Nervous systems

Keywords (reading p. 960-976)

• Nervous system functions
• Structure of a neuron
• Sensory, motor, inter- neurons
• Membrane potential
• Sodium potassium ATPase
• Action potential
• Depolarization
• Hyperpolarization
• Voltage gated ion channels
• Action potential propagation
• Node of Ranvier
• Synapse
• Presynaptic cell
• Postsynaptic cell
• Signal transmission at chemical synapse

Nervous systems

• Functions: sensory input, integration, motor output
• These functions overlap

Example of overlapping function

• Sensory input - visual signal (involves peripheral nervous system)
• Integration - processing of signal by central nervous system (in vertebrates brain and spinal cord)
• Motor output - muscular output (involves peripheral nervous system)

Neurons, the cells of the nervous system

• Structure of a neuron
• Cell body
• Dendrites (input)
• Axon (output)

Structural diversity of neurons

• Sensory neuron - long axon with cell body connected to axon
• Motor neuron - long axon with cell body connected to dendrites
• Interneurons - found in brain, highly branched axons and/or dendrites

Neurons conduct electrical signals

• Very fast
• How do cells convey electrical signals

Membrane potential

• Living cells have an electrical potential across their membranes
• The inside of the cell is more negatively charged than the outside
• This difference in charge is called the membrane potential
• Usually between -50 to -100 mV

What is the basis for the membrane potential?

• Two causes:
• 1) differences in ionic composition of intracellular and extracellular fluid
• 2) selective permeability of the plasma membrane

Ionic composition of intracellular and extracellular fluid

• Cation (positively charged ions) composition
• Intracellular fluid - primary cation is K⁺, Na⁺ is low
• Extracellular fluid - primary cation is Na⁺, K⁺ is low

Ionic composition of intracellular and extracellular fluid

• Anion (negatively charged ions) composition
• Intracellular fluid - proteins, amino acids, sulfate, phosphate (A^-)
• Extracellular fluid - Cl^-

Recall that the membrane can have channels that allow facilitated diffusion to occur

• Cell membranes have many more K⁺ channels than Na⁺ channels

What will happen to K⁺? Na⁺?

Flow of K⁺ >> Na⁺ therefore net loss of positive charge from cell

Gradient between extracellular and intracellular fluid favors loss of K⁺ from the cell

• Negatively charged ions will want to follow to balance the loss of (+) charge, but since the intracellular anions are large molecules like amino acids and proteins, they cannot diffuse out.

This makes the inside of the cell more negatively charged than the outside

• But, there is also a gradient favoring the diffusion of Na⁺ into the cell from the outside
• This could prevent negative charge from building up inside, but it doesn’t. Why not?

Two reasons:

• Low Na⁺ permeability due to few open Na⁺ channels
• Sodium-potassium ATPase

Sodium-potassium ATPase

• Active transport (antiport)
• Each pumping cycle pumps 3 Na⁺ out and 2 K⁺ in at the expense of 1 ATP.

Excitable cells

• Most cells have a stable membrane potential of around -70 mV
• Excitable cells can generate changes in their membrane potentials
• Excitable cells include neurons and muscle cells

Action potential

• Excitable cells can change their membrane potential
• When signaling becomes more positive (depolarization)
• The depolarization is called an action potential
• The action potential is the basis for electrical signaling

Hyperpolarization

depolarization

Action potential

Action potentials occur because of voltage gated ion channels

• If the stimulating potential causes the membrane potential to rise about 15-20 mV an action potential results.
• This is due to the opening of voltage gated ion channels
• voltage gated channels open briefly then shut

Resting state

Initially only Na⁺ channels open

• Since there is a large concentration of Na⁺ outside the cell, Na⁺ rushes in making the intracellular fluid less negatively charged
• This causes the peak of the action potential

Voltage gated K⁺ channels also open

• But they are much slower than Na⁺ channels
• They are fully open after the peak of the action potential
• K⁺ flows out of the cell, and the membrane potential becomes negative again

Undershoot

Tetrodotoxin

• Produced by pufferfish
• Blocks Na⁺ channels
• What would be the effect of ingesting tetrodotoxin?

Propagation of the action potential

• Action potential "travels" along the axon to the other end of the cell
• The speed of transmission can be as high as 100 meters per second or 225 mph.
• Propagation is a series of new action potentials that travel along the axon

Propagation: what happens at the level of the ion channels

• First action potential gives rise to a depolarization further along the axon

• Depolarization at second segment results in the opening of voltage gated Na⁺ channels and a second action potential occurs
• Second action potential triggers a third action potential, etc.

High performance axons

• Faster signal conduction allows more rapid coordination between sensory input and motor output.
• 2 ways to increase action potential transmission speed
• Increase axon diameter
• Nodes of Ranvier

Nodes of Ranvier

• Axons of vertebrates are myelinated
• Insulating layer on axon results from Schwann cells
• Small gaps of exposed axon surface are present between Schwann cells

Nodes of Ranvier

• Depolarization and action potential only occurs in the nodes
• Passive conduction of depolarization from node to node.
• By "jumping" from node to node transmission is faster

How do neurons communicate with other cells?

• Cell/cell communication occurs at synapses
• Examples:
• Synapse between sensory receptor and sensory neuron
• Synapse between motor neuron and muscle cell
• Synapse between neurons

Synapse between neurons

• Transmitting cell = presynaptic cell
• Receiving cell = postsynaptic cell
• Two types of synapse
• Electrical
• Chemical

Electrical synapse

• Action potential (electrical signal) spreads directly.
• Cytoplasm of the two neurons is joined by gap junctions
• Allows rapid transmission from neuron to neuron

Chemical synapse

• Narrow gap between the neurons called the synaptic cleft
• Action potential results in release of neurotransmitter by presynaptic cell
• Neurotransmitter causes depolarization of postsynaptic cell and can result in another action potential

Chemical synapse: a closer look

• Depolarization at the synaptic terminal results in Ca⁺⁺ influx
• Ca⁺⁺ causes vesicles containing neurotransmitter to fuse with presynaptic membrane
• Neurotransmitter diffuses into synaptic cleft
• Neurotransmitter binds to ion channels on the post synaptic membrane

What happens when neurotransmitter binds to ion channels on the post synaptic membrane?

• Ion channels open
• This results in either a depolarization or hyperpolarization (inside becomes more negative)
• Depolarization is stimulatory
• Hyperpolarization is inhibitory

How do the channels close again?

• Enzymatic degradation of the neurotransmitter
• Uptake of neurotransmitter by other neurons