Neurons &

Nervous Systems

nervous systems

connect distant parts

of organisms;

vary in complexityFigure 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 neuronsFigure 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

cell body

axon hillock

axon

axon terminals

Figure 44.2

variation in

# of connections,

length of transmissionFigure 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 potentialFigure 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 channelsFigure 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 pumpFigure 44.5

K+ channels maintain resting potentialFigure 44.6

membrane depolarization

by gated

Na+ channelsFigure 44.8

membranehyperpolarization

by gated

Cl- channelsFigure 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 potentialFigure 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 propagationFigure 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 RanvierFigure 44.12

saltatory propagationFigure 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 junctionFigure 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 functionFigure 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 neuro-

muscular junctionFigure 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 Cl- channels 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 hillockFigure 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 receptorsFigure 44.17

a metabotropic receptorFigure 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