Nervous System - Chapter 4 PDF

Chapter 4 | Nervous system Part 1- Excitable tissues  CHAPTER 4 NERVOUS SYSTEM 1- excitable tissues Introduction Structural and functional units of the nervous system Neuron is the structure unit...

Chapter 4 | Nervous system Part 1- Excitable tissues  CHAPTER 4 NERVOUS SYSTEM 1- excitable tissues Introduction Structural and functional units of the nervous system Neuron is the structure unit of the nervous system, while reflex action is the functional unit, it occurs via reflex arc, Receptor, afferent, center, efferent & effector organ Classifications of the nervous system. Anatomical classification: Central (CNS): includes Brain (Cerebrum, subcortical centers, cerebellum, brain stem) Spinal cord (31 segments, each consists of horns & columns) Peripheral (PNS): includes 12 pairs of cranial nerves 31 pairs of spinal nerves Physiological classification: Sensory part: Transmit sensations from receptors to sensory centers, it includes Somatic, Visceral & Special sensations Motor part: Transmit orders from motor centers to effector organs to control voluntary (Somatic part) & involuntary movements (Autonomic part). The common physiological classification includes 3 main divisions: CNS: Somatic (sensory & motor) ANS (Autonomic nervous system) Special senses 50  CHAPTER 4 NERVOUS SYSTEM 1- excitable tissues Nerve properties 1- Excitability: Nerve is an excitable tissue; able to respond to various stimuli. stimulus is a sudden change in the environment, it may be: according to its type: Electrical, chemical, mechanical, thermal. according to its intensity: Threshold stimulus: minimal intensity, required to give response. Subthreshold stimulus: stimulus having intensity < threshold. Suprathreshold stimulus: stimulus having intensity > threshold Response of the nerve to stimuli occurs in the form of Electrical changes, called action potential (n. impulse), accompanied by Excitability, Metabolic and Thermal changes. 2- Conductivity: Once nerve impulse generated, it is conducted along the nerve fiber according to its type. Unmyelinated → Sweeping conduction Myelinated → Saltatory conduction 51  CHAPTER 4 NERVOUS SYSTEM 1- excitable tissues 3- All or none law: Nerve either responds maximally or not respond at all (=Threshold stimulus produces maximal response) provided that other factors affecting excitability remain constant. All or None law is obeyed by: Single nerve fiber. Single motor unit. Cardiac muscle Unitary smooth muscle. All or None law is not obeyed by: Whole skeletal muscle. Nerve trunk. Multiunit smooth m. 4- Summation: Subthreshold stimuli produce only local responses called local excitatory states not sufficient to produce action potential. If subthreshold stimuli are applied successively within short periods ( resting channels & K+ channels are wide open. 3- Supernormal phase: Period during which excitability is higher than normal → weaker stimulus can excite the nerve, it coincides with –ve after potential. 4- Subnormal phase: period during which excitability is lower than normal, → stronger stimulus is needed to excite the nerve, it coincides with +ve after potential During propagation of n. impulse, more energy is needed, which is derived from breakdown of ATP & creatine phosphate (CP)→ metabolic and thermal changes 55  CHAPTER 4 NERVOUS SYSTEM 1- excitable tissues Mechanism of neuromuscular transmission: Mechanism of skeletal m. contraction (Excitation-contraction coupling):  It is the process by which depolarization of muscle fiber initiate contraction.  When end plate potential reaches the firing level → action potential is generated & propagates on both sides and inside along the T-tubules → Release of Ca2+ from sarcotubular system → Ca2+ in sarcoplasm → 1. Binding of myosin head to actin active sites 2. Bending of cross-bridges & sliding of the actin over myosin 3. Detachment of myosin head from active actin site & another cycle can occur by rebinding to the next sites of actin 56