Anatomy & Physiology Bio 2401 Lecture. Instructor: Daryl Beatty Nervous System Introduction Part 1

Transcription

1 Anatomy & Physiology Bio 2401 Lecture Instructor: Daryl Beatty Nervous System Introduction Part 1

2 Nervous System Introduction Chapter 11 Section A Sequence 4.1 DB Nervous system 1 Intro Presentations 4.2, 4.3, 4.4 and 4.5 will follow.

3 Nervous System Functions 1. Sensation Monitors changes/events occurring in and outside the body. Such changes are known as stimuli and the cells that monitor them are receptors. 2. Integration The parallel processing and interpretation of sensory information to determine the appropriate response 3. Motor output. The activation of muscles or glands (typically via the release of neurotransmitters (NTs))

4 Nervous System Figure 11.1

5 Overview of the Nervous System Figure 14.1

6 Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves Carries messages to and from the spinal cord and brain

7 Overview of the Nervous System Figure 14.1

8 Central nervous system (CNS) Brain and spinal cord Integrative and control centers Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic sensory fiber Skin Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Somatic nervous system Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Sympathetic division Mobilizes body systems during activity Parasympathetic division Conserves energy Promotes housekeeping functions during rest Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS Sympathetic motor fiber of ANS Parasympathetic motor fiber of ANS Heart Bladder Figure 11.2

9 Comparison of Nervous & Endocrine Similarities: They both monitor stimuli and react so as to maintain homeostasis. Differences: The Nervous System is a rapid, fast-acting system. The Endocrine acts slower via hormones, and its actions are usually much longer lasting.

10 Divisions of Nervous System Central Nervous System The brain + the spinal cord The center of integration and control Peripheral Nervous System The nervous system outside of the brain and spinal cord Consists of: 31 Spinal nerves Carry info to and from the spinal cord 12 Cranial nerves Carry info to and from the brain

11 Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Sensory afferent fibers carry impulses from skin, skeletal muscles, and joints to the brain Visceral afferent fibers transmit impulses from visceral organs to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs

12 Motor Division: Two Main Parts Somatic nervous system Conscious (voluntary) control of skeletal muscles Autonomic nervous system (ANS) Involuntary Automatic Regulates smooth muscle, cardiac muscle, and glands Divisions sympathetic and parasympathetic

13 Division of Autonomic System Sympathetic Nervous System Fight or Flight Parasympathetic Nervous System Rest and Digest These 2 systems are antagonistic.

14 Explain to your neighbor. What are the divisions of the Nervous system? What does each do?

15 Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons excitable cells that transmit electrical signals Neuroglia Supporting cells that surround and wrap neurons, and perform other functions (Pilots & ground support)

16 Neuron

17 Supporting Cells: Neuroglia The supporting cells (neuroglia or glial cells): Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth

18 Supporting Cells: Neuroglia Much more numerous than Neurons 6 types of supporting cells 4 are found in the CNS: Astrocytes Microglia Ependymal Cells Oligodendrocytes

19 Astrocytes Most abundant, versatile, and highly branched glial cells They cling to neurons and their synaptic endings, and cover capillariesstar-shaped, abundant, and versatile

20 Astrocytes Functionally, they: Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Control the chemical environment Act as K+ and NT buffers

21 Astrocytes Figure 11.3a

22 Microglia Microglia small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons Why does the CNS have its own immune system?

23 Ependymal Cells Ependymal cells range in shape from squamous to columnar Line the ventricles of the brain Some are ciliated which facilitates the movement of cerebrospinal fluid

24 Oligodendrocytes, Schwann Cells, and Satellite Cells Oligodendrocytes branched cells that wrap CNS nerve fibers Schwann cells (neurolemmocytes) surround fibers of the PNS Satellite cells surround neuron cell bodies with ganglia

25 Oligodendrocytes, Schwann Cells, and Satellite Cells Figure 11.3d, e

26 Neurons (Nerve Cells) PLAY InterActive Physiology : Nervous System I, Anatomy Review, page 4 Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membrane function in: Electrical signaling Cell-to-cell signaling during development

