Neuro I White

7/24/2019 Neuro I White

1/30

The Nervous System Part I - Chapter 11Develops from Ectoderm

Functions of NS - receive, process, and decide weather or not to respond to stimuli- think about it - everything you do, everything you are, everything you think, feel,

believe....its all in your mind...ah, brain

Sensory Input - information is gathered by the nervous system

Integration - this information is processed by the nervous system then it "decides" if to

respond or not

Motor Output - The response that the nervous system makes as a reaction to the

information that it receives

Effector Organs - The structures that carry out the motor output of the nervous

system, e.g. muscles

Central Nervous System (CNS) - Brain + Spinal Cord


2/30

Nerve - humungous bundle of neurons (mostly the axons)

1. Spinal Nerves - to and from the spinal cord

2. Cranial Nerves - to and from the brain (and exit the cranial vault)

.

Peripheral Nervous System (PNS) - the rest of the nervous system

(1) Sensory or Afferent - info towards CNS

(a) Somatic Afferent/Sensory - from skin, muscles, bones, joints

(b) Visceral Afferent/Sensory - from organs

(2) Motor or Efferent - info towards target (effector) organs

(a) Somatic Efferent/Motor - From CNS to skeletal muscles, voluntary

(b) Autonomic or Visceral Efferent/Motor - From CNS to glands, heart, smooth

muscle - automatic, not voluntary

(1) Sympathetic - "Fight or Flight", adrenalin, etc.

(2) Parasympathetic - digestion, urination, defecation, etc.


3/30

Figure 11.2

Central nervous system (CNS)

Brain and spinal cord

Integrative and control centers

Peripheral nervous system (PNS)

Cranial nerves and spinal nerves

Communication lines between the

CNS and the rest of the body

Parasympathetic

division

Conserves energy

Promotes house-

keeping functions

during rest

Motor (efferent) divisionMotor nerve fibers

Conducts impulses from the CNS

to effectors (muscles and glands)

Sensory (afferent) divisionSomatic and visceral sensory

nerve fibersConducts impulses from

receptors to the CNS

Somatic nervous

system

Somatic motor

(voluntary)Conducts impulses

from the CNS to

skeletal muscles

Sympathetic division

Mobilizes body

systems during activity

Autonomic nervous

system (ANS)

Visceral motor

(involuntary)Conducts impulses

from the CNS to

cardiac muscles,

smooth muscles,

and glands

Structure

Function

Sensory (afferent)division of PNSMotor (efferent)

division of PNS

Somatic sensory

fiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

StomachSkeletal

muscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS


4/30

Neuroglial or Glial Cells - about 50 times more abundant than neurons

(1) Astrocytes - In CNS - cause endothelia of brain capillaries to form the blood brain

barrier

- most abundant glial cell of CNS

- regulate exchanges between neurons and capillaries

- regulate migration of newly formed neurons

- help make synaptic connections between neurons

- regulate brain capillary permeability

- remove leaked potassium ions (potentially toxic ions)

- recycle neurotransmitters released from neuron

(2) Microglia - In CNS - macrophages of CNS - Macrophages are phagocytes

(3) Ependyma - In CNS - ciliated, line ventricles and central canal of Spinal Cord

(4) Oligodendrocytes - In CNS - make myelin sheath in CNS

(5) Satellite Cells In PNS - surround soma ...function unknown

(6) Schwann Cells - In PNS -make myelin sheath


5/30

Neuron = nerve cell

Anatomy of neurons

(1) Soma - cell body of neuron

(2) Nissl bodies - Rough ER - so lots of proteins being made

(3) Neurofibrils - cytoskeleton for keeping cell's shape

(4) Dendrite - info/message/impulse all move towards soma - THESE ARE THE

RECEPTIVE/INPUT REGIONS

- up to 100,000 in some neurons


6/30

Anatomy of neurons CONTd

(5) Axon - info/message/impulse all move away from soma

(a) Axon collateral - branches from the main axon

(b) Axolemma - cell membrane of axon

(c) Axoplasm - cytoplasm of axon

(d) Axon hillock - base of axon, where action potentials are generated

(e) Termial branches - branches near the end of an axon up to 10,000 in some

neurons

(f) Terminal = synaptic knob - distal foot-like end of axon

You may have glanced over the red numbers above, but think about it...one

neuron can have 100,000 dendrites and 10,000 terminal branches which means it

connects to 110,000 other neurons


7/30Figure 11.4b

Dendrites

(receptive regions)

