Cells of the Nervous System

• Neurons – cells that send signals within the body

• Supporting cells:– Glial cels – glue neurons together,control

extra-cellular environment of cell– Astrocytes contribute to the blood brain

barrier,– Schwann cells : forms myelin sheath in

PNS, loss of myelin sheath results in MS

Membrane Potential

• All mammalian cells have a negative resting potential

• What factors help maintain this negative resting potential?

• Neurons have ionic channels in the membrane that allows the membrane potential to change

• How is the resting potential maintained in a neuron if there are ionic channels that allow for diffusion of ions?

The Action Potential

• Neurons have gated channels that allows for a change in the resting potential

• Different stimuli open different channels:• Excitatory stimulus opens the Na+

channels and leads to depolarization• Inhibitory stimulus opens K+ channels

and leads to hyper-polarization

What is happening with the gated channels at rest?

What do you notice about the response of these gates to the excitatory stimulus?

Why is there no sodium entering the cell anymore?

Use your knowledge of the timing of these gates to understand why there will be no response to a new stimulus during this phase of the action potential.

Propagation of the Action Potential

• Generated at the axon hillock• What is the “all or none” concept

associated with an action potential?• How is it that you can feel different

degrees of stimulus when action potentials are generated in an all or none fashion?

• How is it that an action potential moves in only one direction?

Speed of Propagation

• How is the speed of propagation of an action potential regulated?

• Larger diameter axons conduct faster action potentials

• Saltatory conduction…..

The Synapse• What is a synapse?• The space between neurons• Presynaptic cell = transmitting cell• Postsynaptic cell = receiving cell• Electrical synapse – action potential spreads

directly via gap junctions• How is an action potential propagated

between synpases that do not use gap junctions to directly send the impulse?

• Chemicals called neurotransmitters are used

EPSP and IPSP• What channel is opened by an excitatory

synapse?• Na+• What channels is opened by an inhibitory neurot

ransmitter?• K+• How is the regulation of the postsynaptic

membrane’s gated channels different from the gated channels found along the axon?

• These channels are chemically sensitive gates as opposed to voltage sensitive gates

Summation

Neurotransmitters

• Neurotransmitters:– Can cause IPSP or EPSP– Are discharged from synaptic vessicles– Are rapidly removed by enzymes

• Acetylcholine – found at neuromuscular junctions– Cholinesterase = enzyme that removes

ach from synapse

• Epinephrine • Norepinehrine• Dopamine – too much associated with

schizophrenia, too little associated with Parkinson's disease

• Serotonin – associated with learning, sleep and moodiness– LSD binds to serotonin receptors and is a

hallucinogenic

• GABA – inhibitory neurotransmitter• Endorphins – natural analgesic in brain• Some neurons release gas molecules such

as NO and CO (Viagra stops an enzyme that slows NO effects on erectile tissue)

Neurotoxins

• Nerve gas, malathion destroys cholinesterase

• DDT increases permeability to Na+• Tetrodotoxin – found in puffer fish and

red tide toxin– Blocks Na+ channels

• Botulin – prevents release of ach• Curare – blocks ach receptor

Functional Organization of the Nervous System

Neuronal Circuits

• Reflex cause an automatic response does not invovle the brain

• Ganglion = a cluster of nerve cell bodies found outside of the CNS

• Interneurons – neurons that integrate sensory input with motor output

• Interneuron branches can carry signals to different parts of spinal cord or brain– Convergent circuits bring information from different

sources– Divergent circuits send information to different regions

The Brain• Brainstem: automatic

activities– Medulla oblongata

controls homeostatic functions, and conducts data to other parts of the brain

– Pons – helps medulla with regulation

– Midbrain – integration of sensory information from auditory and visual pathways

• Cerebellum: movement and balance

• Hand-eye coordination uses cerebellar function

• Diencephalon – intrgrating centers:– Thalamus –

sensory input and motor output center, integration to other brani centers

– Hypothalamus – homeostatic regulation: thermostat, hormonal regulation of anterior pituitary and production of posterior pituitary hormones.

Cerebrum most complex integration center: thought

Note the cell body that is

connected to the axon. This will be part of a ganglion to coordinate

stimuli