ANATOMY

Unit 1 Notes:Nerve

Impulses & Reflexes

(1) What is a Nerve Impulse?

• Chain reaction of chemical events, allowing your neurons to send “messages”.

• Synonymous Terms:– Neuron Message– Nervous System Response– Depolarization & Repolarization

(2) The Basic Steps

1. Neurotransmitter binds to neuron.

2. Depolarization.

3. Repolarization.

4. Repeat #2 and 3 until reaching axon terminal.

5. Release of another neurotransmitter from axon terminal.

6. Regain resting potential.

NeurotransmitterBinds

Depolarization & RepolarizationDown The Axon

Release a neurotransmitter +

Stimulate next move

(3) Resting Potential

• “Resting Potential”:The natural state of a neuron when it’s not doing anything.

• At Rest There is…– Potassium (K+) on the inside of axon.– Sodium (Na+) on the outside of axon.–More cations on the outside than inside.

*This means that the outside is naturally more positive than the inside (making the axon polar).

(4) Depolarization

• De = withoutPolar = uneven charge

• Depolarization: To remove the imbalance of charge.

• What Happens?– Sodium pumps open.– Sodium ions fly thru channels into axon.– Temporarily increases positive charge inside

axon, removing polarization.

(5) Repolarization

• Re = repeatPolar = uneven charge

• Repolarization: To regain the charge imbalance.

• What Happens?– Potassium pumps open.– Potassium ions fly thru channels out of axon.– Temporarily increases positive charge back on

the outside of the axon, regaining polarization.

(6) Reaching the Terminals

• A neurotransmitter will be released.

• This neurotransmitter will:– Bind to another neuron or organ.–Make something else happen (muscle

contraction or another nerve impulse).

(7) Regaining Resting Potential

• Even though the axon is polarized at the end, the ions are on the wrong sides of the axon.

• Must actively transport ions:– Sodium must go back out.– Potassium must go back in.

• Requires ATP.

…Let’s Review All That Again…

(8) Myelin Sheath

• Where Is It?– Covers the outside of the axon.– Wraps around Na/K pumps.

• What Is Its Purpose?– Block some of the Na/K pumps.– Increases the speed of nerve impulse.

• How Does It Work?– Just like main-roads versus the freeway.– Main Roads = Axon without myelin sheath.– Freeway = Axon with myelin sheath.

(9) Reflexes

• Automatically programmed response to a specific stimulus.

• All reflexes are involuntary, however do not need to travel through the individual nervous system divisions.

• Reflex Arc Path:–Stimulus triggers nerve impulse.– Impulse sent to CNS, but at spinal cord

routes directly back out to effector organ.

(10) Types of Reflexes

• Somatic:A reflex which stimulates a response in the skeletal muscle.–Example: Retracting your arm from a hot

stove.

• Autonomic:A reflex which stimulates a response in smooth muscle.–Example: Your stomach releasing gastric

juice and churning once food is dropped in.