Nervous System I

1. Chapter 10 Nervous System I Basic Structure and Function Anatomy & Physiology ivyanatomy.com

2. Functions of the Nervous System Sensory Function: • Sensory receptors detect changes in the environment (stimuli) • Information is carried to the CNS on sensory (afferent) neurons Motor Function: • Nerve impulses are transmitted from the CNS to PNS on motor (efferent) neurons • effectors (muscles or glands) within the PNS cause a change (effect) Integrative Function: • Nervous system maintains homeostasis – detects and responds to changes in blood pressure, body temp, heart rate, etc. • Higher intellect: problem solving, thoughts, memory, judgment

3. Subdivisions of Nervous System • The Central Nervous System (CNS) • brain and spinal cord. • The Peripheral Nervous System (PNS) • 12 pairs of cranial nerves and • 31 pairs of spinal nerves • Nerves may be motor (efferent), sensory (afferent), or both (mixed) The Central Nervous System is Red, while the Peripheral Nervous System is Blue.

4. Divisions of the PNS • Sensory (Afferent) Division • Transmits impulses from receptors in the PNS to CNS • Motor (Efferent) Division • Transmits impulses from CNS to effectors in the PNS

5. Somatic Nervous System Somatic Division is associated with voluntary (skeletal) activities and senses that detect shapes, textures, sounds, and other external and internal forces acting on the body. • Somatic Sensory • Senses you’re consciously aware of • Vision, taste, olfaction, smell, hearing • Touch, vibration, pain • Somatic Motor • Controls voluntary (skeletal) activities • Skeletal muscles – voluntary control

6. Autonomic Nervous System Autonomic Nervous System regulates functions of the internal organs Heart rate, digestion, sexual arousal, urination • Autonomic (Visceral) Sensory • Senses that monitor vital conditions within the body • O2/CO2 levels, pH, Blood Pressure, Body Temp • Autonomic Motor – involuntary control • Smooth muscles • Cardiac muscles • glands

7. Divisions of the Autonomic Nervous System The Autonomic Nervous System (ANS) is divided into two branches • Sympathetic Division • Fight-or-Flight Response • Prepares body for emergency • Parasympathetic Divsion • Rest-and-Digest • Maintains body activities at rest

9. Cells of the Nervous System Neural Tissue contains two Cell Types: • Neurons • Integrate, regulate, and coordinate body functions • Transmit nerve impulses (action potentials) • Neuroglia(glia = “glue”) • Neuroglia provide neurons with nutritional, structural, and functional support

10. Anatomy of a Neuron Neurons vary in shape and size • 3 Parts of a Neuron • Dendrites – receive inputs from other neurons or other stimuli • Cell Body (Soma) • Axon – transmit nerve impulses • away from the cell body towards other neurons, muscles, or glands Dendrite axon terminal axon Cell Body (soma) Schwann Cell Myelin sheath Nucleus

11. Anatomy of a Neuron • Dendrites • Dendrites transmit information towards the cell body • A cell may have many dendrites, few dendrites, or no dendrites • Dendritic spines – additional contact points on dendrites • A neuron may add more spines, increasing its sensitivity to incoming stimuli, or It may remove spines to decrease its sensitivity to stimuli. Segment of a dendrite (green) with dendritic spines (yellow)

12. Anatomy of a Neuron • Cell Body (Soma) • Contains organelles such as nucleus, mitochondria, Golgi Apparatus, neurofilaments, and Rough ER • Chromatophilic Substances (Nissl Bodies) – mostly Rough ER, protein synthesis • Cell body produces proteins for the cell

13. Anatomy of a Neuron • Axon • Transmits electrical impulses (action potentials) away from the neuron • Each neuron has only 1 axon, but it may divide into several branches, called collaterals. • Axon Hillock (trigger zone) – specialized site connected to the cell body, where electrical impulses (action potentials) are initiated • Axon terminal – end of the axon. • Contains enlarged synaptic knob (bouton) • Neurotransmitters are stored within secretory vesicles within the synaptic knob.

14. Anatomy of a Neuron • Axon • Neurofibrils – microtubules that support long axons • Neurofibrils aid in axonal transport – transports proteins from the cell body through the axon • Axoplasm – cytoplasm of the axon • Axolemma – cell membrane of the axon

15. Myelination of Axons • Myelin Sheath • Thick fatty coating of insulation surrounding some axons • Myelin sheath greatly enhances the speed of nerve impulses • Schwann Cells form the myelin sheath in the PNS • Oligodendrocytes form the myelin sheath in the CNS Schwann cell forming the myelin sheath around an axon within the PNS

16. Myelination in the PNS • Schwann Cells • Schwann Cells myelinate neurons in the PNS • Schwann Cells wrap around the axon in a jelly-roll fashion, forming a thick layer of lipid insulation, called the Myelin Sheath • Neurilemma– The cytoplasm and nucleus of the Schwann Cell are pushed outwards, forming an outer layer, called the neurilemma • Nodes of Ranvier – gaps of exposed axon between adjacent Schwann Cells Neurilemma (at the nucleus) Myelin sheath axon Mitochondrion within axon

17. Myelination of Axons • Not all axons are myelinated • Myelinated axonsin the PNS have a series of Schwann cells lined up along the axon, each having a wrapped coating of myelin insulating the axon • Unmyelinated axonsin the PNS are encased by Schwann cell cytoplasm, but there is no wrapped coating of myelin surrounding the axons Schwann Cell Myelin sheath axon axon Schwann Cell

18. Myelination in the CNS • Oligodendrocytes • Oligodendrocytes myelinate axons in the PNS • Each oligodendrocyte myelinates multiple axons • White Matter – mass of myelinated axons within the CNS • Gray Matter – unmyelinated nervous tissue in the CNS Oligodendrocyte myelinating several axons.

