The Nervous System

1. The Nervous System Chapter 7

2. Functions of the Nervous System • Master controlling and communicating system of the body • Maintains body homeostasis with electrical signals • Provides for sensation, higher mental functioning, emotional response • Activates muscles and glands • Three overlapping functions to accomplish control: • Sensory input (stimuli) • Integration (process and interpret) • Motor/Output (effects a response) • Functions are integrated into a loop… keeps modifying until homeostasis is reached or environmental condition changes

3. Structural Classification • Central Nervous System: brain & spinal cord (all neurons & neuroglia within) • Occupy dorsal body cavity • Integrating and command center • Peripheral Nervous System: outside the CNS • Consists of nerves extending from brain & spinal cord • Connects CNS to rest of body • Spinal nerves & cranial nerves • Carry impulses from sensory receptors to CNS & CNS to effectors

4. Functional Classification • Sensory (afferent) division: conveys impulses to the CNS from sensory receptors • Motor (efferent) division: carries impulses from the CNS to effector organs (muscles and glands); effect a motor response • Somatic Nervous System • Voluntarily controls skeletal muscles • Skeletal muscle reflexes (involuntary) • Autonomic Nervous System • Regulates events that are automatic/involuntary (activity of smooth & cardiac muscles and glands) • Sympathetic (stimulates) & parasympathetic (inhibits) nervous system

7. Nervous Tissue: Supporting Cells • Neuroglia (a.k.a. glia or glial cells) • generally support, insulate, and protect the delicate neurons • Cannot transmit nerve impulses • Never lose ability to divide • Brain tumor = “glioma” • Types: • Central Nervous System • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes • Peripheral Nervous System • Schwann cells • Satellite cells

8. Astrocytes • Star-shaped cells (“astro”) • Most abundant (nearly half of neural tissue) • Braces and anchors neurons to blood capillaries • Aid in exchanges between the neurons and the blood capillaries (nutrient regulation) • Protect neurons from harmful substances in blood • Help control chemical environment • Form scar tissue

9. Microglia • Phagocytes that monitor the health of nearby neurons • Dispose of debris (including dead brain cells & bacteria)

10. Ependymal Cells • Form epithelial-like membrane that covers parts of brain and forms inner lining that encloses spaces within brain and spinal cord (central canal) • Help with blood/brain barrier • Ciliated, help circulate cerebrospinal fluid

11. Oligodendrocytes • Have processes wrapped around nerve fibers that produce myelin sheaths (fatty insulating coverings) in brain & spinal cord • Myelin is important for conducting electrical impulses!

12. Supporting Cells of the PNS • Schwann Cells • form myelin sheaths around nerve fibers that are found in the PNS • Satellite cells • Protective, cushioning cells

13. Neurons • Nerve cells • Specialized to receive information and transmit it to other cells! (electrochemical message) • Sensory/integrative/motor functions of nervous system! • Common features of neurons: • Cell body • Metabolic center of the neuron • Contains nucleus & organelles • Nissl bodies (rough ER) & neurofibrils(maintain cell shape) • Processes • Also called “fibers”, can be microscopic or 3-4 feet long • Dendrites: receive incoming impulses (can have many) • Axons: generate nerve impulses & conduct away from cell body (each neuron only has one) • Arises from Axon hillock • Neurilemma: cell membrane of neuron

14. Neurons

15. Myelin Sheaths • Myelin: white, fatty (lipid-protein) material, waxy appearance • Protects and insulates fibers • Increases transmission rate of nerve impulses • Schwann cells myelinate axons in PNS and form myelin sheath • Coil of wrapped membranes enclosing axon • Neurilemma: part of Schwann cell external to myelin sheath • Nodes of Ranvier: gaps between myelin sheaths formed by different Schwann cells • Oligodendrocytesmyelinate axons in CNS • Can myelinate as many as 60 different fibers, no neurilemma

17. Nodes of Ranvier • Spaces between myelin sheaths • Give astrocytes something to hold on to and provides easier transfer of nutrients & materials • Ion transfer • Impulse jumps from node to node

18. Terminology of CNS • Nuclei: clusters of neuron cell bodies, well protected • i.e. Caudate nucleus (region of brain) • Tracts: bundles of nerve fibers (neuron processes) • Each sense has own tract • Sensory tracts go towards brain, motor tracts come from brain • White matter: myelinated fibers (tracts) • Gray matter: unmyelinated fibers and cell bodies (nuclei)

19. Terminology of PNS • Ganglia: small collections of cell bodies outside CNS • Nerves: bundles of nerve fibers (neuron processes)

