Structure and Function of the Neurologic System

1. Structure and Function of the Neurologic System Chapter 14

2. Overview of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Peripheral nervous system (PNS) • Cranial nerves • Spinal nerves • Pathways • Afferent (ascending) • Efferent (descending)

3. Overview of the Nervous System • Peripheral nervous system (PNS) • Somatic nervous system • Motor and sensory pathways regulating voluntary motor control of skeletal muscle • Autonomic nervous system • Motor and sensory pathways regulating the body’s internal environment through involuntary control of organ systems • Sympathetic • Parasympathetic

4. Cells of the Nervous System • Neuron • Variable size and structure throughout the nervous system • Cellular constituents • Microtubules, neurofibrils, Nissl substances, and ribosomes • Some neurons like olfactory neurons continue to divide

5. Neuron • Three components • Cell body (soma) • Located mainly in the CNS • Densely packed cell bodies in the CNS are nuclei • Densely packed cell bodies in the PNS are ganglia • Dendrites • Receptive portion of the neuron

6. Neuron • Axons • Carry nerve impulses away from the cell body • Axon Hillock • Cone-shaped, Nissl-free area, where the axon leaves the cell body • Initial segment of the axon • Area of the axon having the lowest threshold for stimulation, so the action potentials begin at this point

7. Neuron • Axons • Myelin • Segmented layer of lipid material • Insulating substance • Formed and maintained by the Schwann cell • Endoneurium • Delicate layer of connective tissue around each axon • Neurilemma • Thin membrane between the myelin sheath and the endoneurium

8. Neuron • Axons • Nodes of Ranvier • Regular interruptions of the myelin sheath • Saltatory conduction • The flow of ions between segments of myelin rather than along the entire length of the axon • Divergence • Ability of branching axons to influence many neurons • Convergence • Branches of numerous neurons converging on one or a few neurons

9. Structural Classification of Neurons • Based on the number of processes extending from the cell body • Unipolar • Pseudounipolar • Bipolar • Multipolar

10. Functional Classification of Neurons • Sensory • Transmit impulses from sensory receptors to the CNS • Associational • Transmit impulses from neuron to neuron • Motor • Transmit impulses from the CNS to an effector organ

11. Neuroglia

12. Neuroglia • “Nerve glue” • Support the neurons of the CNS • Astrocytes • Oligodendroglia (oligodendrocytes) • Microglia • Ependymal cells

13. Neuroglia

14. Nerve Injury and Regeneration • Wallerian degeneration • Occurs distal to the cut • Swelling appears • Neurofilaments hypertrophy • Myelin sheath shrinks and disintegrates • Axon portion degenerates and disappears

15. Nerve Injury and Regeneration • Proximal to the cut • Swelling and dispersal of the Nissl substance • Cell increases in metabolic activity, protein synthesis, and mitochondrial activity • New terminal sprouts project from the proximal segment • Process limited to myelinated axons • Generally only in the PNS • Depends on location, type of injury, the inflammatory response, and scar tissue formation

16. Neuroglia

17. Nerve Impulse • Neurons generate and conduct electrical and chemical impulses by selectively changing the electrical portion of their plasma membranes and influencing other nearby neurons by the release of neurotransmitters

18. Synapses • Region between adjacent neurons is called a synapse • Impulses are transmitted across the synapse by chemical and electrical conduction • Presynaptic neurons and postsynaptic neurons • Connections between pre- and postsynaptic neurons • Axoaxonic • Axosomatic • Axodendritic • Dendrodentritic

19. Synapses

20. Synapses • Neurotransmitters • >30 substances • Excitatory (excitatory postsynaptic potential) • Inhibitory (inhibitory postsynaptic potential) • Synaptic boutons • Synaptic cleft • Summation • Temporal summation • Spatial summation

21. Central Nervous System • Forebrain • Cerebral hemispheres • Midbrain • Corpora quadrigemina, tegmentum, and cerebral peduncles • Hindbrain • Cerebellum, pons, and medulla

22. Central Nervous System • Reticular formation • Network of connected nuclei that regulate viral reflexes, such as cardiovascular and respiratory function • Together with the cerebral cortex is referred to as the reticular activating system

23. Central Nervous System

24. Central Nervous System • Forebrain • Telencephalon • Cerebrum • Gyri, sulci, and fissures • Gray matter and white matter • Cerebral nuclei (basal ganglia)

25. Forebrain

26. Forebrain

27. Central Nervous System • Diencephalon • Epithalamus • Thalamus • Hypothalamus • Subthalamus

28. Central Nervous System • Midbrain • Corpora quadrigemina (tectum) • Superior and inferior colliculi • Tegmentum • Red nucleus and substantia nigra • Basis pedunculi

29. Central Nervous System • Hindbrain (metencephalon) • Cerebellum • Pons • Myelencephalon • Medulla oblongata

30. Central Nervous System

31. Spinal Cord • Lies within the vertebral canal and is protected by the vertebral column • Connects the brain and the body • Conducts somatic and autonomic reflexes • Provides motor pattern control centers • Modulates sensory and motor function

32. Spinal Cord

33. Spinal Cord

34. Spinal Cord

35. Reflex Arc • Receptor • Afferent (sensory) neuron • Efferent neuron • Effector

36. Reflex Arc

37. Upper and Lower Motor Neurons • Upper motor neurons • Efferent pathways primarily relaying information from the cerebrum to the brain stem or spinal cord • Synapse with interneurons • Lower motor neurons • Neurons having direct influence on muscles • Cell bodies originate in the gray matter of the spinal cord, but their axons extend into the PNS

38. Neuromuscular Junction

39. Motor Pathways • Lateral corticospinal • Corticobulbar • Basal ganglia • Vestibulospinal

40. Sensory Pathways • Anterior spinothalamic • Lateral spinothalamic

41. Protective Structures • Cranium • Eight bones • Frontal • Occipital • Temporal (2) • Parietal (2) • Sphenoid • Ethmoid • Galea aponeurotica

42. Protective Structures • Meninges • Protective membranes surrounding the brain and spinal cord • Dura mater • Arachnoid • Pia mater

43. Meninges

44. Protective Structures • Cerebrospinal fluid (CSF) and the ventricular system • CSF is a clear, colorless fluid similar to blood plasma and interstitial fluid • 125 to 150 mL • Produced by the choroid plexuses in the lateral, third, and fourth ventricles • Reabsorbed through the arachnoid villi

45. Protective Structures • Vertebral column • 33 vertebrae • 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal • Intervertebral disks • Nucleus pulposus

46. Vertebral Column

47. Vertebral Column

48. Blood Supply to the Brain • 800 to 1000 mL per minute • CO2 is the primary regulator for CNS blood flow • Internal carotid and vertebral arteries • Arterial circle (circle of Willis)

49. Blood Supply to the Brain

50. Blood Supply to the Brain