The Nervous System

1. The Nervous System

2. The nervous system is the master controlling and communicating system of the body. Every thought, action, and emotion reflects its activity. Its cells communicate by electrical and chemical signals, which are rapid and specific, and usually cause almost immediate responses.

3. Functions of the Nervous System • Sensory input – gathering information • To monitor changes occurring inside and outside the body. • Changes = stimuli • Integration • To process and interpret sensory input and decide if action is needed. • Motor output • A response to integrated stimuli • The response activates muscles or glands

4. For example, when you are driving and see a red light ahead (sensory input), your nervous system integrates this information (red light means “stop”), and your foot goes for the brake (motor output).

6. Structural Classification of the Nervous System • Central nervous system (CNS) • Brain • Spinal cord • Peripheral nervous system (PNS) • Cranial nerves • Spinal nerves

7. Functional Classification of the PNS It is divided into TWO subdivisions: 1-Sensory(afferent) division • Nerve fibers that carry information to the central nervous system from: • sensory receptors in the skin, skeletal musclesand joints (somatic sensory fibers). • Sensory receptors in the visceral organs (visceral sensory fibers) Figure 7.1

8. 2-Motor (efferent) division • Nerve fibers that carry impulsesaway from the central nervous system ( to Muscles &Glands).These impulses effect (bring about) a motor response. It has two subdivisions 1-Somatic nervous system = voluntary, it controls skeletal muscles 2-Autonomic nervous system= involuntary, it controls smooth &cardiac muscles &glands This also is divided into sympathetic & parasympathetic

9. Organization of the Nervous System Figure 7.2

10. Despite the complexity of the nervous system, there are only two functional cell types Neurons - excitable nerve cells that transmit electrical signals Neuroglia (glial) cells - supporting cells Histology of Nervous Tissue

11. Neuroglia cells - 4 types in the Central NS 1-Astrocytes star shaped with many processes connect to neurons; help anchor them to nearby blood capillaries control the chemical environment of the neurons 2-Microglia - oval with thorny projections - monitor the health of neurons - if infection occurs, they change into macrophages (eating viruses, bacteria and damaged cells)

12. 3-Ependymal cells range in shape from squamous to columnar; many are ciliated line the dorsal body cavity housing the brain and spinal cord form a barrier between the neurons and the rest of the body 4-Oligodendrocytes - have few processes - wrap themselves around axons - form the myelin sheath – an insulating membrane

13. Neuroglia cells - 2 types in the Peripheral NS Satellite cells - surround neuron cell bodies in the periphery - Protective , cushioning cells Schwann cells (neurolemmocytes) - are vital to regeneration of damaged nerve fibers. - adjacent Schwann cells along an axon do not touch one another, so there are gaps in the sheath. These gaps, called nodes of Ranvier occur at regular intervals (about 1 mm apart) along the myelinated axonand form the myelin sheath around larger nerve fibers in the periphery - it acts as an insulators.

14. Neuron (Nerve cell) -The Cells are specialized to transmit messages -Differ structurally but Have common features: • A Cell body with nucleus and the usual organelles, Except centrioles • One or more processes Figure 7.4a

15. Neuron Anatomy • Extensions outside the cell body • Dendrites – conduct impulses toward the cell body • Axons – conduct impulses away from the cell body Figure 7.4a

16. Axons and Nerve Impulses • Axons end in axonal terminals • Axonal terminals contain vesicles with neurotransmitters • Axonal terminals are separated from the next neuron (neuroneural ) junction or the muscle (neuromuscular) junction by a gap calledSynaptic cleft (Synapse).

17. Nerve Fiber Coverings - Most long nerve fibers are covered with a whitish, fatty material called Myelin with waxy appearance. It insulates the fiber &Increases transmission rate - Axons outside CNS are wrapped by Schwann Cells. - Figure 7.5

18. Neuron Cell Body Location • Most are found in the central nervous system • Gray matter – cell bodies and unmyelinated fibers • Nuclei – clusters of cell bodies within the white matter of the central nervous system • Ganglia – collections of cell bodies outside the central nervous system • White matter- collection of myelinated fibers (Tracts) in the CNS. • Fibers outside the CNS are called nerves.

19. Functional Classification of Neurons 1-Sensory (afferent) neurons Carry impulses from the sensory receptors • Cutaneous sense organs • Proprioceptors – detect stretch or tension in muscles and tendons and joints 2-Motor (efferent) neurons that carry impulses from the central nervous system to muscles and glands ,their cell bodies are always in CNS. 3-Interneurons(association neurons) • Their cell bodies are always found in CNS. • Connect sensory and motor neurons in neural pathways.

20. Neuron Classification Figure 7.6

21. Structural Classification of Neurons • Multipolar neurons – many extensions from the cell body. Figure 7.8a

22. Bipolar neurons – one axon and one dendrite Figure 7.8b

23. Unipolar neurons – have a short single process leaving the cell body which is very short ,divides almost immediatly Figure 7.8c

24. Functional Properties of Neurons • Irritability – ability to respond to stimuli. • Conductivity – ability to transmit an impulse. • The plasma membrane at rest is polarized i.e.,Fewer positive ions are inside the cell than outside the cell

25. Starting a Nerve Impulse • a- resting membrane electrical condition. The external face of the membrane is slightly positive, its internal face is slightly negative. The chief extracellular ion is sodium wheras the chief intracellular ion is potassium. The membranr is relatively impermeable to both ions. • b- Stimulus initiates local depolarization by changing permeability to sodium which rush inside the cell changing polarity of the membrane so the inside becomes more positive, the outside become more negative at that site.

