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Nervous System Structure + Reflex Arc (1, 2)

CNS, PNS, Reflex Arc
by

Andrea Weber

on 21 September 2012

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Transcript of Nervous System Structure + Reflex Arc (1, 2)

61 2009-2010 45 2009-2010 33 2009-2010 Various neurotransmitters can be destroyed, or inhibited in the following ways:

1. Broken down by enzymes released from the pre-synaptic neuron.

2. Reuptake of the neurotransmitter into the pre-synaptic membrane.

3. Inactivated by enzymes in the post-synaptic membrane. 59 2009-2010 Structural and functional areas of the cerebrum: 40 2009-2010
Applications:

The effects of many drugs are exerted at the level of the neurotransmitter. Likewise, many diseases interfere with the correct functioning of neurotransmitters. Consider the situations that follow. 28 2009-2010 19 2009-2010 The period of depolarization must be completed and the nerve must repolarize before a second action potential can be conducted.
During the process of repolarization, the neuron cannot respond to any stimuli, nor can it produce an action potential.
The time it takes for the condition of the resting membrane to be restored (repolarization) is called the refractory period. b) Refractory Period 11 2009-2010 63 2009-2010 52 2009-2010 Action Summary 43 2009-2010 Answer: The effect of these drugs would be extreme feelings of anxiety. It is interesting to note that drugs such as Valium, which are given to control anxiety, act to increase the effectiveness of GABA in the brain. Situation 5:
GABA is a neurotransmitter involved in mood and emotion. It is an inhibitory transmitter, keeping mood and emotions modulated. What would be the effect of drugs that suppress GABA? 39 2009-2010 Summation

Spatial

Temporal 29 2009-2010 Acetylcholine: neuromuscular junctions, glands, brain and spinal cord
Norepinepherine: affects brain regions concerned with emotions, dreaming Types of chemical synapse 26 2009-2010 To review everything covered thus far, visit this website and complete the animations and exercises. Review 12 2009-2010 64 2009-2010 50 2009-2010 Divisions of the ANS:

Note the origin of the neurons of the sympathetic vs. the para-sympathetic division.

Note the variations in the length of the pre- and post-ganglionic neurons of the 2 divisions.

Note the effects exerted by the two divisions of the ANS. 48 2009-2010 A neural circuit in the SNS typically involves 1 motor neuron, while a neural circuit in the ANS involves 2 motor neurons and one ganglion (cell body). The SNS involves conscious thought, while functions of the ANS are carried out unconsciously. There are differences in structure between the somatic and autonomic nervous systems. What are they? (Think of 2). The Peripheral Nervous System 47 2009-2010 Sympathetic and Parasympathetic Actions Two basic divisions:
Somatic: Motor and Sensory (i.e. skeletal muscles and reflexes)
Autonomic: Controls the body’s activities that you don’t think about; composed of the Parasympathetic and Sympathetic Systems Peripheral Nervous System 38 2009-2010 Remember that with electrical stimulation, where the all-or-none response applies, summation does not occur. Summation only applies to chemical stimulation. Neurotransmitters released from neurons A and B are both excitatory, but individually are not capable of causing a depolarization. If A and B fire at the same time, enough neurotransmitter is collectively released to cause a depolarization. This effect is called Summation. Neuron C is inhibitory and causes hyperpolarization. Summation: 35 2009-2010 Neurotransmitter re-uptake helps keep binding sites clear 27 2009-2010 A synapse divides 2 neurons
The action potential will not move across the synapse
Neurotransmitters
Released by the signal cell to the receiver cell
Move by diffusion How does a signal move from one neuron to another? 25 2009-2010 Fig. 48.11 Myelin sheathing has bare patches of axon called nodes of Ranvier

