Transcript File - Biology with Radjewski

Nervous System
Chapter 34
34.1
• Neurons and Glia
Nervous System – 2 types of cells
1. Neurons – Nerve Cells
2. Glia – Glial cells
Neurons
• 4 main parts
1. Cell Body
2. Dendrite
3. Axon
4. Axon terminal
Cell body
• Contains the nucleus and the organelles
• Has dendrites extending from it
Dendrites
• Extends from the cell body
• Shrublike
• Bring information from other neurons or
sensory cells to the cell body
• Different degree of branching depending on
the type of neuron
Axon
• Long projection off cell body
– Can extend for example from your spinal cord to
your toe!
• “telephone lines” of the nervous system
• Act on information received by the dendrites
• Generates action potentials (nerve impulse)
down the axon toward a target cell
• A bundle of axons is called a nerve
Axon Terminal
• Swelling of nerve endings
• Is very close to the membrane of the target
cell to form a synapse
» Tiny gap across which 2 neurons communicate either with
electical signals or with chemical signals
» Neuron sending the information is the presynaptic
neuron
» Neuron receiving the information is the postsynaptic
neuron
Glial Cells
• More numerous than neurons
• They release neurotransmitters
» Chemicals associated with nerve impulses
• Provide homeostasis for neurons by clearing
the synapse of neurotransmitters
• Repair neurons and remove dead neurons
• 3 different kinds of glial cells
– Astrocytes, microglia, and Shwann cells
Astrocytes
• Surround the smallest, most permeable blood
vessels in the brain
• Contributes to the blood-brain barrier that
prevents toxic chemicals from reaching the
brain (it’s not perfect)
• This barrier usually prevents antibodies from
entering the brain – so the brain gets its
immune defenses by another glial cell, called
microglia
Microglia
• Glial cell that acts as macrophages and
mediators of inflammatory responses
Schwann Cells
• Forms a multilayered wrap on axons, which
forms a lipid-rich sheath called myelin.
• Myelinated axons have a white appearance,
giving rise to white matter
– Areas of the brain that do not appear white (areas
rich in cell bodies) area gray and are called gray
matter
• Nodes of Ranvier are between the schwann
cells
• ALD (Lorenzo) and Multiple Sclerosis are
diseases where the myelin is affected
3 Functional Categories of Neurons
1. Afferent neurons
–
Carry sensory information into the nervous system
(coming from sensory cells) that transduce sensory
stimuli into action potentials (nerve impulses)
2. Efferent neurons
–
Carry commands to physiological and behavioral effectors
such as muscles and glands (example – motor neurons
carry commands to muscle cells)
3. Interneurons
–
Integrate and store information and communicate
between afferent and efferent neurons (most neurons in
brain are interneurons)
Neural
Networks
Neural
Networks
Neural
Networks
34.2 Electrical Signals
Action Potentials
• Nerve impulses
• Carry information along neurons
• Sudden and large changes in membrane
potential (difference in electrical charge across
the plasma membrane) that travel along axons
and cause the release of chemical signals at
the axon terminal
Voltage
• Measure of the difference in electrical charge
between 2 points
• Represents potential energy because opposite
charges will move together if given a chance
• In wires, electrical current is carried by
electrons
• In solutions and across cell membranes,
electric current is carried by ions
Ions
• Major ions involved are
– Sodium (Na+)
– Potassium (K+)
– Calcium (Ca(2+)
– Chloride (Cl-)
• In cells these ions are kept at different
concentrations inside and outside the cell
• The result of differing these concentrations is
the voltage across the cell membrane, known
as a membrane potential
Resting Membrane Potential
• Occurs in an inactive neuron (not sending or
receiving a signal)
• Typically between -60 and -70 millivolts (mV)
– The minus sign refers to a electrically negative cell
compared to the outside of the cell
• Action Potentials (nerve impulses) are generated
when there is sudden change in this voltage to where
it is more positive inside than outside
• How is this done?
