The Nervous System
•
Nervous System allows organisms to
respond to external and internal stimuli Consists of:
–
–
–
Brain and spinal cord – Central Nervous
System
Peripheral Nerves – Peripheral Nervous
System
Neurons – functional unit of the nervous
system, specialized cells for transmitting
electrical and chemical signals
Anatomy of a Nerve cell:
1. Cell body - contains nucleus, most of the cytoplasm
2.
3.
and most of the organelles
•
Dendrites and axon extend from the cell body
Dendrites - short and highly branched
•
receive stimulus and send to cell body
Axon - conducts impulses away from the cell body to
another neuron or to a muscle or gland
•
microscopic in diameter but may extend a meter
or more in length
•
may divide forming branches – axon collaterals
•
divides at the end to form terminal branches that
end in synaptic terminals
•
synaptic terminals release neurotransmitters
(chemicals) that transmit impulse across the tiny
gap between neurons – synapse
4. Myelin sheath – fatty mat’l surrounding
the axons of neurons outside the CNS
(sheath made of neuroglia in CNS)
speed up transmission of impulse
– composed of Schwann cells that form
insulation
– Nodes of Ranvier – gaps between Schwann
cells (myelin sheath)
•
•
•
•
Nerve – consists of hundreds or thousands of
axons wrapped together in connective tissue
in the CNS, bundles of axons are called tracts
or pathways instead of nerves
Ganglia – outside the CNS, cells bodies are
usually grouped together in masses called
ganglia
inside CNS, collection of cell bodies called
nuclei
Types of Neurons
1. sensory (afferent) neurons – conduct impulses
into CNS from the periphery (sensory impulses)
2. Interneurons (association neurons) – afferent
neurons usually transmit impulses to
interneurons
–
–
–
–
located within CNS
neurons that integrate input and output
integration involves sorting and interpreting incoming
sensory information and determining the appropriate
response
forms connecting lines b/w sensory and motor
neurons
3. motor (efferent) neurons – transmit messages
from CNS to effectors (musc. or gland)
4. sensory receptors, afferent and efferent neurons
are part of the Peripheral Nervous System
Neuroglia (glial cells) – “nerve glue” – support cells
for neurons in the CNS – three main types:
1. Microglia – found near blood vessels,
phagocytes that migrate and remove foreign
and degenerated material
2. Astrocytes – star shaped glial cells that have a
variety of functions:
–
–
–
3.
some are phagocytic and remove invading
microorganisms and debris from nervous tissue
help regulate concentration of potassium ions in
extracellular fluid of nervous tissue
regulate concentration of neurotransmitters
Oligodendrocytes – glial cells that envelope
neurons in CNS, forming insulating sheaths –
speeds transmission of impulse
How Neurons Work
Membrane potential (resting potential)
difference in electrical charge across the
plasma membrane
• Resting Potential (not conducting an impulse)




more negatively charged inside the cell compared to
the interstitial fluid outside
membrane of neuron is polarized due to unequal
distribution of ions – as a result the cell can produce
an action potential (impulse)
Electrochemical potential
-70mV
slight excess of positive ions outside the membrane
and slight excess of negative ions inside the
membrane
 Na+ concentration is 10x greater outside the cell
and K+ concentration 10x greater inside the cell
 ion pumps, ion channels and gates cause a
specific distribution of ions across the cell
membrane
 sodium-potassium pumps in the membrane pump
Na+ out and K+ into cell – both are pumped against
their concentration gradient (ATP) – for every 3
Na+ pumped out, 2 K+ are pumped in (more
positive ions outside than in)
 K+ tends to leak out by diffusion through ion
channels causing further negative charge inside
as compared to outside of cell
 ion channels that allow the passage of Na+ are
closed at resting potential
•
•
•
•
•
•
Stimulation – all or none response (action
potential)
Threshold stimulus – minimum amount needed
for depolarization to occur
causes Na+ ion channels to open allowing Na+
to rush into interior of cell (depolarization)
disturbs adjacent areas – Na+ channels open
causing a depolarization wave – action potential
polarity across membrane is momentarily
reversed
K+ channels also open but more slowly allowing
repolarization
•
•
•
•
Repolarization – after action potential passes, membrane begins to
repolarize
Na+ channels close and membrane become impermeable to Na+
open K+ channels allow K+ leak out of the neuron repolarizing the
membrane
impulse is actually a series of depolarization and repolarization waves
sweeping down the axon (takes place in less than 1 millisecond)
• resting conditions must be reestablished by
sodium-potassium pumps
• Absolute Refractory Period – millisecond during
which membrane is depolarized and cannot
conduct an impulse
• Relative Refractory Period – enough Na+ channel
gates have been reset that axon can carry impulse
but threshold is higher
Impulse conduction video
Impulse conduction
•
•
•
•
•
•
impulse conduction is slower in unmyelinated
axons – continuous conduction – entire axon
must depolarize
diameter of axon affects speed of transmission
larger diameters transmit faster
vertebrate neurons are myelinated – speeds
up transmission
depolarization occurs only at the nodes of
Ranvier – action potential “jumps” from one