27 Components of Neurons Cell body (soma) One or more specialized, slender processes (axons/dendrites) An input region (dendrites/soma) A conducting component (axon) A secretory (output) region (axon terminal)

28 Neurons (Nerve Cells) Figure 11.4b

29 Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolus Is the major biosynthetic center Is the focal point for the outgrowth of neuronal processes Has no centrioles (hence its amitotic nature) Has well-developed Nissl bodies (rough ER) Contains an axon hillock cone-shaped area from which axons arise

30 Processes Armlike extensions from the soma Called tracts in the CNS and nerves in the PNS There are two types: axons and dendrites

31 Dendrites of Motor Neurons Short, tapering, and diffusely branched processes They are the receptive, or input, regions of the neuron Electrical signals are conveyed as graded potentials (not action potentials)

32 Axon Most neurons have a single axon a long (up to 1m) process designed to convey info away from the cell body. Originates from a special region of the cell body called the axon hillock. Transmit action potential from the soma toward the end of the axon where they cause NT release. Often branch sparsely, forming collaterals.

33 Axons: Structure Slender processes of uniform diameter arising from the hillock Long axons are called nerve fibers Usually there is only one unbranched axon per neuron Rare branches, if present, are called axon collaterals Axonal terminal branched terminus of an axon

34 Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminals Movement along axons occurs in two ways Anterograde toward axonal terminal Retrograde away from axonal terminal

35 Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission

36 Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons

37 Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNS A Schwann cell: Envelopes an axon in a trough Encloses the axon with its plasma membrane Has concentric layers of membrane that make up the myelin sheath Neurilemma remaining nucleus and cytoplasm of a Schwann cell

38 Myelin Sheath and Neurilemma: Formation PLAY InterActive Physiology : Nervous System I, Anatomy Review, page 10 Figure 11.5a c

39 Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge PLAY InterActive Physiology : Nervous System I, Anatomy Review, page 11

40 Conduction Velocity Conduction velocities of neurons vary widely Effect of axon diameter Larger diameter fibers have less resistance to local current flow and have faster impulse conduction Effect of myelination Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons

41 Conduction Velocity Effects of myelination Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about 30 times faster (300 mph!) Voltage-gated Na + channels are located at the nodes APs appear to jump rapidly from node to node

42 Stimulus Size of voltage (a) In a bare plasma membrane (without voltage-gated channels), as on a dendrite, voltage decays because current leaks across the membrane. Stimulus Voltage-gated ion channel (b) In an unmyelinated axon, voltage-gated Na + and K + channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gates of channel proteins take time and must occur before voltage regeneration occurs. Stimulus Myelin sheath Node of Ranvier 1 mm (c) In a myelinated axon, myelin keeps current in axons (voltage doesn t decay much). APs are generated only in the nodes of Ranvier and appear to jump rapidly from node to node. Myelin sheath Figure 11.15

43 Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs Impulse conduction slows and eventually ceases

44 Multiple Sclerosis: Treatment Some immune system modifying drugs, including interferons and Copazone: Hold symptoms at bay Reduce complications Reduce disability

45 Unmyelinated Axons A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons

46 Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced There is no neurilemma

47 Regions of the Brain and Spinal Cord White matter dense collections of myelinated fibers Gray matter mostly soma and unmyelinated fibers

48 Neuron Classification Structural: Multipolar three or more processes Bipolar two processes (axon and dendrite) Unipolar single, short process

49 Neuron Classification Functional: Sensory (afferent) transmit impulses toward the CNS Motor (efferent) carry impulses away from the CNS Interneurons (association neurons) shuttle signals through CNS pathways

50 Comparison of Structural Classes of Neurons Table

51 Comparison of Structural Classes of Neurons Table

52 Comparison of Structural Classes of Neurons Table

53 Neurophysiology Neurons are highly irritable Action potentials, or nerve impulses, are: Electrical impulses carried along the length of axons Always the same regardless of stimulus The underlying functional feature of the nervous system