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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