28. c- Depolarization and generation of an action potential. If the stimulus is strong enough , depolarization causes membrane polarity to be completely reversed and an action potential is initiated. • d- Propagation of action potential. Depolarization of the first membrane patch causes permeability changes in the adjacent membrane and the events described in(b) are repeated. Thus the action potential propagates rapidly along the entire length of the membrane.

29. e- Repolarization.potassium ions diffuse out of the cell restoring the negative charge on the inside of the membrane and positive charge on the outside surface.repolarization occurs in the same direction as depolarization. • f- Initial ionic condition restored bythe sodium-potassium pump . Three sodium ions are ejected for every two potassium ions carried back into the cell

30. Nerve Impulse Propagation • The nerve impulse is is an all-or-none response, like firing a gun. It is either propagated over the entire axon ,or it does not happen at all. • Impulses travel faster when fibers have a myelin sheath(saltatory conduction). • Until repolarization occur,a neuron can not conduct another impulse. Figure 7.9c–e

31. HOMEOSTATIC IMBALANCE • 1-A number of chemical and physical factors impair impulse propagation. Sedatives and anesthetics block nerve impulses by altering membrane permeability to sodium. As we have seen, no Na+ entry—no AP.     • 2-Cold and continuous pressure interrupt blood circulation (and hence the delivery of oxygen and nutrients) to neuron processes, impairing their ability to conduct impulses. For example, your fingers get numb when you hold an ice cube for more than a few seconds, and your foot “goes to sleep” when you sit on it. When you remove the cold object or pressure, impulses are transmitted again, leading to an unpleasant prickly feeling.

32. Continuation of the Nerve Impulse between Neurons • Impulses are able to cross the synapse to another nerve by: • Neurotransmitter is released from a nerve’s axon terminal • The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter • The response is very brief because the neurotransmitter is quickly removed either by reuptake by the axonal trminal or by enzymatic breakdown. This limits the period to less than the blink of an eye. • An action potential is started in the dendrite of the next neuron propagating to cell body and its axon. • Notice : impulse transmission is an electrochemical

33. How Neurons Communicate at Synapses Figure 7.10

34. The Reflex Arc • Reflexes are rapid, predictable, and involuntary responses to stimuli • Reflex arc follows a direct route from a sensory neuron, to an interneuron,then to an effector neuron. • Reflex arc have a minimum 5 elements

35. Simple Reflex Arc Figure 7.11b, c

36. Types of Reflexes • One classification: • - Autonomic reflexes eg. • Salivary gland secretion • Heart and blood pressure regulation • Changes in size of the pupil • Digestive system regulation • - Somatic reflexes • Activation of skeletal muscles • Other classification: • - Spinal reflexes ,involve spinal cord as the flexor reflex • -Cranial reflexes requires the brain as light reflex.

37. Exaggerated, Distorted or Absent indicate nervous system disorder. Reflex changes often occur before the pathological condition become obvious. Importance of REFEXES

38. Central Nervous System (CNS) • CNS develops from the embryonic neural tube. • By the fourth week the anterior end begins to expand and brain formation begins, The rest of the tube becomes the spinal cord. • The central canal becomes enlarged in 4 regions of the brain to form the ventricles which are: -Four chambers within the brain. -Filled with cerebrospinal fluid(CSF).

39. Regions of the Brain • Cerebral hemispheres • Diencephalon • Brain stem • Cerebellum Figure 7.12

40. The cerebral hemispheres form the superior part of the brain. Together they account for about 83% of total brain mass. • Picture how a mushroom cap covers the top of its stalk, and you have a fairly good idea of how the paired cerebral hemispheres cover and obscure the diencephalon and the top of the brain stem .

41. Cerebral Hemispheres (Cerebrum) Figure 7.13a

42. Nearly the entire surface of the cerebral hemispheres is marked by elevated ridges of tissue called gyri (ji′ri; “twisters”), separated by shallow grooves called sulci (sul′ki; “furrows”). The singular forms of these terms are gyrus and sulcus. Deeper grooves, called fissures, separate large regions of the brain. The more prominent gyri and sulci are similar in all people and are important anatomical landmarks. The median longitudinal fissure separates the cerebral hemispheres .Another large fissure, the transverse cerebral fissure, separates the cerebral hemispheres from the cerebellum below

43. Several sulci divide each hemisphere into four lobes— frontal, parietal, temporal,and occipital. The central sulcus, which lies in the frontal plane, separates the frontal lobe from the parietal lobe. Bordering the central sulcus are the precentral gyrus anteriorly and the postcentral gyrus posteriorly. More posteriorly, the occipital lobe is separated from the parietal lobe by the parieto-occipital sulcus (pah-ri″ĕ-to-ok-sip′ĭ-tal).

44. Layers of the Cerebrum • Gray matter • Outer layer • Composed mostly of cell bodies of the neurons Figure 7.13a

45. White matter • Fiber tracts inside the gray matter • Example: corpus callosum connects between the two hemispheres. Figure 7.13a

46. Basal nuclei , or basal ganglia –islands of gray matter buried deep within the white matter of the cerebral hemispheres, They help regulate voluntary motor activities in relation to starting or stopping movements sent to skeletal muscles by the primary motor cortex.Disorders of the basal nuclei result in either too much or too little movement as exemplified by Huntington’s chorea and Parkinson’s disease, respectively.

49. Specialized Area of the Cerebrum • Gustatory area (taste) • Visual area • Auditory area • Olfactory area • Speech/language region • Language comprehension region

50. Specialized Area of the Cerebrum Figure 7.13c