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NERVOUS SYSTEM INTRODUCTION

NERVOUS SYSTEM INTRODUCTION. BIO 137 Anatomy & Physiology I. Nervous System. One of body’s 2 main control systems Detects a stimulus inside or outside of body & coordinates a response Acts by electrical signals, called nerve impulses Nervous tissue is excitable

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NERVOUS SYSTEM INTRODUCTION

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  1. NERVOUS SYSTEM INTRODUCTION BIO 137 Anatomy & Physiology I

  2. Nervous System • One of body’s 2 main control systems • Detects a stimulus inside or outside of body & coordinates a response • Acts by electrical signals, called nerve impulses • Nervous tissue is excitable • Controls most of body’s activities to help maintain homeostasis

  3. Organs of the Nervous System • Brain • Spinal Cord • Nerves • Cranial • Spinal • Peripheral

  4. Functions of the Nervous System • 3 main functions • Sensory function • Integrative function • Motor function

  5. Sensory Function of the Nervous System • Sensory receptorsare located at the ends of peripheral nerves • Sensory receptors detect a change outside or inside the body • Monitor internal, external and environmental factors • Light and sound, temperature, oxygen concentration • Information from sensory receptors is converted into a nerve impulse, which is transmitted from peripheral nerves to the CNS

  6. Integrative Function of the Nervous System • The nervous system processes sensory information by analyzing it and making decisions for an appropriate response • ‘Sensory information is integrated‘

  7. Motor Function of the Nervous System • Conscious and subconscious decisions are made and acted upon • Response is carried out by effectorsthrough cranial and spinal nerves • Muscles and/or glands

  8. Divisions of Nervous System • Central Nervous System, CNS • Brain • Spinal cord • Peripheral Nervous System, PNS • Peripheral nerves • Cranial nerves • Spinal nerves • PNS connects CNS to body parts

  9. Divisions of Nervous System

  10. Peripheral Nervous System Divisions • Sensory Division • Sensory receptors in the PNS pick up sensory information from the body and deliver it to the CNS as a nerve impulse • Motor Division • Neurons carry information from the CNS to effectors

  11. PNS: Motor Division • SomaticDivision • Oversees voluntary activities • Skeletal muscle contraction • Autonomic Division • Controls visceral activities • Involuntary activities • Including: Heart rate, blood pressure, breathing, body temperature

  12. PNS:Motor:Autonomic • Sympathetic • Active under stressful situations • ‘Fight or Flight’ • Parasympathetic • Active under normal, restful conditions • Returns body to homeostatic levels following a stressful experience

  13. Divisions of the Nervous System • Ganglia are small masses of neuronal cell bodies located outside the brain and spinal cord, usually closely associated with cranial and spinal nerves. • There are ganglia which are somatic, autonomic, and enteric (that is, they contain those types of neurons.)

  14. Histology of Nervous System • 2 main types of cells • Neurons • A single nerve cell • Neuroglia • Support and nutrition of brain cells • ‘neuron helpers’ • Found in both CNS and PNS

  15. Neuronal Structure • Dendrite(s)– receive information from within or outside the body • Cell body– contains nucleus, organelles • Axon hillock - Axon arises from here on the cell body • Axon– transmits received information as a nerve impulse • A neuron may have many dendrites but only 1 axon • Axon may have branches towards its end called collaterals • Ends of each axon collateral are called axon terminals which end at a synaptic knob • Vesicles of neurotransmitter located here • Synapse exists between end of an axon and what it innervates

  16. Neuronal Structure

  17. Flow of Information in a Neuron • Dendrite(s) • Cell Body • Axon hillock • Axon • Collaterals • Axon Terminals • Synaptic knob

  18. Axons can terminate at three locations 1. Muscle 2. Gland 3. Another neuron The site of communication between two neurons or between a neuron and another effector cell is called a synapse. Axon Terminals

  19. The Synapse: Chemical Transmission • Neurotransmitter is stored in the synaptic end. • The synaptic cleft is the gap between the pre and post-synaptic cells.

  20. Classification of Neurons • Neurons can be classified structurally and functionally • Structural classification • Relationship of number of dendrites to the axon • Functional classification • Where information is carried • Carry info to CNS, entirely in CNS or out of CNS

  21. Structural Classification of Neurons • Multipolar • Many dendrites • Most common in CNS; motor neurons in PNS • Bipolar • One dendrite and one axon extend from cell body • Eyes, ears, nose • Unipolar • One process extends from the cell body • Touch, stretch receptors

  22. Structural Classification of Neurons

  23. Functional Classification of Neurons • Sensory (Afferent) Neurons • Carry info from a body part to the CNS • Detect change outside or inside the body • Usually unipolar • Interneurons • Lie entirely in the CNS • Link other neurons together (within CNS or CNS to motor neurons) - Integrators • Usually multipolar • Motor (Efferent) Neurons • Carry impulses from the CNS to effectors (muscles, glands) • Usually mulitpolar

  24. Functional Classification of Neurons

  25. Neuroglia • Neurogliado not generate or conduct nerve impulses. • They support neurons by: • Forming the Blood Brain Barrier (BBB) • Forming the myelin sheath (nerve insulation) around neuronal axons • Making the CSF that circulates around the brain and spinal cord • Participating in phagocytosis

