Chapter 11 fundamentals of the nervous system and nervous tissue

Nervous system
the master controlling and communicating system of the body

functions of the nervous system
sensory input-stimuli
integration-interpretation
motor output-response

central nervous system CNS
brain and spinal cord
integration and comand center

peripheral nervous system PNS
anything other than the brain and spinal cord
out on the edges
carries messages to and from the spinal cord communicates to the brain whats going on inside and outside the body

sensory (afferent) division of the PNS
sensory afferent fibers-carry info to the brain and spinal cord (skin, skeletal muscles, joints)
visceral afferent( one direction) fibers (viseral organs)

motor (efferent) division of the PNS
effector organs (brain sends its messages out to effector ogans via PNS)
paired nerves that help regulate the body

somatic nervous system (motor)
conscious control of skeletal muscles

autonomic nervous system (motor) ANS
regulates smooth muscle cardiac muscle and glands

sympathetic (ANS)
active
response to danger/scary adrenoline heart respitory elevated fight or flight gets everything kicked up

parasympathetic (ANS)
rest
everything goes down

the two principal cell types of the nervous system are
neurons
supporting cells

neurons
excitable cells that transmit electrical signals (depolarize throw action potential)

supporting cells
cells that surround and wrap neurons
called neuroglia or glial cells
don’t throw AP
structually and functionally important to cell

supporting cells (CNS) astrocytes= star
most abundant, versatile, and highly branched glial cells
they cling to neurons, synaptic endings, and cover capillaries
support and brace neurons (are soft tissue)
anchor neurons to nutrient supplies
guide migration of young neurons
control the chemical enviroment

blood brain barrier
don’t want the brain to come in direct contact or it will kill cells

supporting cells (CNS) microglia
small oval support cells with spiny processes
phagocytes that moniter the health of neurons (mobile move around look for what doesn’t belong)

supporting cells (CNS) ependymal cells
range in shape from squamous to columnar
line the central cavities of the brain and spinal column
circulate cerebral spinal fluid

supporting cells (CNS) oligodendrocytes
branched cells
wrap CNS nerve fibers
myelin sheath

supporting cells (PNS) satellite cells
surround neuron cell bodies

support cells (PNS) schwann cells
myelin sheath
surround nerve fibers of the PNS

neurons (nerve cells)
structural units of the nervous system
extreme longevity
amitotic
high metabolic rate

nerve cell body
contains the nucleus and a nucleolus
is the major biosynthetic
is the focal point for the outgrowth of neuronal processes
has no centrioles (hence its amitotic nature)
has well developed nissl bodies (rough ER)
contains an axon hillock-cone-shaped area from which axons arise

dendrites
receive input
graded potentials not action potentials
unmyelinated

axons
axon hillock generate and transmit action potentials
long axons are called nerve fibers
axon collaterals=branch
axon terminals branches 10,000 not unusual
myelinated

myelin sheath
fatty (protein-lipoid) segmented sheath around most long axons

myelin sheath functions to
protect the axon
electrically insulate fibers from one another
increase the speed of nerve impulse transmission

myelin sheath and neurilemma formation
formed by schwann cells in the PNS

schwann cell
envelopes an axon in a trough
encloses the axon with its plasma membrane
has concentric layers of membrane that make up the mtelin sheath

neurilemma
remaining nucleus and cytoplasm of a schwann cell

node of ranvier (neurofibral nodes)
between adjacent schwann cells
(1mm axon collaterals can emerge)

node of ranvier (unmyekinated axons)
schwann cells partially enclose 15 or more axons
no coiling

Axons of the CNS
myelinated and unmyelinated fibers
myelinated formed by oligodendrocytes (up to 60 axons)
nodes of ranvier are widely spaced
white matter (myelinated fibers)
grey matter (nerve cell bodies and unmyelinated fibers)

multipolar neurons
many processes extend from cell body all are dendrites except for a single axon
abundunt motor neurons
bridge gaps

bipolar
two processes extend from the cell body one is a fused dendrite the other is a axon
rare
special sense organs ex eyes ears

unipoloar
one extension
extends from cell body forms central and peripheral processes which comprise an axon

neurophysiology
sensative easy to stimulate

action potentials
electrical impulses carried along the length of axons
always the same regardless of stimulis
the unerlying functional feature of the nervous system

electrical current and the body
reflects the flow of ions rather than electrons (RMP)

types of plasma membrane ion channels
passive or leakage channels always open
chemically gated channels -open with binding of a specific neurotransmitter (no ATP reguired flow down)
voltage gated channels-open and close in response to membrane potential (constantly leak a little)
mechanically gated channels-open and close in response to physical deformation of receptors (stretched, squashed)

opperation of a chemically gated channel
Na+ and K+ gated channels
(NOT THE Na+ K+ PUMP)
neurotransmitter regulated
diffusion

operation of a voltage-gated channel
Na+channels
voltage controlled
when gate channels are open
ions move quickly along their electrochemical gradients
an electrical current created
voltage changes across the membrane

resting membrane potential (Vr)=RMP
it is generated by different concentrations of Na+ K+ Cl- and protien anions (A-)
-70mV
ionic differences are the consequence of
differential permeability of the neurilemma to Na+ an K+
operation of the sodium-potassium pump

membrane potential signals
used to recieve integrate and send information
membrane potential changes are produced by
changes in membrane permeability to ions
alterations of ion concentrations across the membrane
two types of signals (coming in short distances)
graded potentials (further you are peters off pond ripple)
usually incoming short distances-dendrites
action potentials (all exactly the same strength)
long distance axons

deploarization
the inside of the membrane becomes less negative or more positive

repolarization
the membrane returns to its RMP -70

hyperolarization
the inside of the membrane becomes more negative than the resting potential

graded potentials (info flows in)
short lived local changes in the membrane potential
decrease in intensity with distance
magnitude varies directly with the strength of the stimulis hence GRADED a little, a medium, a lot
sufficiently strong graded potentials can initiate AP
have to have a strong enough graded potential to communicate to the neuron to throw an action potential