19. Myelination in the CNS Gray Matter of the Cerebral Cortex - unmyelinated tissue White Matter of the Cerebrum - myelinated tissue

20. Structural Classification of Neurons dendrites • Multipolar Neuron • Many dendrites and 1 axon • Includes most neurons in the CNS and motor neurons axon

21. Structural Classification of Neurons • Bipolar Neuron • 1 dendrite and 1 axon • Includes some special sensory neurons, such as photoreceptors and olfactory neurons. dendrite axon

22. Structural Classification of Neurons • Psuedounipolar Neuron • Contains a single process that acts as an axon • Peripheral process – conducts information from the PNS • Central process – conducts information toward the CNS • Example includes sensory neurons within the Dorsal Root Ganglia (DRG) peripheral process central process

23. Functional Classification of Neurons • Sensory (afferent) neuron • Transmit impulses from the PNS towards the CNS • Most afferent neurons are unipolar. Some are Bipolar. • Motor (efferent) neuron • Transmit impulses from the CNS towards effectors in the CNS • Somatic Motor Neurons – voluntary control • Autonomic Motor Neurons – involuntary control. • Interneuron (association) • Completely within the CNS • Interneurons link together in the CNS • Interneurons connect sensory neurons to motor neurons

24. Central Nervous System Peripheral Nervous System Sensory receptor Sensory neuron interneuron Effector (muscle or gland) Motor neuron interneuron

25. Neuroglia General Functions of Neuroglia: • Provide structural and metabolic support for neurons • Guide developing neurons into position • Remove excess ions and neurotransmitters • Strengthen synapses • Neuroglia outnumber neurons 10 to 1 Neuroglia in the CNS vs. PNS • Neuroglia of the CNS include: astrocytes, ependymal cells, microglia, and oligodendrocytes • Neuroglia of the PNS include: satellite cells and Schwann Cells

26. Neuroglia of the CNS Astrocytes: • Star-shaped cell • Attaches neuron to blood vessels • Astrocytes aid in metabolism, strengthen synapses, and participate in the Blood-Brain-Barrier Ependymal Cells • Simple cuboidal epithelium with cilia • Lines ventricles of the brain and central canal of spinal cord • Cover choroid plexuses (capillary networks within CNS) • Regulate the composition of cerebrospinal fluid (CSF)

27. Neuroglia of the CNS Microglia: • Normally small cells until activated • Enlarge into macrophages with infection • Phagocytize foreign material Microglia (green) surrounding nerve processes (red) Oligodendrocytes: • Form the myelin sheath within the CNS • Provide structural support Oligodendrocyte myelinating several axons within the CNS

28. Neuroglia of the PNS Satellite Cells: • Surround and support clusters of cell bodies (ganglia) within the PNS Schwann Cells: • Form the myelin sheath in the PNS • Greatly increase nerve impulse speed

29. Neuroglia and Axonal Regeneration • Mature neurons do not divide • If cell body is injured, the neuron usually dies Neuron Regeneration in the PNS: • If a peripheral axon is injured, it may regenerate • Axon separated from cell body and its myelin sheath will degenerate • Schwann cells and neurilemma remain • Remaining Schwann cells provide guiding sheath for growing axon • If growing axon establishes former connection, function will return; if not, function may be lost Neuron Regeneration in the CNS: • CNS axons lack neurilemma to act as guiding sheath • Oligodendrocytes do not proliferate after injury • Regeneration is unlikely

30. Disorders of Neuroglia Multiple Sclerosis: • Autoimmune disease that destroys the myelin sheath of motor neurons. • The damaged myelin sheath is replaced with connective tissue, leaving behind scars (scleroses) • The scars block transmission of underlying neurons, so muscles no longer receive stimuli • Muscles atrophy and wither over time White matter lesions (scleroses) of Multiple Sclerosis

31. The Synapse Neurons communicate with each other at synapses. • A synapse is a site at which a neuron transmits a nerve impulse to another neuron • Presynaptic neuron sends impulse (usually) by releasing neurotransmitters into the synaptic cleft • Postsynaptic neuron receives impulse • Synaptic cleft separates the 2 neurons

32. Synaptic Transmission A nerve impulse (action potential) travels down the axon to the axon terminal. The action potential opens calcium channels causing calcium to diffuse into the synaptic knob. The calcium influx triggers the exocytosis of neurotransmitters from synaptic vesicles into the synapse. The neurotransmitters diffuse across the synapse and bind to receptors on the post-synaptic cell Neurotransmitter either exerts an excitatoryor inhibitory effect, depending on the neurotransmitter and the receptor.

