6.5 Neurons & Synapses

Structure of the Human Nervous System

• Composed of cells called neurons that carry rapid electrical impulses.

Motor Neuron Structure & Function

• Parts of a motor neuron:– Cell body = does most of the typical cell functions, contains the nucleus– Dendrites = branched extensions which connect with other neurons– Axon = thin long fiber which extends from the cell body carrying impulses– Myelin sheath = an

insulating membrane– Nodes of Ranvier =

gaps in the myelin sheathswhere the axon is exposed.

– Motor End Plates= nerve endings thatconnect with a newnerve, muscle, or gland

Nodes of RanvierNucleus

Motor Neuron Structure & Function

• How is the overall structure of a motor neuron related to its function?

Sending a Signal

Figure 48.5

Dendrites

Cell body

Nucleus

Axon hillockAxon

Signal direction

Synapse

Myelin sheath

Synapticterminals

Presynaptic cell Postsynaptic cell

The Nerve Impulse

• Nerve impulse is the signal that the nerve sends through the axon.

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

• Resting potential: “the state of being where an area of a neuron is ready to send an action potential (but is not currently sending).– During resting potential, the neuron is called

polarized.

• The resting potential depends on the ionic gradients that exist across the plasma membrane.

CYTOSOL EXTRACELLULARFLUID

[Na+]15 mM

[K+]150 mM

[Cl–]10 mM

[A–]100 mM

Permanently Located

in Cytoplasm

[Na+]150 mM

[K+]5 mM

[Cl–]120 mM

–

–

–

–

–

+

+

+

+

+

Plasmamembrane

Figure 48.10

The concentration of Na+ is higher in the extracellular fluid than in the cytosol

The opposite is true for K+

Sodium-Potassium Pump

• We are actively pumping Na+ out of the cytoplasm, and K+ into the cytoplasm through our Sodium-Potassium pump.– This requires ATP.– Sends 3 Na+ out,

takes 2 K+ in.• http://highered.mcgr

aw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Resting Potential

• Action Potential: The signal conducted by a “self-propagating wave of ion movement in and out of the neuron membrane”

• When a stimulus depolarizes the membrane…– Na+ channels open, allowing Na+ to diffuse into the cell

• As the action potential goes away…– K+ channels open, and K+ flows out of the cell

• This action potential “depolarizes” the cell, making the cytoplasm & extracellular fluid equally charged

– +– + + + + +

– +– + + + + +

+ –+ – + + + +

+ –+ – + + + +

+ –+ – – – – –

+ –+ – – – – –

– – – –

– – – –

– –

– –

+ +

+ +

+ ++ + – – – –

+ ++ + – – – –

– –– – + + + +

– –– – + + + +Na+

Na+

Na+

Actionpotential

Actionpotential

ActionpotentialK+

K+

K+

Axon

An action potential is generated as Na+ flows inward across the membrane at one location.1

2

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

3 The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

K+

Review

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

Synaptic Transmission

• Once the impulse reaches the axon terminal, there must be a chemical communication between the neurons.

• Synaptic Cleft = area between axon terminal and postsynaptic neuron or cell

• Neurotransmitter = any chemical used for synaptic transmission– Ex. Acetylcholine– Neurotransmitters

are held in vesicles

Steps:

• 1. Calcium ions diffuse into the axon terminal.

• 2. Vesicles with neurotransmitter fuse with cell membrane, releasing its contents into the synaptic cleft.

• 3. Neurotransmitters diffuseacross synaptic cleft to the post-synaptic neuron or muscle/glandcell.

• 4. Neurotransmitters bind witha receptor protein.

Steps

• 5. When the neurotransmitterbinds with the receptor, it opens an ion channel and Na+ diffusesinto the cell….

• 6. depolarizing it, and instigating an action potential.

• 7. Neurotransmitter is broken down and released from receptor.

• 8. Na+ ion channels close.• 9. Neurotransmitters are taken

up again by vesicles into axon terminal.

Synaptic Transmission

• http://www.youtube.com/watch?v=hqWv0iEF4ho

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__transmission_across_a_synapse.html