in PNS ganglia

Excitability of membrane, 12.12

Excitable cells communicate with one another by electrical signals Action potentials

_____________________ = voltage difference across the membrane

________________________________ = voltage difference between inside and outside of cell membrane when not responding to stimulus In many neurons and muscle fibers = -70 to –90 mV Inside of cell negative (w.r.t outside of cell)

Current is flow of charged particles in cells = ions are the charged particles

AP occur in neurons because: Many different ion channels Ion channels open and close in response to

specific stimuli Stimulus is a change in environment strong enough to

initiate an AP phospholipid bilayer = good insulator

Current flow thru ion channels

Ion channels Ions move across membrane down electrochemical

gradient: High to low concentration Positive negative, negativepositive

Opposites attract Current changes membrane potential (voltage

across the membrane) AP travels (or propagates along cell) due to flow of

ions thru channels Ion channels open and close due to gates

Gate= part of protein channel: shuts or opens pore 4 types of ion channels

Types of ion channel

1. ____________ – gates randomly alternate between open and closed more K+ ion leakage channels (than Na+)

2. ____________ (12.11a)- open in response to change in membrane potential generation & conduction of AP

3. _________________ – open/closes due to mechanical stimulation Vibration, pressure, tissue stretch

Types continued

4. _______________ (12.11b)– opens/closes due to specific chemical stimulus Neurotransmitters, hormones, specific ions

Ex. Acetylcholine: opens cation channels so Na+ and Ca2+ can move in, K+ can move out

Ligand can: Open or close by binding a portion of the protein

channel Indirectly activate by signaling a G-protein (18.4)

Resting membrane potential

Figure 12.12 Small build up of negative ions inside

Equal buildup of positive ions on ECM side Separation of such electrical charges = potential

energy (in volts… usually mV) Potential energy- potential to move

The > difference in charge, the > the membrane potential (voltage) Neuron resting mem potential: -40 to-90mV (-70mV) If cell exhibits membrane potential then is “polarized”

Most body cells polarized, potential varies

Summary: resting mem. potential

polarized typically around -70mV inside negative, outside positive higher [Na+] outside than inside higher [K+] inside than outside

2 conditions allow maintenance of resting membrane potential in excitable cells: Unequal distribution of ions across the plasma

membrane ECF rich in Na+ and Cl- Cytosol- main ion is K+

Anions are phosphates & amino acids in proteins Relative permeability of plasma membrane to Na+

and K+ At rest in neuron or muscle fiber, permeability to K+ is

50-100X greater than Na+ due to leak channels

Sodium (Na+)

Electrical & concentration gradients promote Na+ inflow Negative interior attracts cations (more Na+ECF) Na+ leak is slow, but would eventually destroy

gradient Na+/K+ pump counteracts the Na+ slow leak from

affecting the resting membrane potential

Graded potential

small deviation from membrane potential that makes the membrane more or less polarized (Na+ and Ca++ in, and K+ out)

occur in the dendrites and cell body of the motor neuron, if reach the axon:

voltage-gated ion channels openAP

Action potential, fig 12.14-16

Sequence of rapidly occuring events that happen in 2 phases: Depolarizing phase- negative membrane potential

decreases toward zero & eventually becomes positive.

Repolarizing phase- restores resting mem. potential to –70mV

2 types of voltage-gated ion channels open then close (Na+ gates, K+ gates) Channels present mainly in axon & axon terminals

AP – basic sequence of events

1st: Na+ channels open Na+ rush into cell

Begins depolarization phase

2nd: K+ channels open K+ flow out

Begins repolarizing phase

Together these 2 phase last 1 msec

All or none principle

Depolarization must reach a certain level for an AP to occur Threshold – the membrane potential that must be reached

in order to trigger an AP -55mV in most neurons

The voltage gates will open AP that is always the same size occurs Analogy: hit the first domino:

Strong or weak hit, as long as it knocks over… All or none- action potential happens or it doesn’t (all

dominos fall or none do)

Depolarization Threshold reached,Na+ channels open rapidly Gradient favors Na+ inward movement

Membrane potential –55mV = +30 mV Depolarized=MORE positive inside than outside

Na+ channels: 2 separate gates Activation gate and Inactivation gate

At resting state: inactivation open, activation gate is CLOSED Na+ cannot move into cell thru these channels

Activated state: both gates open, Na+ in + feedback: as more depolarized, more open

Shortly after activation gates open, inactivation CLOSE Channel now: inactivated state Less than 1 msec, 20,000 Na+ in, change mem

potential considerably BUT, [Na+] hardly changes because of millions of Na+

present in nearby ECF Na+/K+ pump can easily bail out Na+ to then maintain

low Na+ inside cell

Repolarization Depolarization also opens voltage gated K+

channels K+ channels open more slowly

K+ channels open when Na+ closing This causes REPOLARIZATION

Na+ channels inactive, Na+ inflow slows K+ channel open, K+ outflow accelerates

Membrane potential ∆ +30 to –70 mV Inactivated Na+ channels return to resting state If outflow K+ large enoughhyperpolarization:

Membrane more permeable to K+ than at resting (-90mV)

