Chapter 48

Chapter 48

Nervous System Overview

Nervous System Function

1.) Sensory input: the NS has receptors that gather sensory input (touch, taste, pain, smell, sight, etc)

2.) Integration: input is conducted to the brain/spinal cord

3.) Motor output: the NS causes something to happen in effector cells (muscle cells, gland cells, etc). The NS triggers the appropriate response.

Nervous System Structure

1.) Our nervous system is composed of nerve cells or neurons.

2.) Structure of a neuron:

a.) Cell body: the part of the neuron containing the nucleus & organelles

Nervous System Structure

b.) Branching from the cell body are fibers called dendrites & axons.

i.) Dendrites are short and receive signals from other neurons.

ii.) Axons are long & transmit signals to other neurons.

Nervous System Structure

c.) Axons are covered with an insulating material called myelin. This forms what is termed a myelin sheath around the axon.

3.) Axons either lead to other neurons or to effector cells (like muscle cells).

a.) B/w each axon & the next neuron or effector is a space called the synapse.

Nervous System Structure

b.) To transmit signals across the synapse, axons release neurotransmitters.

i.) FYI: the cell receiving the NT is the postsynaptic cell & the cell sending it is the presynaptic cell.

Nervous System Function

The basics: a stimulus causes a change in a neuron which transmits that change down its axon to other neurons. This can lead to the brain which stimulates the appropriate neurons to trigger the appropriate response.

So – what is the change in the neuron???

Neuron Signaling

1.) All cells have a difference in electrical charges across their membranes = membrane potential.

a.) The membrane potential of a neuron at rest = 70 mV = resting potential.

Neuron Signaling

2.) Membrane potential exists due to different ion concentrations on either side of the membrane.

a.) A key way in which cells maintain their membrane potential is thru the action of the Na+/K+ pump.

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#

Neuron Signaling

3.) Changes in membrane potential = neuron impulses/signaling!

4.) Stimuli can cause the gated ion channels in the neuron’s membrane to open or close.

a.) This can cause ions to move across the membrane & change the membrane potential.

The Action Potential

An action potential is a nerve impulse – it is a change in membrane potential that spreads down the length of neuron.

Here’s how it happens:1.) A stimulus causes Na+ channels in the

neuron’s membrane to open.

a.) Na+ rushes into neuron.

The Action Potential

b.) Na+ influx cause the inside of the neuron to be less negative – called depolarization.

2.) If the stimulus is strong enough, enough Na+ will rush in & cause the neuron to reach its threshold potential (~ -50 mV).

3.) If depolarization reaches threshold potential, an action potential is generated!!!

The Action Potential

a.) This means that depolarization spreads from one area of the neuron to the next, down the axon.

b.) Action potentials are all or none events – they always create the same pattern of depolarization (to the same charge) once they are triggered, no matter how strong the stimulus is.

The Action Potential

4.) Depolarization is followed by a rapid repolarization:

a.) Opened Na+ channels start to close & K+ channels start to open.

b.) Na+ can’t enter & K+ rushes out – this returns the cell to its negative state.

The Action Potential

5.) During this time, there is a short refractory period – a time during which another action potential cannot be generated.

6.) The more intense a stimulus = the more frequent the action potential.

The Action Potential

7.) The action potential is propagated down the axon:

a.) Na+ ions moving into the neuron in one area depolarize the next section of the axon.

b.) The refractory period keeps the action potential moving in one direction.

The Action Potential

http://outreach.mcb.harvard.edu/animations/actionpotential.swf

The Action Potential8.) We conduct action potentials very rapidly due to

the myelin sheath surrounding the axon.

a.) There are small gaps in the myelin called nodes of Ranvier – these are where the ion channels are located.

b.) Action potentials can only be generated here – this causes the action potential to “jump” from node to node = called saltatory conduction.

http://www.brainviews.com/abFiles/AniSalt.htm

Communication at Synapses

1.) Synapses can occur b/w 2 neurons, b/w sensory receptors & neurons or b/w neurons and effectors (like muscle cells or gland cells).

2.) At chemical synapses, the action potential triggers the release of neurotransmitter (NT) molecules across the synapse which bind to the postsynaptic cell.

Communication at Synapses

Here’s how…

1.) NT molecules are stored in vesicles in presynaptic cell.

2.) An action potential triggers Ca ion channels to open & Ca floods into the presynaptic cell.

3.) Ca causes the vesicle to fuse with the membrane at the synapse.

Communication at Synapses

4.) The vesicles release their NT into the synapse.

5.) The NT molecules bind with receptors on the postsynaptic cell’s membrane.

a.) The binding of the NT molecules either causes depolarization or hyperpolarization.

Communication at Synapses

b.) Depolarization = excitatory postsynaptic potential = action potential

c.) Hyperpolarization = inhibitory postsynaptic potential

Communication at Synapses

Click on “Chemical Synapse” at the following link:

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html#

The following link has a good representation of chemical synapses associated with how you learn:

http://outreach.mcb.harvard.edu/animations/synaptic.swf

Neurotransmitters

There are many types of NTs. They cause different responses in different types of cells.

Some examples:

1.) Acetylcholine: functions b/w neurons & muscle cells to trigger contraction

Neurotransmitters

2.) Dopamine & serotonin: function in the brain & are associated with mood, attention, sleep & learning.

a.) Imbalances in NT can lead to disorders – depression, etc.

3.) Epinephrine & norepinephrine (adrenaline): have a variety of affects associated with “fight or flight” response

Neurotransmitters

4.) Endorphins: function b/w neurons to dull pain perception.

5.) GABA: functions in brain to inhibit impulses.

Neurotransmitters

NTs functioning in different parts of the brain & b/w different neurons have different effects.

Some of the most interesting things to look at are how drugs affects NTs and can cause addiction.

The Science of Addiction

Check out the following sites:

1.) First go here & go through the slides on the reward pathway:http://learn.genetics.utah.edu/units/addiction/reward/

2.) Then go here and review synaptic communication: http://learn.genetics.utah.edu/units/addiction/reward/neurontalk.cfm

3.) Now go here to the “Mouse Party” to see what drugs do at the synapse level:

http://learn.genetics.utah.edu/units/addiction/drugs/mouse.cfm

Main Overview

So…a stimulus detected by a sensory receptor triggers an action potential in a neuron. This action potential is propagated down the axon & passes to other neurons – maybe going all the way to the brain – by the release of NT at synapses.

A response can be triggered in the form of action potentials passing back the opposite way to an effector.