LE NERVOUS SYSTEM
Reflexes: Sensory nuerons: trasmit info from sensors that detect
external stimuli/internal conditionsInfor sent to CNS where
interneurons analyze/interpret sensory inputMotor output leaves CNS
via motor nuerons which communicate w/ effector cellsLe Neuron
Structure
Neurons organelles located in cell body. Dendrites are highly
branched extensions that receive signals from other neurons. The
axon is a loner extension that transmits signals to other cells
(other neurons or effector cells). The conical region of axon where
it joins cell body is axon hillock, region where signals that
travel down axon are generated. Many axons are enclosed in myelin
sheath layer. Near the end, the axon divides into several branches,
each ending in a synaptic terminal. The site of communication
between a synaptic terminal and another cell is a synapse. Info can
be passed by neurotransmitters. Gilia are supporting cells that are
essential for sturctural integrity of nervous system: 1)Astrocytes:
structural support for neurons/regulate extracellular
concentrations of ions/neurotransmitters, can facilitate info
transfer at synapse between neighboring neurons (cellular
mechanism: learning/memory). Astrocytes next to active neurons also
cause nearby blood vessels to dilate, increasing blood flow and
enabling neurons to obtain O2 and glucose more quickly. During
development, astrocytes induce formation of tight junctions between
cells that line capillaries in brain/spinal cord resulting in the
blood-brain barrier-which restricts passage of certain stuff into
CNS 2)Radial Glia: (embryo) form tracks along which newly formed
neurons migrate from neural tubegiving rise to CNS, can act as stem
cell. So can astrocytes. 3)Oligodendrocytes (CNS) and Schwann cells
(PNS): glia that form the myelin sheaths around axons. Neurons
become myelinated when Schwann cells or oligodendrocytes wrap
around axons in my layers of membrane-mostly lipid so it is poor
conductor of electrical currents & provides for electrical
insulation of axon. Gaps between adjacent Schwann cells are nodes
of Ranvier. Le Resting Potential/Gated Ion ChannelsResting
potential-membrane potential of neuron not transmitting a signal,
depends on ionic gradients across membrane. Concentrations of Na+
and Cl- are higher in extracellular fluid than cytosol. The reverse
is true for K+. Neurons have gated ion channels which open/close in
response to 1)stretch-gated ion channels: found in cells that sense
stretch when membrane is mechanically deformed 2)ligand-gated
channel: found at synapses and open/clsoe when chemical like a
neurotransmitter binds to channel 3)voltage-gated ion
channels-found in axons and open/close when membrane potential
changes.
Dem Action PotentialsIf cell has gated ion channels, its
membrane potential may change in response to stimuli. Stimuli can
trigger a hyperpolarization, an increase in magnitude of membrane
potential (inside of membrane becomes more negative, may be caused
by opening of gated K+ channels). Other stimuli trigger
depolarization, reduction in magnitude of membrance potential (may
be due to opening of gated Na+ channels)Depolarizations are graded
only up to a certain membrane voltage, called a threshold. The
action potential [all or none] is strong enough to produce a
depolarization that reaches the threshold. Voltage-gated Na+ and
voltage-gated K+ channels produce action potential. Na+ channel
opens first. Na+ channel has an activation gate/inactivation gate
whereas K+ has one activation gate. 1)Resting State: Activation
gates of Na+ and K+ channels are closed. 2)Depolarization: Stimulus
opens activation gates on some Na+ channels. Na+ influx thru
channels depolarizes membrane; if depolarization reaches threshold,
it triggers an action potential. 3)Rising Phase of Action
Potential: Depolarization opens more Na+ channels, while K+
channels remain closed. Na+ influx makes inside of membrane
positive w/ respect to outside. 4)Falling Phase of Action
Potential: The Inactivation gates close, blocking Na+ influx. K+
channels open permitting K+ efflux, making inside of cell negative.
5)Undershoot: Both Na+ gates close, K+ channels are still open, but
eventually closeresting state. The refractory period sets a limit
on the max frequency at which action potentials can be generated.
The action potential functions as a long-distance signal by not
diminishing from the cell body to the synaptic terminals; it does
so by regenerating itself along axon. 1)Action potential is
generated as Na+ flows inward across membrane at one location.
2)Depolarization of action potential spreads to neighboring region
of membrane, re-initiating action potential there. To the left of
this region, the membrane is repolarizing a K+ flows outward.
3)Depolarization/repolarization repeats in next region so action
potential is propagated along axon. The larger the axons diameter,
the faster the conduction. Myelin increases the conduction speed of
action potentials by insulating the axon membrane. Insulation
causes the depolarizing current to spread further along interior of
axon, bringing distant regions of membrane to threshold sooner.
Action potentials are not generated in regions between the nodes of
Ranvier-Saltatory conduction. Dem Neurons Communicate at Dem
SynapsesMany synapses are chemical synapses which involve release
of chemical neurotransmitter by presynaptic neuron. Presynaptic
neuron synthesizes the neurotransmitter and packages it in synaptic
vesicles (stored in synaptic terminals which interact with
postsynaptic neuron). 1)When action potential depolarizes plasma
membrane of synaptic terminal, it 2)opens voltage-gated Ca2+
channels in membrane. 3)Elevated Ca concentration in terminal
causes synaptic vessicles to fuse w/ presynaptic membrane.