Subthreshold stimulus – stimulus of such weak intensity its not strong enough to initiate AP

Refractory period – time period in which an excitable cell cannot respond to stimulus that is usually adequate to evoke an AP Absolute r.pd. – time during which a 2nd AP cannot be

initiated even with very strong stimulus coincides with Na+ channel activation & inactivation

Relative r. pd - 2nd AP can be initiated BUT only by a larger than normal stimulus voltage gated K+ channels still open after inactivated Na+

channels returned to resting state

Nerve impulse propagation

Nerve impulse must travel from trigger zone to axon terminals: Propagation or conduction = ability to conduct AP along the

p.m. Na+ ions flow in depolarization opens Na+ channels in

adjacent segments of membrane Nerve impulse self-propagates along the membrane (like

row of dominos) Nerve is in refractory behind the leading edge of impulse,

so normally the impulse moves in one direction.

Saltatory conduction, fig 12.16

Saltatory = leaping Propagation of AP along the exposed parts of a

myelinated axon. AP appears at successive Nodes of Ranvier

Seems to leap Uneven distribution of voltage-gated channels

Myelin sheath few there along the myelinated portion and many at the node

Current flows thru ECF surrounding sheath & thru cytosol inside axon until reaches next node Ionic flow continues down myelinated axon

Saltatory conduction (2)

Consequences: Leaping conduction

Impulse leaps from one area of axolemma to the next Smaller number of channels in general because

only opening channels at the nodes is more energetically efficient Only a small area of axolemma has to depolarize and

repolarize

Signal transmission at synapse

Synapse- functional junction between 2 neurons, neuron & effector Can be chemical or electrical

Both differ structurally and functionally allow info to be communicated, filtered and integrated Synapses can change

To allow learning Diseases and neurological disorders can result Sites of action for theraputic & addictive chemicals

Presynaptic- sending message Postsynaptic- receiving message

Electrical synapses AP conduct directly between adjacents cells at gap

junctions: tunnels to allow ion flow visercal smooth muscle, cardiac muscle, developing

embryo, some in CNS Advantages to electrical:

Faster communication than chemical Pass directly from pre to postsynaptic cell

Synchronization of activity of a group of neurons or muscle fibers In unison due to connection by gap junctions Heart beat, coordination of smooth muscle contraction in GI-

peristalsis

Chemical synapse

Occurs since p.m. of pre & post synaptic cell are not touching Synaptic cleft- space between, filled w/ interstitial fluid

Nerve impulses cannot conduct across Presynaptic releases neurotransmitter:

Diffuses across Binds receptor on p.m. of postsynaptic neuron Postsynaptic potential is produced

Presynaptic converts electrical signal to chemical signal

Synaptic delay – about 0.5 msec

Typical chemical synapse, 12.17

Impulse arrives at synaptic end bulb Depolarizing phase opens voltage-gated Ca2+

channels present at end bulb [Ca2+] in ECF, Ca2+ flows inward

[Ca2+] inside presynaptic signals to trigger exocytosis of synaptic vesicles Vesicle merge w/ neuron plasma membrane Neurotranmitters released into synaptic cleft

Each vesicle several thousand neurotransmitters

NT diffuse cleft & bind to postsynaptic receptors

Excitatory postsynaptic potential

EPSP Depolarizing postsynaptic potential Often result of opening of cation channels

Na+ (this inflow being greater than the following) K+ Ca2+ Single EPSP not always cause an impulse BUT

makes cell more excitable b/c partially depolarized

Inhibitory postsynaptic potential

IPSP Hyperpolarization of postsynaptic membrane Generation of AP more difficult than usual

because membrane potential is more negative than at resting

Often result of opening ligand-gated channels: Cl- (diffuse in) K+ (diffuse out)

Nerve regeneration, fig 12.20

Plasticity: change based on experiences New dendrites, new proteins, new synapses

Limited regeneration (replicate or repair) PNS: dendrite & myelinated axon can be repaired if:

Cell body intact, schwann cell active, there is slow scar tissue formation

Neurolemma remains though part axon & sheath deteriorated

CNS: little or no repair Inhibitory influence of neuroglia

Oligodendrocytes have no neurolemma like schwann CNS myelin is a factor in inhibiting regeneration Scar tissue due to rapid astrocyte proliferation creates barrier

Adults: absence of growth stimulating cues (unlike fetus)

Neural circuits, fig 12.19

Complicated networks CNS contains billions Functional group- processes specific kind of info

Simple series circuit= presynaptic neuron stimulated only one postsynaptic neuron, 2nd stimulates one other, and so on

Diverging circuit= presynaptic neuron synapses w/ several postsynaptic Also stimulate several cells along the circuit Amplify signal

Figure 12.19 continued…

Converging circuit= several presynaptic neurons synapse w/ a single postsynaptic Receiving input from several diff. Sources Motor neuron receiving info from many areas of the

brain Reverberating circuit= stimulate presynaptic

postsynaptic to send a series of nerve impulses. Inhibitory neuron may turn off after time Breathing, coordinated muscular activities, waking,

short term memory Parallel after-discharge circuit= single presynaptic cell

stimulates group of neurons each which synapse with common postsynaptic Precise activities such as math calculation