Download Neurons & Nervous Systems

Neurons
&
Nervous Systems
nervous systems
connect
distant parts
of
organisms;
vary in complexity
Figure 44.1
Nervous System Components
• neurons
– obtain information
– transform information into signals
– transmit information
– integrate (process) information
– transmit responsive information
Nervous System Components
• glial cells (outnumber neurons)
– provide nutrients to neurons
– maintain ionic environment for neurons
– remove debris
– guide neuron development
– insulate neurons
Schwann cells insulate peripheral neurons
Figure 44.3
Nervous System Components
• glial cells (outnumber neurons)
– provide nutrients to neurons
– maintain ionic environment for neurons
– remove debris
– guide neuron development
– insulate neurons
– form blood-brain barrier
brains
vary in
complexity
Nervous System Components
• ganglia
– clusters of neurons
– information processing centers
• brain
– large, dominant pair of ganglia
• spinal cord
– with brain, forms central nervous system
(CNS) of vertebrates
Nervous System Components
• peripheral nervous system
– connects sensory systems to CNS
– connects CNS to effectors
Neurons
•
•
•
•
several functionally distinct parts
vary in size, complexity, organization
generate nerve impulses (action potentials)
communicate with other cells through
synapses
– axon terminal plasma membrane releases
neurotransmitters
– target cell plasma membrane binds
neurotransmitters
– targets include neurons, muscle, gland cells
dendrites
Figure 44.2
cell body
axon hillock
axon
axon terminals
variation in
# of
connections,
length of
transmission
Figure 44.2
Neuronal Networks
• collect information, process information and
respond to information
• consist of at least
– sensory neuron
– motor neuron
– muscle cell
• most neurons form 1000’s of synapses,
participate in multiple neuronal networks
resting potential
Figure 44.4
Nerve impulses
• cytoplasm is more negative than environment
• voltage difference is measured across the
plasma membrane
– membrane potential
– resting potential in unstimulated neurons
• -60 mV
– action potential
• a brief reversal of membrane polarity
• can be transmitted along a neuronal axon
membrane potential
• electrical potential (voltage) is the tendency of
charged particles to move between two
locations
• membrane potential represents the tendency of
ions to cross the membrane
– ions cannot freely cross the hydrophobic
membrane
– ion channels and pumps enable ion flow
across the cell membrane
– dominant ions are Na+, Cl-, K+ & Ca2+
Pumps and Channels
• channels permit diffusion of ions across the
membrane
– channels are more or less selective
– channels may be open or gated
• open channels are unrestricted
• voltage-gated channels respond to voltage
changes
• chemically-gated channels respond to
specific chemical signals
Na & K channels
Figure 44.5
Pumps and Channels
• pumps actively transport ions across the
membrane
– sodium-potassium (Na+-K+) pump
• dominant neuronal plasma membrane
pump
• pumps Na+ out, K+ in
• maintains cytoplasmic K+ higher and Na+
lower than external
Na-K pump
Figure 44.5
K+ channels maintain resting potential
Figure 44.6
membrane
depolarization
by
gated
Na+ channels
Figure 44.8
membrane
hyperpolarization
by
gated
Cl- channels
Figure 44.8
Pumps and Channels
• gated channels can alter membrane polarity
– opening Na+ channels depolarizes the
membrane
– opening K+ or Cl- channels hyperpolarizes
the membrane
• transmission and processing of information
occurs through changes in neuronal membrane
potentials
Nerve Impulses (Action Potentials)
• opening gated channels results in ion flow
– ion flow in a neuron dissipates over distance
– ion flow cannot transmit a signal to a distant
target
• localized ion flow can stimulate nearby
voltage-gated channels
– if enough Na+ enters, neighboring channels
will open
– if each channel triggers its neighbor, a signal
can travel the length of a neural axon
action potential
Figure 44.9
Nerve Impulses (Action Potentials)
• an action potential
– results from a 1-2 millisecond opening of
Na+ channels
– membrane potential rises rapidly (spike)