4)Vesicles release neurotransmitter into synaptic cleft.
5)Neurotransmitter binds to receptor portion of ligand-ion channels
in postsynaptic membrane, opening them and allowing ions to diffuse
across-direct synaptic transmission and result is a postsynaptic
potential. 6)Synpatic transmission ends when neurotransmitter
diffuses out of synaptic cleft, taken by another cell, or degraded
by enzyme. Indirect Synpatic Transmission (slower but longer
onset):1)Neurotransmitter binds to receptor not part of an ion
channel, activating a signal transduction pathway involving a 2nd
messenger in postsynaptic cell.Dem Neurotransmitters
Le Specialized Nervous SystemVertebrate CNS: narrow central
canal of spinal cord and four ventricles of the brainfilled w/
cerebrospinal fluid, which circulates slowly thru central
canal/ventricles and drains into veins, assisting in supply of
nutrients/hormones to diff parts of brain and in removal of wastes.
In mammals, the fluid cushions the brain & spinal cord. In
cross sections of the brain and spinal cord, there is the white
matter (myelin sheaths give axons whitish appearance) and the gray
mattermainly dendrites, unmyelinated axons, & neuron cell
bodies. PNS: transmits info to and from CNS, regulates
movement/internal environ. Vertebrate PNS consists of light-right
pairs of cranial and spinal nerves and associated ganglia. Somatic
Nervous System: carries signals to and from skeletal muscles
(response to external stimuli and is voluntary) Autonomic Nervous
System: regulates internal environ by controlling smooth and
cardiac muscles and organs of digestive, cardiovascular, excretory,
and endocrine systems Sympathetic: fight or flight response
Parasympathetic: promote calming and a return to self-maintenance
functions Enteric: consists of network of neurons in digestive
tract, pancreas, and gallbladder that control organs secretions as
well as activity in smooth muscles
Le Brain Stuff
Forebraintelencephalon [cerebrum(cerebral hemispheres: cerebral
cortex, white matter, basal nuclei)]and diencephalon
[Diencephalon(thalamus, hypothalamus, epithalamus)]
Midbrainmesencephalon [Midbrain(part of
brainstem)]Hindbrainmetencephalon [pons(part of brainstem),
cerebellum)] and myelencephalon [medualla oblongata (part of
brainstem)]BrainstemMedulla Oblongata, Pons, Midbrain-homeostasis,
coordination of movement, conduction of info to higher brain
centersMedulla Oblongata: (automatic,homeostatic) breathing, heart
and blood vessel activity, swallowing, vomiting, digestion Pons:
regulates breathing centers in medulla Information transmission is
one of the most important functions of medulla dn pons since all
axons carrying sensory info to and motor instructions from higher
brain regions pass thru brainstem. The midbrain contains centers
for receipt and integration of several types of sensory info. It
also sends coded sensory info along neurons to specific regions of
forebrain.
The reticular formation(RAS) is a network of neurons present in
the core of the brainstem. Part of the RAS regulates sleep and
arousal. RAS acts as a sensory filter, selecting which info reaches
cerebral cortex; the more info that reaches cerebral cortex, the
more alert/aware a person is. The pons and medulla contain centers
that cause sleep when stimulated, and the midbrain has a center
that causes arousal.
CerebellumDevelops from part of metencephalon, important for
coordination and error checking during motor, perceptual, and
cognitive functions (learning, decision making, consciousness),
recieves sensory info about position of joints and length of
muscles and auditory/visual systems. Cerebellum also integrates
sensory/motor info to coordinate movement and balance.
DiencephalonEpithalamus: pineal gland and choroid plexus cluster
of capillaries that produce cerebrospinal fluidThalamus: main
input/output center for motor info leaving cerebrum. Incoming info
from senses is sorted in thalamus and sent to proper cerebral
centers for processing, also receives imput that regulate
emotion/arousal. Hypothalamus: regulates hunger, thirst,
sexual/mating behaviors, fight-flight responses, pleasure
Biological clocks-maintain circadian rhythms, regulate
physiological phenomena, including hormone release, hunger,
heightened sensitivity to external stimuli. In mammals, biological
clock is the suprachiasmatic nuclei (SCN). Bio clocks require
external cues to remain synchronized w/ environmental cycles.
CerebrumDevelops from telencephalon, divided into right and left
cerebral hemispheres and each hemisphere consists of basal nuclei
located deep within white matter-basal nuclei are important centers
for planning and learning movement sequences. In humans, cerebral
cortex is the most complex part of brain; sensory info is analyzed,
motor commands are issues, and language is generated in there.
Neocortext forms the outermost part of mammalian cerebrum,
consisting of 6 parallel layers of neurons arranged tangential to
brain surface. The left side of cortext recieves info from, and
controls movement of. The corpus callosum enables communication
between right/left cerebral cortices.