node to the next – saltatory conduction
transmission is much faster than in
unmyelinated axons
Transmission across Synapses
•
•
synapse – gap between axon of one neuron and
dendrites of the next or between a neuron and an
effector
synapse between neuron and muscle cell is
called a neuromuscular junction or motor end
plate
•
•
•
•
•
•
neurotransmitters act as chemical messengers to
conduct the signal across the synapse
synaptic vesicles contain neurotransmitter
When action potential reaches axon terminal,
calcium ions begin to diffuse in – Ca+ influx
vesicle fuses to presynaptic membrane and
dumps transmitter into synaptic cleft – diffuses
across synapse
neurotransmitter binds to highly specific receptors
on the postsynaptic membrane (specific to the
type of neurotransmitter) – binding begins
depolarization and impulse continues
enzymes in cleft decompose neurotransmitter to
free up receptor sites for next impulse or the
neurotransmitter is actively transported back into
presynaptic vesicles (reuptake)
Neurotransmitters each have a different function:
•
Excitatory – stimulate neurons
– acetylcholine (stimulate muscle contraction)
– norepinephrine, dopamine, and serotonin (affect mood)
•
Inhibitory – stop depolarization
– Gamma-aminobutyric Acid (GABA) – inhibits neurons in
brain and spinal cord
•
excitatory postsynaptic potential (EPSP) – if a
neurotransmitter is excitatory, it results in an EPSP)
– causes partial depolarization bringing neuron closer to
firing
– one EPSP is probably too weak to trigger an action
potential – EPSPs can be added together (summation) –
results in firing of neuron
•
inhibitory postsynaptic potential (IPSP) – occur when
neurotransmitter causes postsynaptic membrane to
hyperpolarize – brings membrane potential farther away
from threshold and a stronger stimulus would be necessary
to fire it
Organization of Neurons
•
•
•
CNS contains millions of neurons organized into
neural networks
neurons are organized into neural circuits
(specific pathways) within each network
Neural Circuits in all of the networks share many
organizational features:
–
Convergence – a single neuron is controlled by
converging signals from two or more presynaptic
neurons – important mechanism by which CNS can
integrate information from various sources
•
Divergence – a single presynaptic neuron stimulates many
postsynaptic neurons
•
Reverberating circuit – an axon colateral synapses with an
interneuron in a sequence that can send new impulses through the
circuit
example of positive feedback
new impulses generated again and again until fatigue or stopped
by inhibition (impt in cyclic behaviors such as breathing)
•
•
Central Nervous System
•
•
•
Vertebrate Nervous System is divided into
the Central Nervous System (brain and
spinal cord) and the Peripheral Nervous
System (sensory receptors and the
nerves which act as communication lines)
Parts of the body are linked to the brain
by cranial nerves and to the spinal cord by
spinal nerves
Afferent neurons “inform” the CNS of
changing conditions, efferent nerves
transmit the “decisions” of the CNS
•
•
•
•
•
Evolution of the Vertebrate Brain
all vertebrates have same basic brain
structure
different parts are specialized in different
vertebrate classes
evolutionary trend is toward increasing
complexity
all vert. brains start off as three bulges
that swell at the anterior end of the neural
tube – these eventually become the
hindbrain, midbrain, and forebrain
Hindbrain
develops into medulla, pons, and cerebellum
(brain stem)
1. medulla – continuous with spinal cord
– serves as a vital center - regulates
respiration, heartbeat, blood pressure,
swallowing, coughing, and vomiting
2. cerebellum – coordinates muscle activity,
responsible for muscle tone, posture and
equilibrium
3. pons – bridge connecting the spinal cord
and medulla with upper parts of the brain
Midbrain
•
•
•
•
most prominent part of brain in fish and
amphibians
acts as main association area (receives sensory
info, integrates it, and sends decisions to motor
nerves)
contains optic lobes for visual interpretations
birds, reptiles, and mammals
–
–
consists of superior colliculi (center for reflexes such
as pupil constriction) and inferior colliculi (centers for
some auditory reflexes)
also contains a center to help maintain muscle tone
and posture
Forebrain
develops into thalamus, hypothalamus, cerebrum,
and olfactory bulbs
1. thalamus – acts as a relay center for motor and
sensory messages – in mammals, all sensory
info passes through thalamus before sent to
cerebrum
2. hypothalamus – contains olfactory centers and
acts a principal integration center for regulation
of internal organs
–
–
–
controls body temp in reptiles, birds, and mammals
regulates appetite, water balance, and emotional
response
links the nervous system to the endocrine system and
produces certain hormones
3. Olfactory bulbs – impt in chemical sense of smell
4. Cerebrum – divided into right and left hemisphere
–
–
–
–
–
most of cerebrum made of white matter (consists
mainly of myelinated axons that connect to various
parts of the brain)
Cerebral Cortex – layer of gray matter which makes up
the outer portion of the cerebrum (contains cell bodies
and dendrites)
most prominent part of brain in mammals
surface area is increased by convolutions
functions:
• conscious interpretations of sensations
• voluntary muscle movement
• memory
• reasoning and problem solving
• intelligence and personality