Ch 3: Nervous system - Excitable cells/ neuronal signalling

Plasma membrane of all cells are … electrically

Polarized

Membrane potential

A separation of opposite charges across the plasma membrane

Water-soluble ions

Can cross membrane only through channels specific to them

(Leak channels or gated channels)

Leak channels

Open all the time

Gated channels

Voltage gated

Chemically gated

Mechanically gated

Thermally gated

Na+ - K+ pump

Makes a small contribution to resting membrane potential

3 Na out of cell for ever 2 K it pumps in

Cell loses more positive charges than it gains

(Actively maintain Na+ and K+ concentration gradients)

K+ concentration

Higher inside the cell

Favours movement of K+ out of cell

Inside becomes more negative

K+ acting alone would establish equilibrium potential of -90mV

Na+ concentration

Higher outside cell

Favours movement into cell

Na+ acting alone would establish equilibrium potential of +60 mv

Polarization

Any state where the membrane potential is other than 0mV

Depolarization

Membrane becomes less polarized than at resting potential

Repolarization

Membrane returns to resting potential after having been depolarized

Hyperpolarization

Membrane becomes more polarized than at resting potential

Depolarization proceeds slowly at first until it reachs

Threshold potential

Repolarization forces push potential too far, causing a brief phase called

Hyperpolarization

Nerve and muscle cells are … cells

Excitable cells (can change their resting membrane potential to produce electrical signals)

Neurons uses signals to

Receive, process, initiate, and transmit messages

Muscle cells use signals to

Initiate muscle contraction

Action potentials are propagated from the … to the …

Axon hillock

Axon terminals

Axon hillock

First portion of axon

The region of the cell body from which the axon leaves

Neuron’s trigger zone

Axon terminals

Release chemical messengers that simultaneously influence other cells with which they come into close association

Output zone of the neuron

Two types of propagation

Contiguous conduction

Saltatory conduction

Contiguous conduction

Conduction in unmyelinated fibres

Action potential spreads along every portion of membrane

Saltatory conduction

Rapid conduction in myelinated fibres

Impulse jumps over sections of the fibre covered with insulating myelin

Refractory period determines

The maximum number of new action potentials that can be initiated and propagated along a fibre in a given period of time

Length of refractory period varies for different types of neurons

Longer the refractory period, the greater the delay before a new action potentials can be initiated

All-or-none law

Once threshold is reached, the resultant action potentials always goes to maximal height

A triggering event that fails to depolarize the membrane does not trigger an action potential at all

Myelin

Composed primarily of lipids and acts as an insulator on the nerve

Produced by oligodendrocytes in brain/spinal cord

Produced by Schwann cells in nerves running between CNS and PNS

Nodes of Ranvier lack myelin (AP occur)

A stronger stimulus

Does not produce a larger action potentials

Does trigger a greater number of action potentials per second

The speed at which an action potential travels down the axon depends on two factors

1. Whether fibre in myelinated

2. Diameter of the fibre

Why does saltatory conduction propagate action potentials more rapidly than contiguous conduction

AP is regenerated only at unmyelinated axonal nodes and not between

Myelinated fibres conduct impulses about 50x faster than unmyelinated fibres of comparable size

When a neuron terminates on a muscle or gland, the neuron is said to … the structure

Innervate (supply with nerves)

Steps happening at synapse

1. AP propagated to terminal of presynaptic neuron

2. Ca2+ enters synaptic knob (presynaptic terminal)

3. Neurotransmitter binds to receptor sites on post synaptic neuron

4. Neurotransmitter binds to receptor sites on postsynaptic neuron

5. Specific ion channels open the sub synaptic membrane

Two types of synapses

Excitatory synapses (Na+ in, glutamate)

Inhibitory synapses (Cl- in, GABA)

The response to a given neurotransmitter-receptor combination is

Always constant and produces the same response (some always excitatory or inhibitory, others are variable)

Mechanisms for removal of neurotransmitter

1. Diffusion away from synaptic cleft

2. Inactivation by specific enzyme within sub synaptic membrane

3. Being actively taken back up into the axon terminal by transport mechanisms in presynaptic membrane

Convergence

Where a given neuron has many other neurons synapsing on it (at dendrites)

Single cell is influenced by many other cells

Divergence

When a single cell synapses with and influences many other cells (at terminals)