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Nervous System
Nervous System
1.
2.
3.
4.
Cells of the Nervous System
•
Two major cell types
1. Neurons
•
The Neuron
Receive & Integrate
Inputs
Electrical and chemical communication
2. Glia
•
•
Parts of the neuron
Organization of the nervous system
Neural communication
Autonomic Nervous system
Important for development and support
Myelin
Neurons are the fundamental unit of the
nervous system
Impulse conduction
Neurotransmitter
Secretion
1
Neurons – diversity of types
Organization of nervous system
Other sensory input
dendrites
cell body
Motor Output
Behaviour
Processing
Sensory Input
axon
Patterned
activity
Filters
Purkinje Cell
Cortical Pyramidal Neuron
Experience
Simple
Nerve
net
Complex
Neural
ganglia
Interconnected
Ganglia
Segmentation
Anterior specialization
‘brain’
• Even ‘simple’ nervous systems can display
very complex behaviour
• eg nematodes, leeches, and flies can all
display learning!
Hydra
Leech
# of neurons
nematode
300
Human
100 x 109
2
Central
Nervous
System
Brain
Segmented
Nerve
Cord
Fruit Fly Embryo
Peripheral
Nervous
System
Human
Division of the
Motor Nervous System
Autonomic
Sympathetic
Parasympathetic
Heart rate
Cardiovascular function
Pupil dilation
Lung function
Voluntary
Things you control
Organization of the Vertebrate
Nervous System
Brain
Spinal Cord
Sensory Neurons
(afferent)
Sensory
Receptors
Motor Neurons
(efferent)
Autonomic
Voluntary
(somatic)
Autonomic Nervous System
• Parallel Systems that innervate the same target
• Opposing effects on the target
•Usually in balance
Sympathetic
Parasympathetic
Dilate pupil
Constrict pupil
Increase heart rate Decrease Heart Rate
3
Figure 6-4
The Reflex Arc
Sensory
receptor
Monosynaptic
Sensory
receptor
Interneuron
Polysynaptic
Summary & Key Points
1. The fundamental cell of the nervous
system is the neuron.
2. The neuron has functionally specialized
regions
3. Evolution of the nervous system includes
increasing cell number and
interconnection
4. Subdivisions of the vertebrate nervous
system
Neural Communication
Basic Concepts
1. Neurons use electrical and chemical
mechanisms
4
Neural Signalling
Within
neurons
Between
neurons
Bioelectric Potentials
This connection is
called the synapse
• Neurons have an electrical potential
(voltage) across the cell membrane
• The inside of the cell is more negative
than the outside
– called the Resting Membrane Potential
electrical
chemical &
electrical
Measuring Membrane Potential
amplifier
microelectrode
Reference
electrode
Membrane potential
0 mV
Bioelectric Potentials
• Action Potentials are rapid changes in
resting membrane potential that travel
down the axon
• Action Potentials initiate synaptic
transmission at the nerve terminal
cell
-80 mV
Bathing solution
time
5
Action Potentials
Action Potential
• What are they?
– Rapid reversal of the resting membrane
potential
Overshoot
Rising phase or
depolarization
Falling phase or
Repolarization
0 mV
cell
0 mV
Threshold Potential
-80 mV
-80 mV
Resting membrane
potential
Undershoot or afterhyperpolarization
3 ms
Na+
Voltage-gated channels
Na+
K+
K+
K+
K+
Na+
Membrane Potential
Section of
Axon
Membrane Potential
Na+
time
time
6
Action Potential Conduction
Action Potential Conduction
Stimulate Action Potential
axon
Axon hillock = initial segment
Region of neuron where AP usually starts
Action Potential Conduction
Record
voltage
Na+
Stimulate Action Potential
1. APs constant amplitude at all points
along the axon
7
Sequence of Events leading to AP
propagation
Myelinated nerve
1. Stimulus opens Na+ channels & cause AP
2. Depolarizing current flows down the axon
3. Local depolarization opens Na+ channels
downstream & initiate a new AP
4. Na+ channels close (inactivate) & K+ channels
open
5. Local depolarization opens Na+ channels
downstream and initiate a new AP
Myelin
Node of Ranvier
Myelin Formed by:
Schwann cells (periphery)
Oligodendrocytes (central)
Na+
Myelin
Saltatory conduction
8
Myelin
Mulitple Sclerosis
Myelin increases speed of conduction by:
1. Increasing membrane resistance
•
Reduces ‘leakiness’→ ↑ length constant
2. Voltage-gated channels only at Node of
Ranvier
•
• Demyelination of axons
– Impaired AP conduction
– Symptom depends on nerves affected
• Optic nerve → blindness
• Motor nerves → weakness or paralysis
APs generated only at the Node
Neural Signalling
Within
neurons
Between
neurons
This connection is
called the synapse
Definitions
1. Synapse : the functional contacts
between neurons and other cells
2. Synaptic Transmission: the process used
by neurons to relay information from one
cell to the next
electrical
chemical &
electrical
9
Types of Synapses
Direction of information flow
1. Electrical
¾ Direct flow of electrical current from one cell
to the next through gap junctions
2. Chemical
¾ Secrete neurotransmitter molecules that
activate receptors
Chemical synapse
Presynaptic nerve
terminal
Synaptic vesicles
Synaptic cleft
Neurotransmitter
receptors
Chemical Synapses
Features:
1. Use chemical neurotransmitters
2. There is a space between the pre- and
postsynaptic neuron, called the synaptic
cleft
3. Neurotransmitter is stored in synaptic
vesicles
Postsynaptic membrane
10
Real synapse
Chemical Synapses
Drosophila neuromuscular junction
Features:
4. Can be excitatory or inhibitory
•
Depends on neurotransmitter and receptor
5. Can activate ion-channels or signaling
pathways
6. The amount of transmitter released is
variable and can be modulated
Synaptic vesicles
Synaptic contacts
Active Zone
Sequence of Events at a Chemical
Synapse
1.
2.
3.
Na+
Ca++
4.
5.
6.
7.
8.
Action potential arrives & depolarizes nerve terminal
Voltage-gated Ca++ channels open & Ca++ flows into
the nerve terminal
Ca++ causes synaptic vesicles to fuse with the
plasma membrane
Neurotransmitters are released into synaptic cleft
Neurotransmitter binds to receptors
Opens ion channels and positive current flows into
postsynaptic cell
Current flow gives postsynaptic potential
If postsynaptic potential = threshold → Action Potential
Depolarization
11
Na+
Ca++
Depolarization
12