• Sodium Potassium Pump!
Sodium Potassium Pump Review
• Form of active transport, uses ATP
• Na+ ions are pumped out of the cell and exchanged
for potassium ions from the outside of the cell
• Remember this exchange is uneven. The sodium
potassium pump is constantly pumping Na+ out and
K+, but the concentration of Na+ is higher outside
than inside and the concentration of K+ is higher
inside than outside.
– These concentration gradients will be used to generate the
resting potential and changes in the resting potential
– How does the resting potential change?
A stimulus occurs (light, pinch, etc.)
• That triggers a voltage gated Na+ channel to
open which brings Na+ ions into the cell
– Sodium is going into the cell because it moves
from H  L and there is more sodium outside the
cell than inside the cell
• So now the inside of the cell becomes less
negative (70mV  50mV) – this is called
depolarization
– When the inside of the neuron becomes less
negative (more positive)
What happens next?
• Additional voltage gated Na+ channels open,
causing a rapid spike of depolarization – an
action potential.
– The action potential is traveling down the axon.
• The depolarization triggers voltage gated K+
ions to open, which allows K+ to flow out of
the cell (from H  L) – this is called
hyperpolarized
– Membrane is becoming even more negative
Chapter 49
Section 1 Neurons and Nerve
Impulses
Action Potential
Click below to watch the Visual Concept.
Visual Concept
Action Potentials
• Signal strength does not change during travel
• All or nothing
– Positive feedback mechanism to ensure that action
potentials always rise to their maximum value
• Self regenerating – 1 action potential stimulates
another etc.
• Cannot go in reverse due to the refractory period
(time during which membrane is returning to resting
potential)
• Travel faster in myelinated axons and in largerdiameter axons
– Squid axons are big, so their response time is rapid!
Communication between neurons
• Once an action potential reaches the axon terminal, it releases
neurotransmitters into the synaptic cleft. These neurotransmitters bind
to receptors proteins and open the ion channels of the new neuron cell.
• If enough ion channels are opened, the action potential will continue
through the new neuron. If not, the nervous signal will be terminated.
• After the neurotransmitters have opened the ion channels, they will be
cleared out of the synaptic cleft by being reabsorbed by the neuron that
released them or broken down by enzymes.
Chapter 49
Section 1 Neurons and Nerve
Impulses
Release of Neurotransmitter
Click below to watch the Visual Concept.
Visual Concept
Chemical Synapse
• Neurotransmitters released from a
presynaptic cell bind to receptors in the
membrane of a postsynaptic cell
• The neurotransmitter used by all vertebrate
neuromuscular synapses is acetylcholine
(ACh)
• 7 steps
7 steps to a chemical synapse
1. Action potential arrives at axon terminal.
2. Na+ channels open; depolarization causes voltage gated
Ca2+ channels to open
3. Ca2+ enters the cell and triggers fusion of acetylcholine
vesicles with the presynaptic membrane
4. Acetylcholine molecules diffuse across the synaptic cleft and
bind to receptors on the postsynaptic membrane
5. When binding occurs, they open up their channels and
depolarize the postsynaptic membrane
6. The spreading depolarization fires an action potential in the
postsynaptic membrane
7. Acetylcholine is broken down
Neurotransmitter Action – how
does it stop?
• They must be cleared from synapse
• Enzymes may destroy the neurotransmitters
• Or neurotransmitters might simply diffuse
away from the cleft
Types of Neurotransmitters
• More than 50 recognized
• Acetylcholine is used in the brain with motor
neurons
• Others are GABA, dopamine, norepinephrine,
serotonin, endorphins
Drugs interfere
• Drugs can interfere with neurotransmitter
release
• Toxins from Clostridium destroy proteins
necessary for the binding of vesicles to the
presynaptic membrane.
• These toxins cause botulism and tetanus –
fatal diseases that involve muscle impairment
due to loss of neurotransmitter release