Cell body

(biosynthetic center

and receptive region)

Nucleolus

NucleusNissl bodies

Axon

(impulse generating

and conducting region)

Axon hillock

NeurilemmaTerminal

branches

Node of Ranvier

Impulse

direction

Schwann cell

(one inter-

node)

Axon

terminals

(secretory

region)(b)


8/30

Axonal Transport the way structures get from the soma to the terminal and back

(1) Antegrade - away from soma towards terminal

- mitochondria and synaptic vesicles (carry neurotransmitters) to terminal

- fast = 20 - 400 mm/day

- enzymes & cytoskeletal components

- slow - 0.5 - 10 mm/day

(2) Retrograde - back towards soma

- empty synaptic vesicles (organelles) back to soma for refill

- manner in which diseases get to CNS: tetanus, herpes, rabies, polio


9/30

Myelin Sheath - phospholipid insulation around axon which speeds up impulse

- Neurolemma - outermost layer of sheath consisting of exposed cell membrane of

Schwann Cell or Oligodendrocyte

- Nodes of Ranvier - breaks in sheath along the axon

- Saltatory Conduction - "jumping conduction" - action potentials jump from node to

node to speed up impulse from 2 m/s in Unmyelinated neurons to 120 m/s in

myelinated neurons (60 times faster)


10/30Figure 11.5a

(a) Myelination of a nerve

fiber (axon)

Schwann cell

cytoplasm

Axon

NeurilemmaMyelin sheath

Schwann cellnucleus

Schwann cell

plasma membrane

3

A Schwann cell

envelopes an axon.

The Schwann cell then

rotates around the axon,wrapping its plasma

membrane loosely around

it in successive layers.

The Schwann cellcytoplasm is forced from

between the membranes.

The tight membrane

wrappings surrounding

the axon form the myelin

sheath.


11/30

Structural Classification of Neurons & where found

(1) Multipolar- 1 axon, several dendrites; most common neuron (99%) type in

nervous system (NS)

(2) Bipolar- 1 axon, 1 dendrite; sensory such as olfactory, retina, inner ear

(3) Unipolar - 1 process that leaves soma; carry sensory info to spinal cord (SC)

Functional Classes of Neurons

(1) Sensory (afferent) - information towards CNS

(2) Interneurons - neurons between # 1 & 3. These "DECICED" weather or not to

respond to stimuli

- 99% of all neurons

(3) Motor- information towards target (effector) organs


12/30Table 11.1 (1 of 3)


13/30Table 11.1 (2 of 3)


14/30Table 11.1 (3 of 3)


15/30Figure 11.1

Sensory input

Motor output

Integration


16/30

Types of Channels - some discussed in transport

- Ligand-Gated Channels - gate or door of channel is opened when a ligand

(chemical) attaches

- Voltage-Gated Channels - gate or door of channel is opened when the charge on

the cell membrane changes such as in an action potential

- Mechanical-Gated Channels - gate/door of channel is opened in response to

mechanical factors such as touch, pressure,

- Leakage or Non-gated Channels - continuously open and leak ions


17/30

Electrochemical Gradient - electrical & chemical components - see notes on

transport

- Electrical Potential - difference in charge on inside and outside of cell.

- inside negative at rest....outside positive (electrical gradients)- more potassium inside, more sodium (and calcium and chloride) on outside

(chemical gradients)

- about 10x sodium outside, 12x calcium outside, 30x potassium inside, 500x

negatively charged proteins inside

- Current - influx or efflux of ions


18/30

- Resting Membrane Potential - The value of the electrical potential in millivolts - in

other words it is the difference in the voltage inside the cell compared to the outside

at rest which simply means the neuron is not generating an impulse

- Polar- at rest cells are polar i.e. positive on outside, negative on inside so they are