20. Functional Classification of Neurons • Grouped according to the direction the nerve impulse is traveling relative to the CNS • Sensory/afferent neurons • Carry impulses from sensory receptors (in the internal organs or the skin) to the CNS (from environment to CNS!) • Cell bodies found in ganglion outside the CNS • Dendrites associated with specialized receptors • Motor/efferent neurons • Carry impulses from CNS to the viscera and/or muscles & glands (tell them to do something in response to the stimuli) • Cell bodies always located in the CNS • Interneurons (association neurons) • Connect motor & sensory neurons in neural pathways • Cell bodies always in the CNS

21. Path of TravelSensory  interneurons  motor

22. Sensory Receptors • Grouped by location • Interoceptors(internal environment) • Exteroceptors(external surface of body) • Pressure, pain, temperature • Proprioceptors (muscles, tendons, & joints) • Control equilibrium, posture, and own movements • Send information to brain on body position • Grouped by structure • Free nerve endings – dendrites sense pain, • usually in skin • Encapsulated – capsule/knob at end of dendrites • Separate, specific cells – cell takes info and stimulates neuron • i.e. photoreceptors (rods/cones) – cell stimulates neurons to take information to brain

23. Sensory Receptors • Grouped by stimuli (selective/specific) • Stimulus produces potential if threshold met! • mechanoreceptors (mechanical stimuli) • Rapid adaptation: Meissner’s corpuscle, Lamellar corpuscle, hair root plexus; slower adaptation: Merkel disc, Raffini corpuscle • thermoreceptors (heat) • free nerve endings • nociceptors (pain) • free nerve endings, not adaptive • photoreceptors (light) • chemoreceptors (chemicals) • pH, ions • osmoreceptors (osmotic pressure)

24. Adaptation • Receptors adapt to stimuli that they are continuously exposed to (can be rapid or slow) • i.e. smell & temperature are quickly adapting, pain is very slow • Less and less integration occurs A dog (red line) tracks a pheasant (yellow line). As the dog keeps leaving the odor to prevent receptor adaptation, it zigzags.

25. Structural Classification of Neurons • Page 235, Figure 7.8 • Based on number of processes extending from cell body • Multipolar neuron: several processes • Motor & associated neurons, most common structural type • Majority of interneurons in CNS are multipolar • Bipolar neuron: two processes(axon & dendrite) • Rare, found only in some special sense organs (eye, nose) • Act in sensory processing as receptor cells • Unipolar neurons: single process emerging from cell body • Sensory neurons found in PNS ganglia

27. Physiology of Nerve Impulses • Two major functional properties of neurons: • Irritability: ability to respond to a stimulus and convert it into a nerve impulse • Conductivity: ability to transmit the impulse to other neurons, muscles, or glands • Neurotransmission: neurons communicating with one another!

28. How does it all work? • Plasma membrane of a resting (inactive) neuron is polarized (resting potential) • Fewer positive ions sitting on the inside than on the outside of the membrane • more negative inside = resting/inactive • Positive ions inside cell: K+ (potassium) • Positive ions outside cell: Na+ (sodium) • Membrane relatively impermeable to both ions • Ion channels closed when resting • Neuron no longer at rest when sodium enters the cell and then potassium leaves • Neuron uses energy to maintain polarization • Na/K pump keeps ions where they are “supposed” to be when at rest • Page 236, Figure 7.9 – Flow chart

29. Resting Potential • Sodium • Potassium Polarized • Negative inside • Positive outside • Will continue resting until receives stimulus (from environment or from another neuron) • Typically is a neurotransmitter • If enough received, neuron will depolarize and “fire”

30. Action Potential 1. Stimuli excite neurons 2. Gates of sodium channels in membrane open with stimulation • Na+ diffuses into the cell (high concentration  low) • Depolarization: polarity of neuron’s membrane reversed as sodium diffuses • More negative outside, positive inside 3. If threshold potential reached, neuron activated to initiate & transmit an action potential (nerve impulse) • all-or-none response • Depolarization (electrical impulse) happens along the axon • Nodes of Ranvier – jumps from node to node

31. Action Potential 4. Na+ channels close and K+ channels open • K+ diffuse out of neuron into tissue fluid rapidly • Repolarization – restores electrical conditions at membrane to the polarized (resting) state • Neuron cannot fire again until repolarized 5. Sodium-potassium pump restores original concentrations • Sodium (goes in) • Depolarized • Potassium (goes out) • Repolarized

32. Myelinated Axons • Fibers with myelin sheaths conduct impulses much faster • Nerve impulse jumps from node to node along length of fiber (salutatory conduction) • Gaps allow ions to cross

33. Action Potential Repolarization Depolarization • All or none response • Once threshold is met, neuron will fire • Some mental illnesses involve neurons firing when they shouldn’t or not firing when they should (bipolar, schizophrenia) • Takes milliseconds to occur Hyperpolarization