Action potentials jump from node to node Saltatory Conduction in Myelinated Axons 22 2009-2010 All or None Response: Increasing the intensity of the stimulus over 2 mV will not elicit a stronger or faster response. Any stimulus over the threshold value will produce a maximum response. Any stimulus under the threshold value will produce no response. The neuron either fires maximally or not at all. Threshold level: A possible stimulus must be above a critical value, called the threshold level, in order to cause an action potential. In the example in your textbook, any stimulus below 2 mV will not elicit a response, while any stimulus at 2 mV or above will cause the neuron to fire. Threshold Levels and the
All-or-None Response 41 2009-2010 Answer: extreme muscle weakness Situation 2:
Myasthenia gravis is a disease in which the acetylcholine receptors are blocked by antibodies. What symptoms would you expect in an individual with this condition? Answer: spastic paralysis Situation 1:
Acetylcholine is inhibited by the enzyme acetylcholinesterase (AChE). Nerve gas inhibits the action of AChE. What symptoms would you expect in an individual affected by nerve gas? 31 2009-2010 Detail of Axon Terminal 30 2009-2010 Transmission Across the Synapse Source: Gray 23 2009-2010 There is a greater frequency of impulses. To indicate “hot”, the brain receives messages at a higher rate that it does to indicate “warm”. There are a greater number of neurons firing to indicate “hot” compared with the number of neurons firing to indicate “warm”. 2 ways: Question:
Keeping in mind the principles of threshold value and all-or-none response, how would the brain be able to detect the intensity of a stimulus (Eg. warm vs. hot), if a neuron fires either maximally or not at all? 14 2009-2010 It is considered in three stages: Resting Membrane
Depolarization
Repolarization and Refractory Period The Electrochemical Impulse 51 2009-2010 Fox, S.I.Human Physiology, 2nd ed., Wm. C. Brown Pblishers, 1987. note the different neurotransmitters used in the sympathetic nervous system vs. the parasympathetic nervous system
compare the origin and the length of the pre-ganglionic neurons in the two divisions Divisions of the Autonomic Nervous System 49 2009-2010 Parasympathetic NS: Sympathetic NS:
restores homeostasis
long pre-ganglionic neuron
short post-ganglionic neuron
both pre- and post-ganglionic neurons release acetylcholine
parasympathetic neurons originate from the cranio-sacral regions of the spinal cord
prepares the body for stress
short pre-ganglionic neuron
long post-ganglionic neuron
pre-ganglionic neuron releases acetylcholine
post-ganglionic neuron releases norepinephrines
sympathetic neurons originate from the thoraco-lumbar regions of the spinal cord There are 2 divisions of the Autonomic Nervous System. They are: 42 2009-2010 Answer: Give a drug that blocks dopamine receptors. A side effect would be Parkinsonian-like symptoms. Situation 4:
Knowing what you do about Situation 3, how would you treat schizophrenia? What side effect would you expect? Answer: Increase the concentration of dopamine in the brain by giving a drug such as L-dopa. A side effect to this strategy is the development of symptoms of schizophrenia. Situation 3:
Parkinson’s disease is caused by the degeneration of dopaminergic neurons (neurons that use dopamine as a neurotransmitter) in the brain. How could this condition be treated (simplistically thinking)? 32 2009-2010 Detail of the Synapse Itself Postsynaptic
membrane Neurotransmitter
molecules
(e.g., Acetylcholine
or Dopamine) 24 2009-2010 Myelin sheathing allows these neurons to conduct nerve impulses faster than in non-myelinated neurons. Many vertebrate peripheral neurons have an insulating sheath around the axon called myelin which is formed by Schwann cells. Myelinated Neurons 20 2009-2010 Threshold potential will trigger an action potential or nerve impulse
The Action Potential is an all-or-none response Nerve Impulse - The Action Potential 15 2009-2010 The resting membrane is polarized. The sodium/potassium pump (red arrow) actively transports Na+ out of the cell and K+ into the cell. Step 1: The Resting Membrane 62 2009-2010 * The nerves in the brain stem control your heartbeat, breathing, and blood pressure. * The Brain Stem connects the brain to the spinal cord. The Brain Stem 55 2009-2010 Shannon Mitchell 2000 This is a basal view of a human brain. Reprinted with permission from Virtual Hospital, University of Iowa. The corresponding website is http://www.vh.org The Brain 54 2009-2010 * The spinal cord is the part of the nervous system that connects the brain to the rest of the nervous system. * The spinal cord sends messages to the brain. The Spinal Cord 53 2009-2010 