Transcript chapter 48

CHAPTER 48
NERVOUS SYSTEMS
Section A: An Overview Of Nervous Systems
Dr. Hesham Sharaf
• A ganglion ‫ عقدة عصبية‬is a cluster ‫ تجمع‬of nerve cell bodies within the PNS.
• A nucleus is a cluster of nerve cell bodies within the CNS.
• Neurons ‫ الخاليا العصبية‬differ in both function and shape.
Dr. Hesham Sharaf
Fig. 48.4, Page 1025
• A Simple Nerve Circuit ‫ – دائرة عصبية‬the Reflex Arc ‫القوس اإلنعكاسى‬.
– A reflex is an autonomic (automatic) response ‫إستجابة عصبية ذاتية‬.
‫حسـى‬
‫خلية عصبية بينية‬
‫حركى‬
Dr. Hesham Sharaf
Fig. 48.3, Page 1024
Knee-jerk reflex
‫حسـية‬
‫حركية‬
‫وسيطة‬
•
Types of Nerve Circuits ‫الدوائر العصبية‬:
There are 3 patterns of organizations:1)
Single presynaptic neuron  several postsynaptic neurons:
takes information from a single source (e.g. eye) to several parts of
the brain.
2)- Several presynaptic neurons  single postsynaptic neuron:
takes information from several sources (e.g. vision, touch and
hearing) to a single postsynaptic neuron in the brain.
3)
Circular paths:
Information flows from one
neuron to another and then
back to its source (for example
memory stored information).
Dr. Hesham Sharaf
•
•
Supporting Cells (Glia) ‫خاليا الغراء العصبى المد ِعمة‬.
Unit 7,
Ch 44
Play a supportive rule, and include the following types of glia ‫خاليا الغراء‬:-
1)
Astrocytes: are found within the CNS and provide structural and
metabolic support as well as forming of tight junctions to help form
the blood-brain barrier. They also communicate with one another via
chemical signals.
2)
Oligodendrocytes: are found within the CNS and form a myelin
sheath ‫ الغمد العازل‬by insulating axons.
3)
Schwann cells: are found within the PNS (within the axon of each
neuron) and form a myelin sheath by insulating axons.
‫الغمد العصبى‬
Fig. 48.5, Page 1026
Dr. Hesham Sharaf
Section B1: The Nature Of Nerve Signals
Every cell has a voltage, or membrane potential, across its
plasma membrane
• A membrane potential ‫ الجهد الكهربائى للغشاء‬is a localized electrical
gradient across membrane.
– Anions are more concentrated
within a cell (e.g. protein mol.).
– Cations are more concentrated
in the extracellular fluid (e.g. Na+).
• Membrane Potentials is
Measurable.
– An unstimulated cell usually
have a resting potential of
-70 mV.
Dr. Hesham Sharaf
• How a Cell Maintains a Membrane Potential.
– Cations.
• K+ the principal intracellular cation.
• Na+ is the principal extracellular cation.
– Anions.
• Proteins, amino acids, sulfate, and phosphate are the principal
intracellular anions.
• Cl– is principal extracellular anion.
• Ungated ‫ مفتوحة‬ion channels: allow ions to diffuse across the plasma
membrane.
•
This diffusion does not
achieve an equilibrium
since sodium-potassium
pump transports these
ions against their
concentration gradients
(see lecture 15 slide 4)
Dr. Hesham Sharaf
Fig. 48.7, Page 1027
Changes in the membrane potential of a neuron give rise
to nerve impulses ‫اإلشارات العصبية‬
I- Excited state of the cell ‫حالة اإلثارة فى الخلية‬:
•
Neurons have the ability to generate large changes in
their membrane potentials.
– Gated ion channels open or close in response to stimuli ‫مؤثرات‬.
• The resulting diffusion of ions leads to a change in the membrane
potential.
• Types of gated ions channels.
– Chemically-gated ion channels open or close in response to a chemical
stimulus.
– Voltage-gated ion channels open or close in response to a change in
membrane potential.
Dr. Hesham Sharaf
Graded Potential: ‫الجهد الكهربائى المتغير‬
It is the changes in membrane potential via the following:-
A.
1)
Hyperpolarization ‫اإلستقطاب المرتفع‬.
Gated K+ channels open  K+ diffuses out of the cell  the
membrane potential become more negative (increase in membrane potential).
1)
Depolarization ‫فقـْد اإلستقطاب‬.
Gated Na+ channels open  Na+ diffuses into the cell  the
membrane potential becomes less negative (decrease in membrane potential).
B.
The Action Potential:
1ATP
Occurs in the Axons only.
–
–
If graded potentials reached
55mV, the threshold potential
‫ مستوى الجهد األمثل‬is achieved.
This triggers an action
potential generating impulse.
T. protein
Low conc.
+ 2
of K
High conc.
+
of Na
High conc.
+
of K
Low conc.
3 of Na+
II- Resting state of the cell: Both Na+ and K+ channels
are closed and the membrane’s resting potential is maintained.
Dr. Hesham Sharaf
Unit 7,
Ch 44