34. Transmission of Signal at Synapse • Electrical impulse becomes chemical signal (“electrochemical” event) – page 238, Fig 7.10 • Neurotransmitter chemical crosses synapse to transmit signal from one neuron to the next 1. Action potential reaches axon terminal & electrical change opens calcium channels 2. Calcium causes vesicles containing neurotransmitter to fuse with membrane & openings form, releasing transmitter 3. Neurotransmitter molecules diffuse across synapse and bind to receptors on membrane of next neuron 4. If enough neurotransmitter released, depolarization of next neuron occurs 5. neurotransmitter is removed from the synapse (diffusion, reuptake into axon terminal, or enzymatic breakdown)

35. Video Links on MBC

37. Neurotransmitters • chemical messengers that carry signals between neurons as well as other cells in the body • released from the end of one neuron and cross the synapse to receptor sites in the next neuronor effector • Certain neurotransmitters increase ion permeability (excitatory) • Others decrease permeability (inhibitory)

39. Acetylcholine • Abbreviated ACh • most common neurotransmitter • located in both the central nervous and peripheral nervous system • first neurotransmitter be identified in 1914 • acts on basic autonomic and muscular functions • Saringas (chemical warfare nerve agent) disrupts its ability to function and often leads to death

40. Glutamate • Excitatory neurotransmitter • Plays a role in cognition, learning, and memory • Main neurotransmitter in CNS of mammals • Must be in correct balance in right place at right time! Excess glutamate in extracellular space can damage neurons • “overexcites” neurons and causes them to open channels, letting substances into cells that shouldn’t be there • released with stroke & head trauma and causes damage • Researching drugs to help prevent damage • Malfunction of glutamate has also been associated with Alzheimer'sDisease

41. GABA • gamma-aminobutyric acid • GABA is the most important and common inhibitory neurotransmitter • Fine-tunes neurotransmission • Stops the brain from becoming overexcited • Too much may cause hallucinations • Also responsible for regulation of muscle tone

42. Dopamine • Generally involved in regulatory motor activity • In the basal ganglia of the brain, involved in mood, drives, pleasurable feelings, sensory perception, and attention • Produced when “feeling good,” also causes addictions (caffeine, cigarettes, drugs, etc.) • With addictive substances, dopamine is increased • More released • Less broken down • More received by receptors • Drugs can mimic dopamine (i.e. marijuana – “dope”)

43. Epinephrine • Also known as adrenaline (when released as hormone) • Causes the feeling of being “revved up” or on edge • Activates a “fight or flight” reaction in the autonomic nervous system • Excitatory neurotransmitter – stimulates nerves to fire • Counteracts with norepinephrine/noradrenaline

44. Serotonin • Attention and other complex cognitive functions (drives), such as sleep (dreaming), eating, mood, pain regulation • Neurons which use serotonin are distributed throughout the brain, stomach and spinal cord • Mood disorders • Antidepressants are serotonin uptake inhibitors (leave serotonin in synapse longer!) • i.e. Prozac was first a diet aid (limited appetite), but also caused mood change due to its affect on serotonin. Important to understand balance! (for example, Prozac would not be a good antidepressant for someone with a history of anorexia.)

45. Neurotransmitter Balance • Most functions in body isn’t just one neurotransmitter – usually a balance • Have to have right amount of each – any of them off can cause mental illness • hard to treat mental illness with medication because don’t know which is off • have to use trial & error

46. Other Neurotransmitter Examples • Nitric oxide: vasodilator • Endorphins – stress or pain, “runners high”

47. Physiology: Reflexes • Reflexes are rapid, predictable, and involuntary responses to stimuli • Occur over reflex arcs and in both CNS and PNS structures • Somatic reflexes: all reflexes that stimulate the skeletal muscles (pulling back from hot stove) • Autonomic reflexes: regulate activity of smooth muscles, the heart, and glands (salivary reflex, pupillary reflex) • Five elements: • Sensory receptor – reacts to a stimulus • Effector organ – muscle or gland eventually stimulated • Sensory and motor neurons to connect the two • CNS integration center - synapse or interneurons between the sensory and motor neurons • Example: patellar (knee-jerk) reflex, pulling hand back (Fig 7.11) • Spinal reflexes – without brain involvement

48. Reflexes Figure 7.11

49. Central Nervous System

50. Central Nervous System • Brain & spinal cord • 100 billion multipolar neurons • First appears as simple tube during embryonic development (neural tube) • Brain formation begins in the fourth week • Functional Anatomy of the Brain • Cerebral hemispheres (cognitive function) • Diencephalon (glands) • Brain stem (autonomic) • Cerebellum (coordination)