  26. Neuroglia • There are 4 types of neuroglia in the CNS: • Astrocytes - support neurons in the CNS • Maintain the chemical environment (Ca2+ & K+) • Oligodendrocytes - produce myelin in CNS • Microglia - participate in phagocytosis • Ependymal cells - form and circulate CSF • There are 2 types of neuroglia in the PNS: • Satellite cells - support neurons in PNS • Schwann cells - produce myelin in PNS

  27. Neuroglia

  28. PNS: Neuroglial Cells • Schwann Cells • Produce myelin on peripheral myelinated axons • Satellite Cells • Support neurons

  29. CNS: Neuroglial Cells • Astrocytes • Star shaped, found between neurons and vessels • Form scar tissue in response to brain injury • maintain blood brain barrier • Regulate nourishment of neurons

  30. CNS: Neuroglial Cells • Oligodendrocytes • Form myelin sheath in CNS neurons • Ependyma • Line ventricles in brain and central canal in cord • Cerebrospinal fluid circulation • Microglia • Support neurons and phagocytize bacteria and debris

  31. Functions of Neurons • Conduct nerve impulses • Axonal transport • Transport of biochemicals produced in the cell body to the end of the axon

  32. Myelination of Axons • Myelin sheath found around some large neuronal axons in PNS & CNS • Myelin has a high lipid content that insulates axons and increases nerve impulse speed • CNS: Myelin formed by Oligodendrocytes • PNS: Myelin formed by Schwann Cells

  33. Schwann Cell Myelination in PNS • Gaps between regions of myelin called Nodes of Ranvier • Nerve impulse jumps from node to node in a myelinated axon • This is called Saltatory Conduction

  34. Multiple Sclerosis • Disease resulting from destruction of myelin sheath in some neurons of brain & spinal cord • Leaves scar tissue called sclerosis • Nerve fibers may also be damaged • Prevents nerve impulse transmission to/from brain • Muscles atrophy without stimulation • Possible causes • *Autoimmune • Viral infection (no direct link to one specific virus) • Symptoms reflect specific neurons in the CNS that are affected

  35. Regeneration of Neuronal Axons • CNS neuronal axons can notregenerate • PNS may allow regeneration of axons • Nerve tissue regeneration is largely dependent on Schwann Cells • Cell body damaged – no regeneration • Axon damaged – may be regenerated • Regeneration is slow and the axon may not always grow in correct location • May still lose some function, but not all

  36. Neuronal Regeneration • The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is the neurolemma (sheath of Schwann). • When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon.

  37. Gray and White Matter • White matter of the brain and spinal cord is formed from aggregations of myelinated axons from many neurons. • The lipid part of myelin imparts the white appearance. • Gray matter (gray because it lacks myelin) of the brain and spinal cord is formed from neuronal cell bodies and dendrites.

  38. Neuronal Physiology • Neurons are excitable cells that have an electrical charge • Cell membrane is polarized (electrically charged) • A difference in charge exists between inside/outside cell • Inside of cell negative relative to outside • The difference in charge is called membrane potential, measured in millivolts • MP due to differences in distribution & permeability of Na+, K+ and other anions between the inside and outside of the neuron

  39. Resting Membrane Potential, RMP • At rest, RMP is -70 millivolts • Inside cell • High potassium, K+ • Anions • Outside cell • High sodium Na+ • These ions can only move through ion channels in the plasma membrane • Cell membrane impermeable to anions

  40. - anions - anions - anions

  41. Resting Membrane Potential, RMP • RMP maintained by normal diffusion (80%) and Sodium/ Potassium ATPase pump (20%) • By normal diffusion, what way would Na+ travel? • By normal diffusion, what way would K+ travel?

  42. RMP: Sodium-Potassium ATPase Pump • 3 Na+ out for every 2 K+ in • ACTIVE TRANSPORT • Results in charge separation between inside and outside a neuron

  43. Nerve Impulse • When a neuron responds to a stimulus, it affects the resting membrane potential • Begins on dendrites or cell body BUT effect is on gated ion channels in the axon cell membrane (K+/Na+) • Gated ion channels open in response to an electrical impulse • Recall which way Na+ and K+ travel by normal diffusion

  44. Membrane Responses to Ion Movement • Hyperpolarized – inside more (–) than resting • Depolarized – inside more (+) than resting • Repolarized – return to -70mV

  45. Stimulus - Depolarization 1. Stimulus is detected • must cause RMP to reach threshold potential, -55mV (depolarized) 2. Voltage gated Na+ channels open, Na+ rushes into cell • Inside more positive than RMP, up to +30mV • Result is beginning of Action Potential - rapid reversal of membrane potential

  46. Repolarization • When the MP reaches +30mV, potassium (K+) channels open (while sodium channels close) • Potassium ions (K+) flow out of the cell until RMP is restored back to -70mV • Repolarization is required before the neuron can be stimulated again.

  47. Action Potential • Sufficient Stimulus • Threshold reached • Depolarization when Na+ gated channels open, Na+ enters cell • Na+ channels close, K+ channels open • K+ exits cell • Repolarization

  48. Action Potentials • Only occurs on the axon • AP is an all or none response • May be summation of many stimuli to reach threshold (graded potentials) • Many AP per single axon • AP goes in one direction • AP can not be stimulated during absoluterefractory period

  49. Nerve Impulse • A nerve impulse is the propagation of action potentials along an axon • Unmyelinated axons • Impulse conducted over entire surface • Myelinated axons • Impulse occurs only at nodes of ranvier

  50. Saltatory Conduction • Regions of axon undergoing AP simulate adjacent areas to reach threshold • Impulse jumps from node to node • Occurs through length of axon • Saltatory conduction

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