POLARIZED

I. Local or Graded Potentials - from dendrites to axon hillock - Sodium floods in, so

inside of cell less negative = Depolarization

- these potentials are the ones that will bring the cell (neuron) to "Threshold" OR

not, therefore, here the "decision" is made to fire the neuron or not

- these happen at the RECEPTIVE portion of the neuron (info comes in at the

dendrites) - so some sort of stimulus causes it


19/30

- Two Major Types of Local Potentials

1. Receptor Potential - because it occurs in a sensory receptor like a touch receptor

- will these bring the cell to "threshold"? Think about a mosquito landing on your

face. Sometimes you don't even know they are there because the insect does

not disturb the touch receptors enough to reach "threshold". Sodium does flood

in, but not enough. THEN, the beast stabs you with HER proboscis and injects

an anticoagulant, all of which causes a boat-load of sodium to flood in and now

the graded potential reaches "threshold" and causes an "ACTION POTENTIAL"

(nerve impulse) so the touch receptor fires and now you feel it.

2. Postsynaptic Potential - in a second neuron when the NT from the 1st neuron

binds to its receptors in the 2nd


20/30

Characteristics of Local Potentials

(1) Graded - varies in magnitude according to stimuli (may or may not bring neuron to

threshold)

(2) Decremental - decreases in magnitude w/ distance from source

(3) Reversible - if stop stimuli, cell will repolarize (becomes more negative again) toresting conditions

(4) Can be Stimulatroy (excitatory) Or Inhibitory

a. Excitatory Postsynaptic Potential (EPSP)

- Excitatory local potential at the postsynaptic neuron

- makes inside of neuron closer to threshold = easier to "fire"- more positive inside (sodium influx) OR less negative (chloride efflux)

or

b. Inhibitory Postsynaptic Potential (IPSP)

- Inhibitory local potential at the postsynaptic neuron- makes inside of cell more negative or farther away from threshold - harder to

fire

- more negative inside (potassium efflux or chloride influx)


21/30

II. Action Potential (AP) - from axon hillock to terminal

- If the local potential is strong enough (stimulus is strong enough), enough

POSITIVE sodium enters to bring the cell to Threshold - the point-of-no-return

and an action potential is generated

Stages of Action Potentials

1. Resting State

- more sodium outside, more potassium inside

- and negative inside, positive outside

- so the resing cell is POLARized

2. Depolarization - sodium influx so inside pos - pos on inside and outside so the

cell membrane is now DEPOLARIZED

3. Repolarization - potassium efflux so inside becomes negative again but now

the chemical gradient is all messed up

4. Hyperpolarization - too much potassium goes out so more negative than

normal resting cell

- The Sodium Pump - will restore the chemical gradient and help keep inside

negative because, 3 "+ Nas out for every 2 "- Ks in

5. back to Resting State


22/30

Action

potential

12

3

4

Resting state

Cell is polarized

Depolarization

Inside more positive as

Na+ floods in

Repolarization

Inside more negative again

As K+ floods out

Hyperpolarization

The big picture

1 1

2

3

4

Time (ms)

ThresholdMembranepotential(mV)

Figure 11.11 (1 of 5)


23/30

Characteristics of Action Potentials

(1) All-or-None - once the cell reaches threshold there is no stopping the AP - The AP

is NOT GRADED

(2) Nondecremental - no decrease in magnitude as it moves along the axon

(3) Irreversible - once you start it, you cannot stop it until it is all over

(4) Excitatory only - always results in NT release (for chemical synapses)

- this is not to say that the released NT is always excitatory, quite the contrary

(5) Propagation of AP

a. Continuous Conduction - on unmyelinated axons - the AP creeps along from

on Na+ channel to the next

- these Sodium channels are found just next to each other along the entire axon

so this may take some time

- as slow as 1 m/s (meters per second)only 2 miles per hour

b. Saltatory Conduction - on myelinated axons - the AP JUMPS (salta) from Node

to Node which speeds up the conduction of the impulse - up to 150 m/s 300

miles per hour


24/30

Refractory Period - time for neuron to reset

(1) Absolute - no AP can be generated - from beginning of AP to about half-way

through repolarization

(2) Relative - from 2nd half of repolarization through hyperpolarization period back to

resting

- although an AP may be generated, it requires a STRONGER THAN NORMAL

stimulus

Synapse - where two neurons meet (or neuron meets muscle cell, etc.)

