Topic 20: Neurons and Synapses (Ch. 48)

BIOL 1030 – TOPIC 20 LECTURE NOTES

Topic 20: Neurons and Synapses (Ch. 48)

I.  function

A.  receive information about the external and internal environment

B.  process and integrate that information

C.  store information as necessary

D.  command responses (mainly by signals to muscles and glands)

II.  how neurons receive and conduct signals

A.  review of neuron structure: dendrites, cell body, axon

B.  neurons keep a resting membrane potential (polarized membrane)

1.  many negative ions (anions) are trapped inside the cell

2.  sodium-potassium pumps in the plasma membrane actively pump out 3 Na+ ions and bring in 2 K+ ions – makes inside even more negative

3.  active pumping sets up diffusion gradients

·  higher K+ concentration in the cell

·  higher Na+ concentration outside the cell

·  diffusion occurs to try and correct the imbalances

·  work must constantly be done to maintain the ion gradients

C.  incoming signals generate action potentials by changing membrane potential beyond a threshold

1.  plasma membrane also has voltage-gated channels for Na+ and K+

2.  small amounts of depolarization do not affect these channels

3.  depolarization beyond a threshold leads to an action potential

·  first, Na+ channels open: Na+ rushes in

·  leads to a rapid change in membrane potential, all the way from about

-70 mV to about +30 mV

·  K+ channels open: K+ rushes out

·  leads to a rapid repolarization

·  neurons have a refractory period between possible action potentials, because it takes time for the sodium-potassium pump to reestablish the ion gradients

D.  action potentials propagate along dendrites, usually through the cell body, and down axons to the synapse(s)

III.  how neurons transmit signals

A.  synapses are intercellular junctions between an axon and either a muscle cell, gland cell, or dendrites of another neuron

B.  cell sending signal: presynaptic cell; cell receiving signal: postsynaptic cell

C.  neurotransmitters carry the signal to the next cell

1.  action potential at end of axon leads to opening of voltage-gated Ca++ channels: Ca++ rushed in

2.  Ca++ stimulates vesicles filled with neurotransmitters to fuse with plasma membrane

3.  neurotransmitters are released into the synapse

4.  neurotransmitters bind with receptors in the cell membrane of the postsynaptic cell

5.  binding causes different effects depending on neurotransmitter type and postsynaptic cell type

6.  example: ACh binding to ion channel in muscle cell allows Na+ in and K+ out, starting an action potential in the muscle cell

D.  some know neurotransmitters and their major functions

1.  glutamate – excitatory (transmits action potentials)

2.  glycine and GABA – inhibitory (reduce likelihood of action potentials)

3.  dopamine – control of body movements

4.  serotonin – regulation of sleep and emotional state

E.  neurotransmitters must be reabsorbed or destroyed, and receptor number can be modified

1.  the normal body responds to unusually high or low amounts of neurotransmitters

·  if neurotransmitter level stays too high, receptor number is reduced and reuptake or destruction mechanisms are increased

·  the opposite occurs if level stays too low

·  neurological disorders are often associated with problems in these responses (including psychiatric disorders)

2.  drug addiction is caused by body responses to drug exposure

·  example: cocaine leads to keeping too much dopamine around too long

·  eventually, dopamine receptor number is reduced

·  cocaine now needed just to feel normal, and higher dose needed to get high

IV.  the role of supporting cells

A.  called neuroglia in central nervous system (CNS – brain and spinal cord)

B.  10x more of them than neurons in CNS

C.  major role is to produce myelin sheaths around axons

1.  myelin sheaths are layers of membrane, insulating the axon

2.  sheaths are interrupted every 1-2 mm by small gaps called nodes of Ranvier

3.  action potentials do not move along myelin-coated regions; instead, they “jump” to the next node of Ranvier

4.  thus, action potentials propagate quicker along myelinated axons

D.  not all neuron cells have myelin sheaths

V.  comparison of animal nervous systems

A.  sponges do not have a nervous system

B.  radiata have neural nets

C.  echinoderms have a neural net with a central neural ring

D.  bilataria typically have a central nervous system, with a centralized control center (brain) located in the head

VI.  vertebrate nervous system organization

A.  central nervous system (CNS) – brain and spinal cord; mainly association neurons

B.  peripheral nervous system (PNS) – everything else; mainly sensory and motor neurons

1.  somatic motor neurons – control skeletal muscles

2.  autonomic motor neurons – regulate smooth and cardiac muscle, and glands

VII.  vertebrate brain – selected regions

A.  medulla oblongata (brain stem) – integrates brain with spinal cord; controls breathing

B.  cerebellum – associated with medulla oblongata; involved in coordination and motion memory

C.  cerebrum – greatly enlarged in humans, generally larger in mammals relative to body mass; motor control, memory, emotion, higher functions (in cerebral cortex)

1.  left cerebral hemisphere controls right side of body, and vice-versa

2.  corpus callosum integrates the hemispheres

3.  hemispheres divided into lobes with different main functions

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