Transcript Nervous Systems

Nervous Systems
Three Main
Functions:
1. Sensory Input
2. Integration
3. Motor Output
Two Main Parts of Vertebrate
Nervous Systems
• Central nervous system (CNS)
– brain and spinal cord
– integration
• Peripheral nervous system (PNS)
– network of nerves extending into different parts
of the body
– carries sensory input to the CNS and motor
output away from the CNS
Two Cell Types in Nervous
Systems
• Neurons
– Cells that conduct
the nerve impulses
• Supporting Cells
– Neuroglia
Figure 48.2x Neurons
Three Major Types of
Nerve Cells
• Sensory neurons
– communicate info about the external or internal
environment to the CNS
• Interneurons
– integrate sensory input and motor output
– makes synapses only with other neurons
• Motor neurons
– convey impulses from the CNS to effector cells
Supporting Cells - Neuroglia
• provide neurons with nutrients, remove
wastes
Two important types in vertebrates
– Oligodendrocytes – myelin sheath in CNS
– Schwann cells -myelin sheath in PNS
Myelin Sheath Formation
Conduction of the Nerve Impulse
• Membrane Potential
– Voltage measured across a membrane due to
differences in electrical charge
– Inside of cell is negative wrt outside
• Resting potential of neuron = -70 mV
Figure 48.6 Measuring membrane potentials
Sodium-Potassium
Pump
Extracellular
P
Intracellular
1. Carrier in membrane binds
6. Dephosphorylation of protein
triggers change to original
conformation, with low affinity
for K+. K+ diffuses into the cell,
and the cycle repeats.
+
ADP
ATP
intracellular sodium.
2. ATP phosphorylates protein
with bound sodium.
K+
Na+
Pi
Pi
Pi
5. Binding of potassium causes
dephosphorylation of protein.
Pi
4. This conformation has higher
affinity for K+. Extracellular
K+ binds to exposed sites.
3. Phosphorylation causes
conformational change in
protein, reducing its affinity for
Na+. The Na+ then diffuses out.
Excitable Cells
• Neurons & muscle cells
• Have gated ion channels that allow cell to
change its membrane potential in response
to stimuli
Gated Ion Channels
• Some stimuli open K+ channels
– K+ leaves cell
– Membrane potential more negative
– hyperpolarization
• Some stimuli open Na+ channels
– Na+ enters cell
– Membrane potential less negative
– depolarization
Gated Ion Channels
• Strength of stimuli determines how many
ion channels open
= graded response
Nerve Impulse Transmission
Action Potentials
• Occur once a threshold of depolarization is
reached
– -50 to –55 mV
• All or none response (not graded)
– Magnitude of action potential is independent of
strength of depolarizing stimuli
• Hyperpolarization makes them less likely
3. Top curve
2. Rising Phase
Maximum voltage reached
Stimulus causes above threshold voltage
Potassium
gate opens
K+
Na+
1. Resting Phase
Equilibrium between diffusion of K+ out
of cell and voltage pulling K+ into cell
Voltage-gated
potassium channel
Membrane potential (mV)
Sodium channel
activation gate opens
Na+ channel
inactivation gate
closes
+50
0
–70
1
3
2
Time (ms)
4. Falling Phase
Undershoot occurs as excess potassium
diffuses out before potassium channel closes
Potassium channel
gate closes
Potassium
gate open
Equilibrium
restored
Potassium
channel
Voltage-gated
sodium channel
Sodium channel
activation gate closes.
Inactivation gate opens.
Na+ channel
inactivation gate
closed
Refractory Period
• During undershoot the membrane is less
likely to depolarize
• Keeps the action potential moving in one
direction
Propagation of Action Potential
• Action potential are very localized events
• DO NOT travel down membrane
• Are generated anew in a sequence along the
neuron
resting
repolarized
depolarized
+ + + + + + + + +
– – – – – – – – –
+ + + + + + + + +
– – – – – – – – –
– – + + + + + + +
+ + – – – – – – –
Na+
+ + + + + + + – –
– – – – – – – + +
K+
+ + – – + + + + +
– – + + – – – – –
Na+
+ + + + + – – + +
– – – – – + + – –
K+
K+
+ + + + – – – + +
– – – – + + + – –
Na+
+ + – – – + + + +
– – + + + – – – –
K+
K+
+ + + + + + + – –
– – – – – – – + +
Na+
– – + + + + + + +
+ + – – – – – – –
Cytoplasm
Cell
membrane
K+
Saltatory Conduction
Transfer of Nerve Impulse to
Next Cell
• Synapse
– the gap between the synaptic terminals of an
axon and a target cell
Transfer of Nerve Impulse to
Next Cell
• Electrical synapses
– Gap junctions allow ion currents to continue
• Chemical synapses
– More common
– Electrical impulses must be changed to a
chemical signal that crosses the synapse
Synapses
Neurotransmitters
Effects of Cocaine
Transporter
protein
Dopamine
Cocaine
Receptor
protein
Neurotransmitter
Synapse
Transporter protein
Receptor protein
1. Reuptake of neurotransmitter by transporter
at a normal synapse.
Drug
molecule
2. Drug molecules block
transporter and cause
overstimulation of the
postsynaptic membrane.
3. Neuron adjusts to
overstimulation by
decreasing the number
of receptors.
4. Decreased number of
receptors make the
synapse less sensitive
when the drug is removed.
Diversity of Nervous Systems
Cnidarian
Human
Earthworm
Central nervous
system
Peripheral
Spinal
nerves
cord
Nerve
net
Cerebrum
Cerebellum
Cervical
nerves
Thoracic
nerves
Arthropod
Lumbar
nerves
Sacral
nerves
Echinoderm
Radial
nerve
Nerve
ribs
Brain
Femoral
nerve
Ventral
nerve cords
Mollusk
Flatworm
Nerve cords
Giant axon Brain
Sciatic
nerve
Tibial
nerve
Associative
neurons