then returns to resting potential
– Na+ channels cannot open for 1-2
milliseconds following an action potential
(refractory period)
Nerve Impulses (Action Potentials)
• an action potential
– travels down an axon without loss of
strength
• depolarization opens Na+ gates
• short range current flow depolarizes
nearby membrane
• neighboring Na+ gates open
action potential propagation
Figure 44.10
Nerve Impulses (Action Potentials)
• action potentials travel rapidly along nerves
– rate of transmission is related to diameter of
axon
• thicker axon propagates signal faster
• propagation rate in vertebrates is enhanced by
glial cells
– Schwann cells form discontinuous sheath
• gaps = nodes of Ranvier
• action potentials fire at nodes
nodes of Ranvier
Figure 44.12
saltatory propagation
Figure 44.12
Nerve Impulses (Action Potentials)
• action potential at a node of Ranvier
– propagates by current flow to next node
– current flow is supported by myelin sheath
– saltatory (jumping) propagation is more
rapid than continuous propagation
Synaptic Communication
• synapse
– presynaptic cell membrane
– postsynaptic cell membrane
– synaptic cleft
• neuromuscular junction
– motor neuron => muscle cell
– one axon, many branches & axon terminals
– axon terminals produce neurotransmitter
a neuromuscular
junction
Figure 44.13
Synaptic Communication
• neuromuscular junction
– presynaptic membrane releases
acetylcholine from vesicles by exocytosis
– acetylcholine diffuses across synaptic cleft
– postsynaptic membrane (motor end plate)
receptors bind acetylcholine and open
Na+/K+ channels
– motor end plate depolarizes
– acetylcholinesterase degrades acetylcholine
in synaptic cleft
acetylcholine function
Figure 44.14
Synaptic Communication
• presynaptic axon
– transmits a signal in response to action
potential arrival
– action potential triggers voltage-gated
calcium channel
– calcium influx causes acetylcholine vesicles
to fuse with presynaptic membrane
Synaptic Communication
• postsynaptic membrane
– motor end plate receives signal, opens
channels, depolarizes
– motor end plate does not fire action
potentials (too few voltage-gated channels)
– motor end plate must transmit enough Na+
to spread depolarization to neighboring
areas
– depolarization of neighboring plasma
membrane fires action potentials
synaptic
transmission
at a
neuromuscular
junction
Figure
44.13
Synaptic Communication
• excitatory & inhibitory neuronal synapses
– different presynaptic neurotransmitters
– different postsynaptic receptors
• excitatory synapses depolarize (EPSP)
• inhibitory synapses hyperpolarize (IPSP)
–e.g. GABA or glycine causes Clchannels to open
Information Processing
• Nerve Impulse (action potential) is all-or-none
• firing action potential depends on sum of all
incoming information
– axon hillock receives EPSP/IPSP from all
dendrites and cell body
– IPSPs oppose depolarization by EPSPs
– axon hillock fires action potential when
membrane depolarizes to threshold
Information Processing
• axon hillock sums EPSPs & IPSPs
– spatial summation adds effects of all
synapses at one time
– temporal summation adds effects of
synapses firing rapidly over time
spatial
and temporal summation
at the
axon hillock
Figure 44.15
Information Processing
• presynaptic excitation and inhibition
– synapse between axon terminal of one
neuron and axon terminal of another
– first neuron regulates amount of
neurotransmitter released by axon terminal
responding to action potential
Information Processing
• neurotransmitter receptors
– ionotropic receptors are ion channels
– metabotropic receptors influence ion
channels indirectly
ionotropic receptors
Figure 44.17
a metabotropic receptor
Figure 44.16
Information Processing
• electrical synapses (gap junctions)
– very rapid
– bi-directional
– excitatory only
– unable to perform termporal summation
– require large membrane surface area
– uncommon in nervous systems that utilize
integration and exhibit learning
Information Processing
• neurotransmitters
– more than 25 known
– each may bind more than one receptor
– response is determined by the receptor
Table 44.1