Fundamentals of the Nervous System and Nervous Tissue

1. Fundamentals of the Nervous System and Nervous Tissue Chapter 12

2. Introduction • The nervous system is the master controlling and communicating system of the body • It is responsible for all behavior • Along with the endocrine system it is responsible for regulating and maintaining body homeostasis • Cells of the nervous system communicate by means of electrical signals

3. Nervous System Functions • The nervous system has three overlapping functions • Gathering of sensory input • Integration or interpretation of sensory input • Causation of a response or motor output

4. Introduction • Sensory input • The nervous system has millions of sensory receptors to monitor both internal and external change • Integration • It processes and interprets the sensory input and makes decisions about what should be done at each moment • Motor output • Causes a response by activating effector organs (muscles and glands)

5. Organization • There is only one, highly integrated nervous system • Basic divisions of the nervous system • Central Nervous Systems • Peripheral Nervous System

6. Organization • In order to discuss the nervous in smaller portions, for convenience the nervous system is divided into two parts • The central nervous system • Brain and spinal cord • Integrative and control centers • The peripheral nervous system • Spinal and cranial nerves • Communication lines between the CNS and the rest of the body

7. Organization of the Nervous System

8. Organization • The peripheral nervous system has two fundamental subdivisions • Sensory (afferent) division • Somatic and visceral sensory nerve fibers • Consists of nerve fibers carrying impulses to the central nervous system • Motor (efferent) division • Motor nerve fibers • Conducts impulses from the CNS to effectors • (glands and muscles)

9. Organization of the Nervous System

10. Organization • The motor division of the peripheral nervous system has two main subdivisions • The Somatic motor • Voluntary motor • Conducts impulses from the CNS to skeletal muscle • The Visceral motor • Involuntary motor • Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands • Equivalent to the autonomic nervous system (ANS)

11. Organization of the Nervous System

12. Peripheral Nervous System

13. Organization of the Nervous System • Somatic sensory • The sensory receptors that are spread widely throughout the outer tube of the body • These include the many senses experienced on the skin and in the body wall, such as touch, pain, pressure, vibration and temperature • Proprioception provides feedback from the stretch of the muscles, tendons and joint capsules - your “body sense”

14. Organization of the Nervous System • Somatic sensory • The special somatic senses are receptors are more localized and specialized • The special senses include; sight, hearing, balance, smell and taste.

15. Organization of the Nervous System • Visceral sensory • The general visceral senses include stretch, pain, and temperature which can be felt widely in the digestive and urinary tracts, reproductive organs, and other viscera • Sensations such as hunger and nausea are also general visceral sensations • The chemical senses such as taste and smell are considered by some as special visceral senses

16. Organization of the Nervous System • Somatic motor • The general somatic motor is part of the PNS that stimulates contraction of the skeletal muscle in the body • Also referred to as voluntary nervous system • Skeletal muscles are widely distributed throughout the body, and therefore there is no special somatic motor category

17. Organization of the Nervous System • Visceral motor • The general visceral motor part of the PNS regulates the contraction of smooth and cardiac muscle and secretion by the body’s many glands • General visceral motor neurons make up the autonomic nervous (ANS) which controls the function of the visceral organs

18. Organization of the Nervous System • Visceral motor • Because we generally have no voluntary control over such activities as the pumping of the heart and movement of food through the digestive tract • The ANS is also called the involuntary nervous system

19. Organization of ANS • The autonomic nervous system has two principle subdivisions • Sympathetic division • Mobilizes body systems during emergency situations • Parasympathetic division • Conserves energy • Promotes non-emergency functions • The two subdivisions bring about opposite effects on the same visceral organs • What one subdivision stimulates, the other inhibits

20. Nervous Tissue • The nervous system consists mostly of nervous tissue whose cells are densely packed and tightly intertwined • Nervous tissue is made up just two main types of cells • Neurons - the excitable cells that transmit electrical signals • Neuroglia - nonexcitable supporting cells that surround and wrap the neurons • Both cell types develop from the same embryonic tissues: neural tube and crest

21. The Neuron • The human body contains many billions of neurons which are the basic structural units of the nervous system • Neurons are highly specialized cells that conduct electrical signals from one part of the body to another • These signals are transmitted along the plasma membrane in the form of nerve impulses or action potentials

22. Neurons • Neurons are the structural units of the nervous system • Neurons are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another

23. The Neuron • Special characteristics of neurons • They have extreme longevity. Neurons can and must function over a lifetime • They do not divide • As fetal neurons assume their roles as communication links in the nervous system, they lose their ability to undergo mitosis • Cells cannot be replaced if destroyed - Some limited exceptions do exist in the CNS as neural stem cells have been identified