APs
excitable membranes
only in muscle fibers and neurons
they do not decrease in strength over distance
they are principle means of neural communication
an action potential in the axon of a neuron is a nerve impulse
1. open up Na+ ion channels
2. close Na+ ion channels
3. open up K+ ion channels

action potential resting state
Na+ K+ channels are closed
leakage accounts for small movements of Na+ and K+
each Na+ channels has two voltage regulated gates (activation and inactivation)
K+ channels have one gate

activation gates
voltage sensative
closed in the resting state

inactivation gates
open in the resting state

action potential depolarization phase
Na+ permeability increases
membrane potential reverses
Na+ gates are opened K+ gates are cloosed
threshold- a critical level of depolarization (-55)
at threshold depolarization becomes self-generating
(sodium flows into cell opens quickly K opens slower)

action potential repolarization phase
Na+ inactivation gates close blocks inflow of Na+
membrane permeability to Na+ declines to resting levels
as Na+ gates close voltage sensative K+ gates open
K+ exits the cell and internal negativity of the resting neuron is restored
(K+ stays open a little longer than it needs to (undershooting) becomes negative -90mv hyperpolarization)

action potential hyperpolarization
K+ gates remain open causing as excessive efflux of K+
undershoot
the neuron is insensative to stimulis and depolarization during this time
Na+ channels reset (activation gates close)
Na+ K+ pump will reset ionic distribution (brings back up to -55mv RMP)

threshold and action potential
all or none phenomenon (+30mv)
the CNS determines stimulus intensity by the frequency of impulse transmission
greater the stimulis faster the AP are sent out the more it “hurts”

absolute refactory period
while generating an AP
Na+ channels are open
cannot respond to stimulis
the absolute refactory period
ensures that each action potential is sperate( can’t blend
enforces one-way transmission of nerve impulses

relative refactory period
following the absolute refractory period when
sodium gates are closed
potassium gates are open
repolarization is occurring
threshold level for AP is elevated
strong stimuli

saltatory conduction (jump faster)
mylonated
voltage-gated Na+ channels concentrated at nodes
APs are triggered only at the nodes and jump from one node to the other
30x faster than continuous conduction (swiming)
sensative areas have more nerve endings

nerve fiber classification
nerve fibers are classified according to
diameter, degree of myelination, speed of conduction
group A
somatic sensory and motor nuerons big thick myelin
300 mph (voluntary almost instantaniously)
group B and C
ANS visceral motor and sensory neurons
B-medium lightly myelinated 40mph
C-small unmyelinated 2pmh (involuntary continous
conduction)
bigger/more myelinated=faster conduction

synapses (all over the place)
a junction that mediates information transfer from one neuron
to another neuron
to an effector cell (muscle or gland)
presynaptic neuron (in neuron)
postsynaptic neuron

chemical synapses
Ca2+ channels
synaptic vesicles- neurotransmitters
undirectional communication
undirectional communication
synaptic delay- time needed to do this (0.3-5.0 ms)

postsynaptic potentials (excititory)
neurotransmitter receptors mediate changes in membrane potential according to
amount of nuerotransmitters released
amount of time the neurotransmitter is bound to receptor
the two types of postsynaptic potentials are
EPSP excitatory postsynaptic potentials
IPSP inhibitory postsynaptic potentials
(Cl or K channels)

excitatory postsynaptic potentials
EPSP are graded potentials that can initiate an action potential in an axon
chemically gated channels
Na+ and k+ flow in opposite directions at the same time
graded potential reaches axon hillock = AP

inhibitory synapses and IPSP’s
IPSP’s inhibit an AP
induce hyperpolarization
K+ Cl-
driven farther from threshold

summation
a single EPSP cannot induce an AP
EPSPs must summate to induce an AP
temporal summation- presynaptic neurons transmit impulses in rapid fire order
spatial summation postsynaptic neuron is stimulated by a large number of terminals at the same time
IPSPs can also summate with EPSP canceling each other out

common neurotransmitters
acetylcholine ACh (acetylcholinesterase AChE)
biogenic amines
amino acids
peptides
endorphines
novel messengers ATP and dissolved gases NO and CO
pain sensation memory and learning cGMP

biogenic amines
broadly distributed in the brain
play roles in emotional behaviors and our biological clock
catecholamines- dopamine norepinephrine (NE) “feel good”
indolamines- serotonin sleep appetite moods and histamine inflamation

neural integration neuronal pools
functional group of neurons that
intergrate incoming information
forward the processed information to its appropriate destination

mutiple sclerosis
an autoimmune disease that mainly affects young adults
sypmtoms: visual disturbances, weakness, loss of muscle
control, and urinary incontinence
nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses
shunting and short-circuiting of nerve impulses occurs

multiple sclerosis treatment
the advent of disease-modifying drugs including interferon beta-1a and -1b