- can be:

(1) Axodendritic - axon to dentrites

(2) Axosomatic - axon to soma

(3) Axoaxonic - axon to axon

(4) Dendrodentritic - dendrite to dendrite (5) Dendrosomatic - dendrite to soma

- Presynaptic Neuron - 1st neuron sending impulse towards the synaps

- Postsynaptic Neuron - 2nd neuron


25/30

Figure 11.16

Dendrites

Cell body

Axon

Axodendriticsynapses

Axosomatic

synapses

Cell body (soma) of

postsynaptic neuron

Axon

(b)

Axoaxonic synapses

Axosomatic

synapses

(a)


26/30

Types of Synapses:

1. Electrical - cells connect by gap junctions - AP goes straight through w/out

stopping

- very fast - seen in some neurons, smooth muscle, cardiac muscle, especially in

glial cells2. Chemical - most common form - as seen in the neuromuscular junction

- neurotransitter is released, diffuses across synaptic cleft and binds to its receptors

- review from muscles: synaptic vesicles, synaptic cleft

1.Alpha Motor Nerve's action potential (impulse) reaches Synaptic Knob

- this opens Voltage-Gated Calcium Channels allowing EXTRACELLULAR

Ca++ to flood into neuron

2. This Ca++ causes Exocytosis of Synaptic Vesicles releasing

Neurotransmitter(NT) into synaptic cleft.

3. NT diffuses across cleft to bind to its receptors

4. these receptors are Ligand-Gated Sodium/Potassium Channels which open

allowing Na+ INFLUX see depolarization below

5. Just after Na+ influx is a K+ Efflux

- this movement of ions traveling along the post synaptic nerve's cell membrane is the

ACTION POTENTIAL


27/30

Stopping Impulses

(1) Enzymatic Degradation - within synaptic cleft, enzymes such as

acetylcholinesterase destroy the NT's

(2) Re-uptake - The NT is pumped back into the synaptic vesicles, removing it from the

synapse- once back in vesicles, the NT's are destroyed by the enzyme Monoamine

Oxidase (MAO)

(3) Diffusion - NT simply diffuses away from synapse

Summation - process by which the local potentials (EPSPs) accumulate to reachthreshold

1. Temporal - (timing) A single neuron fires so quickly, it causes accumulation of

enough EPSP's to reach threshold in the postsynaptic neuron

2. Spacial - (space) Several presynaptic neurons fire at the same time to accumulate

enough EPSP's to reach threshold in the postsynaptic neuron


28/30

Facilitation - a neuron is partially depolarized so even though it does not reach

threshold, it is much easier to do so it takes less stimulus to reach threshold with the

next stimulus.

Synaptic Potentiation - repeated firing at a synapse makes it much easier for thepresynaptic neuron to cause a threshold in the postsynaptic neuron

Presynaptic Inhibition - 1st neuron causes IPSP in 2nd making it more difficult for the

2nd to fire


29/30

Neurotransmitters (NT's) - over 50 identified so far

1. Acetycholine - receptors are known as cholinergic

2. Biogenic Amines = Adrenergics (Why?) = Biogenic Amines

- aka Catacholamines or Catacholinergic

- NTs = Epinephrine (Adrenalin), Norepinephrine (Noradrenalin), and Dopamine- Indolamines - histamine, Serotonin

3. Amino Acids

- glycine and gamma amino butyric acid (GABA) - typically inhibitory

- GABA opens chloride channels allowing the negative chloride to diffuse into the

cell

- glycine is inhibited by strychnine which causes spastic paralysis

- glutamate and aspartate - typically excitatory


30/30

4. Neuropeptides

- beta-endorphin - endogenous OPIATE = pain killer - seen during a "runner's

high", during labor

- same receptors as morphine, heroin, etc.

- Substance P5. Purines - ATP and adenosine act as major NT's

6. Dissolved Gases such as Nitric Oxide

Neuromodulators - hormones, etc. that modify synaptic transmission.

- cause up or down regulation of NT receptors (increase or decrease the number of

receptors making the neuron more or less sensitive to the NT respectively

- change rate of NT production, release, and removal

Documents

Neuro I White