The spinal cord 1. The brain * The Central Nervous System is made of two main organs. * The Central Nervous System controls all of the body’s activities. The Central Nervous System 34 2009-2010 How binding sites work Binding site The reflex arc will be completed before the brain receives any information. Eventually the brain does receive a message, via a second interneuron in the spinal cord. Effector Motor neuron Interneuron Sensory neuron Sensory receptor 17 2009-2010 Resting membrane -70 mV Depolarization +40 mV Resting membrane -70 mV _ _ _ _ _ _ + + + + + + + _ _ _ _ _ _ _ + + + + + + + _ _ _ _ _ _ _ _ + + + + + + + + inside outside Excitation of the neuron causes the Na+ gates in the membrane to open, and the K+ gates to remain closed.
As a result, Na+ rushes into the cell (by diffusion), causing there to be more positive charges INSIDE the membrane than outside.
The charge separation changes from -70mV to +40 mV. This is depolarization. An action potential, or nerve impulse is initiated. Step 2: Depolarization 13 2009-2010 Messages carried throughout the body by nerves. Nerve Impulse 60 2009-2010 2. The cerebellum controls your posture. 1. The cerebellum controls you balance. * The cerebellum is below and to the back of the cerebrum. The Cerebellum Basic Tasks of the Nervous System If necessary, signal effector organs to make an appropriate response. The nervous system is made of: A system that controls all
of the activities of the body. 58 2009-2010 4. The cerebrum controls your movement and identifies the information gathered by your sense organs. 3. The cerebrum controls your speaking. 2. The cerebrum controls your memory. 1. The cerebrum controls your thinking. * The Cerebrum is the largest part of the brain. The Cerebrum 57 2009-2010 3. The Brain Stem 2. The Cerebellum 1. The Cerebrum * The Brain has three main parts… The Three Parts of the Brain 46 2009-2010 Ear Nerves Skin Tongue Eye * Made of nerves and sense organs. * Carries messages between the CNS and the rest of the body. Peripheral Nervous System Parasympathetic
Nervous
System Sympathetic
Nervous
System Motor nerves Sensory nerves Autonomic system Somatic system spinal cord brain Peripheral Nervous System (PNS) Central Nervous System (CNS) 18 2009-2010 active transport active transport K K K K K Na Na Na Na Na Outside Cell Inside Cell (cytoplasm) The Na+ / K+ pump must actively transport Na+ out of the cell and K+ into the cell to restore the polarity of the neuron to -70mV. Repolarization is complete. It takes about 0.001 seconds. Once the charge inside the membrane reaches +40mV, the Na+ gates close, and the K+ gates open. Potassium is now free to diffuse out of the neuron.
We now have a large concentration of Na+ inside the neuron, and a large concentration of K+ outside the neuron. This is the reverse of the ion distribution in the resting membrane: Step 3: a) Repolarization 21 2009-2010 neuron 4. Resting membrane 3. Repolarization / Refractory period 2. Depolarization 1. Resting membrane 4. 3. 2. 1. Membrane potential (mV) _ _ _ _ _ _ +++++++++++ _ _ _ _ _ _ +++++++++ +++++++++ _ _ _ _ _ _ _ _ _ _ _ ++++++++ -70mV +40 mV Review of the electrochemical impulse: What “step” is occurring at each of the numbers below? 56 2009-2010 frontal lobe parietal lobe occipital lobe meninges: between cranium and cerebrum cranium temporal lobe hypothalamus thalamus pituitary medulla oblongata pons cerebellum cerebrum The Brain: 16 2009-2010 diffusion diffusion In the resting membrane, it is theorized that more channels are open to K+ than to Na+, so there are more K+ ions that diffuse out compared with the Na+ ions that diffuse in. (This is represented by the differently sized arrows). Although the Na+ / K+ pump actively transports Na+ out and K+ in, these ions want to diffuse in the opposite direction, down their concentration gradients, as shown by the red arrows. K K K K K K Na Na Na Na Na Na Outside Cell Inside Cell (cytoplasm) Because of the Na+ / K+ pump, the resting membrane would appear as follows: 37 2009-2010 Nicotine fills & activates acetylcholine binding sites producing effects such as increased heart rate and blood pressure 36 2009-2010 Cocaine inhibits the re-uptake of dopamine producing effects such as increased heart rate and blood pressure Monitor both external and internal environments. Process the information and often integrate it with stored information. Stimulus Any change in the environment
internal OR external The nervous system allows
your body to respond to stimuli
through THREE basic tasks The Nervous System accomplishes these tasks by using NEURAL CIRCUITS.

An example of a neural circuit is a REFLEX ARC. REFLEX ARC The Nervous System
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