Neurons: The Messengers

1. Neurons: The Messengers • About 100 billion neurons (nerve cells) in the human brain • Neurons have many of the same features as other cells • Nucleus • Cytoplasm • Cell membrane • What makes neurons unique is their shape and function

2. Neural Communication The body’s information system is built from billions of interconnected cells called neurons.

3. Neuron A nerve cell, or a neuron, consists of many different parts.

4. Parts of a Neuron Cell Body: Life support center of the neuron. Dendrites:Branching extensions at the cell body. Receive messages from other neurons. Axon:Long single extension of a neuron, covered with myelin [MY-uh-lin] sheath to insulate and speed up messages through neurons. Terminal Branches of axon: Branched endings of an axon that transmit messages to other neurons.

5. Types of Neurons • Sensory neurons • Carry information from sensory systems to the brain • Also referred to as afferent • Motor neurons • Carry information from the brain to muscles and glands • Also referred to as efferent • Interneurons • Carry information between other neurons

6. Glial Cells • Cells that insulate and support neurons • Create the myelin sheath • Remove waste products • Provide nourishment • Prevent harmful substances from entering the brain

7. Action Potential Properties All-or-None Response: When the depolarizing current exceeds the threshold, a neuron will fire. If the depolarizing current fails to exceed the threshold, a neuron will not fire. Intensity of an action potential remains the same throughout the length of the axon.

8. Action Potential A neural impulse. A brief electrical charge that travels down an axon and is generated by the movement of positively charged atoms in and out of channels in the axon’s membrane.

9. The Neural Impulse • Ions • Charged molecules • Resting Potential • When more negative ions are inside the neuron than outside • Charge is approximately -70mV • Neuron is not transmitting information

10. Depolarization & Hyperpolarization Depolarization: Depolarization occurs when positive ions enter the neuron, making it more prone to firing an action potential. Hyperpolarization occurs when negative ions enter the neuron, making it less prone to firing an action potential.

11. Threshold Threshold:Each neuron receives depolarizing and hyperpolarizing currents from many neurons. When the depolarizing current (positive ions) minus the hyperpolarizing current (negative ions) exceed minimum intensity (threshold) the neuron fires an action potential.

12. Refractory Period & Pumps Refractory Period: After a neuron fires an action potential it pauses for a short period to recharge itself to fire again. Sodium-Potassium Pumps: Sodium-potassium pumps pump positive ions out from the inside of the neuron, making them ready for another action potential.

13. Synapse Synapse [SIN-aps]ajunction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron. This tiny gap is called the synaptic gap or cleft.

14. The Synapse • Synaptic space (synaptic cleft) • Tiny gap between neurons • Terminal button • Enlarged area at the end of an axon • The synapse • Composed of the terminal button of one neuron, the synaptic space, and the dendrites or cell body of the receiving neuron

15. Transmission Between Neurons • Synaptic vesicles • Sacs in terminal button that release chemicals into synaptic space • Neurotransmitters • Chemicals released by synaptic vesicles • Receptor sites • Location on receptor neuron for specific neurotransmitter

16. Neurotransmitters Neurotransmitters (chemicals) released from the sending neuron travel across the synapse and bind to receptor sites on the receiving neuron, thereby influencing it to generate an action potential.

17. Lock & Key Mechanism Neurotransmitters bind to the receptors of the receiving neuron in a key-lock mechanism.

18. Neurotransmitters

19. Agonists

20. Antagonists

21. Some Well-Known Neurotransmitters • Acetylcholine (ACh) • Released at the neuromuscular junction • Plays an important role in arousal and attention • Loss of ACh producing cells is linked to Alzheimer’s Disease • Dopamine • Affects neurons associated with voluntary movement • Plays a role in learning, memory, and emotions • Loss of dopamine-producing cells causes symptoms of Parkinson’s Disease

22. Some Well-Known Neurotransmitters • Serotonin • Found throughout the brain • Appears to sets an “emotional tone” • Low serotonin levels are implicated in depression • Endorphins • Reduce pain by inhibiting or “turning down” neurons that transmit pain information

23. Reuptake Neurotransmitters in the synapse are reabsorbed into the sending neurons through the process of reuptake. This process applies the brakes on neurotransmitter action.

24. Psychopharmacology • Most psychoactive drugs (and toxins) work by blocking or enhancing synaptic transmission • Botulism • Blocks release of ACh at the neuromuscular junction, causing paralysis • “Botox” is botulism toxin used to prevent facial muscles from making wrinkles

25. Psychopharmacology • Curare • Can stun or kill prey quickly • Blocks ACh receptors causing paralysis • Antipsychotic medications • Block dopamine receptors • Reduces schizophrenic hallucinations • Caffeine • Increases the release of excitatory neurotransmitters by blocking the inhibitory neurotransmitter adenosine

26. Psychopharmacology • Cocaine • Prevents reabsorption of dopamine • Leads to heightened arousal of entire nervous system