24. The Neuron • Special characteristics of neurons • They have an exceptionally high metabolic rate requiring continuous and abundant supplies of oxygen and glucose • Neurons cannot survive for more than a few minutes without oxygen

25. The Neuron • Neurons are typically large, complex cells • Neurons vary in their structure but they all have two fundamental components • Neuron cell body • One or more processes

26. The Cell Body • The cell body of the neuron is also called a soma • The cell bodies of different neurons vary widely in size (from 5 to 140 m in diameter) • However, all consist of a single nucleus surrounded by cytoplasm

28. The Cell Body • Typically large, complex cells, neurons have the following structures • Cell body • Nuclei • Chromatophilic (Nissl) bodies • Neurofibrils • Axon hillock • Cell processes • Dendrites • Axon • Myelin sheath or neurilemma

29. The Cell Body • Cell Body • Nuclei • Chromatophilic (Nissl) bodies • Neurofibrils • Axon hillock • Neuron Processes • Dendrites • Axons • Myelin sheaths • Axon terminals

30. The Cell Body • In all but the smallest neurons, the nucleus is spherical and clear and contains a nucleolus near its center

31. The Cell Body • The cytoplasm contains all the usual cellular organelles with the exception of centrioles (not needed in amitotic cells) as well as Nissl bodies • These cellular organelles continually renew the membranes of the cell

32. The Cell Body • Neurofibrils are bundles of intermediate filaments that run in a network between chromatophilic bodies • These filaments keep the cell from being pulled apart when it is subjected to tensile forces

33. The Cell Body • The cell body is the focal point for the outgrowth of the neuron processes during embryonic development

34. The Cell Body • In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons

35. The Cell Body • Most neuron cell bodies are located within the CNS • However, clusters of cell bodies called ganglia (singular ganglion) lie along the nerves in the PNS

36. Neuron Processes • Bundles of neuron processes in the CNS are called tracts • Bundles of neuron processes in the PNS are called nerves

38. Neuron Processes Motor neuron • Armlike processes extend from the cell bodies of all neurons • There are two types of processes • Dendrites • Axons

39. Neuron Processes Motor neuron • The cell processes of neurons are described here using a motor neuron • Motor neurons represent a typical neuron, but sensory neurons differ from the typical pattern

40. Dendrites • Dendrites are short, tapering, diffusely branching extensions from the cell body • Motor neurons have hundreds of dendrites clustering close to the cell body • Dendrites function as receptive cites providing an enlarged area for the reception of signals from other neurons • By definition, dendrites conduct electrical signals toward the cell body

41. Dendrites • Dendritic spines represent areas of close contact with other neurons • These electrical signals are not nerve impulses but are short distance signals call graded potentials

42. Axons • Each neuron has only one axon • The axon arises from the cone shaped axon hillock • It narrows to form a slender process that stays uniform in diameter the rest of its length • Length varies; short or absent to 3 feet in length

43. Axons • Each axon is called a nerve fiber • Axons are impulse generators and conductors that transmit nerve impulses away from the cell body

44. Axons • Chromatophilic bodies (Nissl) and the Golgi apparatus are absent from the axon and the axon hillock • Axons lack ribosomes and all organelles involved in protein synthesis, so they must receive their proteins from the cell body

45. Axons • Neurofilaments, actin microfilaments, and microtubules are especially evident in axons, where they provide structural strength • These cytoskeletal elements also aid in the transport of substances to and from the cell body as the axonal cytoplasm is recycled and renewed • The movement of substances along axons is called axonal transport

46. Axons • The axon of some neurons are short, but in others it can be extremely long • Motor neurons in the CNS have axons that must reach to the musculature that they control that might be 3-4 feet away • Any long axon is called a nerve fiber and travels in a group of fibers composing a nerve

47. Axons • Although axons branch far less frequently than dendrites, occasional branches do occur along their length • These branches, called axon collaterals, extend from the axon at almost right angles

48. Axons • Axons branch profusely at its terminus • Ten thousand of these terminal branches per neuron is not unusual • These branches end in knobs called axon terminals or boutons

49. Axon • A nerve impulse is typically generated at the axon’s initial segment and is conducted along the axon to the axon terminals, where it causes the release of chemicals called neurotransmitters into the extracellular space • The neurotransmitters excite or inhibit the neurons or target organs with which the axon is in close contact

50. Axon • Axon diameter varies considerably among the different neurons of the body • Axons with larger diameters conduct impulses faster than those with smaller diameters • Neurons follow the law of physics: The resistance to the passage of an electrical current decreases as the diameter of any “cable” increases