33. Cell Membrane Potential • The cell membrane is usually polarized (charged) • Inside the membrane is negatively charged relative to outside the membrane • Polarization is due to unequal distribution of ions across the membrane • Polarization is maintained by a series of ion pumps and ionchannels • All Cells have a membrane potential. Cell membrane

34. Cell Membrane Potential • Potassium (K+) ions: major intracellular positive ions (cations). • Sodium (Na+) ions: major extracellular positive ions (cations). • This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump) but also by ion channels in the cell membrane. • Na+/K+ Pump transports Na+ ions out of cell and K+ ions into cell • Ion channels, formed by membrane proteins, help regulate passage of specific ions into or out of the cell • Many chemical & electrical factors affect opening & closing of gated channels

35. Ion Channels • Non-Gated (Leak) Ion channels, • Channels are always open, allowing specific ions to “leak” down their concentration gradient. • Cells have abundant K+ leak channels, making them permeable to K+.

36. Ion Channels • Mechanically-Gated Ion channels, • Open in response to physical deformation of the cell membrane • Touch, Hearing, Pressure, Vibrations, Etc. Na+ open closed

37. Ion Channels • Ligand-Gated Ion channels, • Open in response to a ligand (neurotransmitter, hormone, or other molecule) Ligand (molecule) Na+ open closed

38. Ion Channels • Voltage-Gated Ion channels, • Open and close due to small changes in the membrane potential (millivolts = mV) • Voltage-gated Na+ channels open when membrane potential reaches -55mV. • Voltage-gated K+ channels open as the membrane potential approaches +35mV -70mV -55mV open closed

39. Ion Pumps • Sodium-Potassium ATPase (Pump) • Active Transport Mechanism (uses ATP) • Pumps 3 Na+out of the cell • Pumps 2 K+ into the cell

40. Membrane Potential 3 Factors Establish the Membrane Potential 1. Na+/K+ ATPase 2. Non-gated K+ channels 3. Negatively charged proteins and DNA within the cell • Sodium-Potassium ATPase (Pump) • Pumps 3 Na+ out of the cell, but only 2 K+ into the cell. • Net positive charges leaving the cell, making inside negatively charged. • The Na+/K+ pump only contributes a small amount (-5mV) to the membrane potential

41. Cell Membrane Potential 3 Factors Help Maintain the Cell Membrane Potential Na+/K+ Pump K+ leak channel • Non-gated Potassium Channels • Cell has many K+ leak channels, making it permeable to potassium. • K+ continually leaks out of the cell, making the inside of the cell more negative. Na+ K+ ATP ADP + P K+

42. Resting Membrane Potential • Resting Membrane Potential (RMP) • RMP = membrane potential of excitable cells (neurons and muscles) while at rest. • For a neuron at rest, the RMP is -70mV inside the cell.

43. Membrane Potential Changes • Resting Membrane Potential (RMP)of neuron = -70mV Opening/Closing gated-Ion channels cause changes in local membrane potential • Depolarization • membrane potential becomes less negative. • e.g. -60 mV -70mv (RMP) • Hyperpolarization • membrane potential becomes more negative. • e.g. -100 mV -70mv (RMP) Time (ms)

44. Local Potential Changes • Graded (Local) Potentials • Local potential changes are graded—the greater the stimulus intensity, the greater the potential change • If degree of depolarization reaches threshold potentialof -55 mV, an action potential results • If degree of depolarization does not reach threshold potential, an action potential will not occur subhreshold potential

45. Summation of Graded Potentials • Summation– Graded potentials may add together (summate) • Spatial summation – stimuli from multiple neurons • Temporal summation – high frequency stimulation from a presynaptic neuron • Combination – stimuli from multiple neurons at a high frequency • If summation reaches threshold potential (-55mV), it initiates an action potential Example of Spatial Summation Example of Temporal Summation

47. Action Potential Depolarization Repolarization -55 mV Threshold potential -70 mV (RMP) stimulus Subthreshold potentials Hyperpolarization

48. 3 Phases of an Action Potential • Depolarization • Voltage-Gated Na+ channels open at -55mV (threshold) • Na+ diffuses into the cell • Repolarization • Voltage-Gated K+ channels open as cell depolarizes towards +30mV • K+ diffuses out of the cell • Na+ channels close • Hyperpolarization • K+ channels remain open, causing an overshoot • Na+/K+ pumps reestablish the RMP.

49. Action Potential • Resting Membrane Potential Na+ K+ At rest, the membrane is polarized (RMP = -70mV). Sodium is mostly outside the cell and potassium is within the cell.

50. Action Potential • Depolarization Na+ Na+ When a stimulus reaches threshold stimulus (-55mV), voltage-gated Na+ channels open. Sodium rapidly diffuses into the cell, depolarizing the membrane up to +30mV.