The Nervous System

1. The Nervous System Nancy G. Morris Volunteer State Community College Campbell Chapter 48

3. Organization of nervous systems • There is great diversity among animals. • All phyla have a nervous “system” except sponges. • In the Hydra’s nerve net, impulses are conducted in both directions causing movement of entire body. • Some cnidarians & echinoderms have modified nerve nets with rudimentary centralization.

4. Organization of nervous systems • Cephalization = evolutionary trend for concentration of sensory & feeding organs on the anterior end of a moving animal (Bilaterial symmetry). • Most bilateral animals have a PNS & a CNS (brain & one or more nerve cords). • Flatworms have a simple “brain” containing large interneurons. • Annelids & arthropods have a well-defined ventral nerve cord & prominent brain. Often coordinate ganglia in each segment to coordinate action. • Cephalopods have the most sophisticated invertebrate nervous system containing a large brain & giant axons.

5. Fig. 48.13 Diversity in Nervous Systems

6. Three overlapping functions of the Nervous System: • Sensory input is the conduction of signals from sensory receptors to integration centers of the nervous system. • Integration is a process by which information from sensory receptors is interpreted & associated with appropriate responses of the body. • Motor output is the conduction of signals from the processing center to effector cells (muscle & gland cells) that actually carry out the body’s response to stimuli.

7. Fig. 48.1 Overview: Vertebrate Nervous System

8. Three majors classes of neurons • Sensory neurons – convey information about the external & internal environments from sensory receptors to CNS; most synapse with interneurons. • Interneurons – integrate sensory input and motor input; located within the CNS; synapse only with other neurons • Motor neurons – convey impulses form the CNS to effector cells

9. Fig. 48.4 Structural Diversity of Neurons

10. A single neuron on the surface of a microprocessor. A cm3 of the human brain will contain more than 50 million neurons.

11. Signals are conducted by nerves with many axons coming from many different neurons surrounded by connective tissue, the perineurium. • Found in both parts of the nervous system: • 1) Central Nervous System(CNS) = comprised of brain & spinal cord; responsible for integration of sensory input & associating stimuli with appropriate motor output • 2) Peripheral Nervous System(PNS)= consists of a network of nerves extending into different parts of the body that carry sensory input to the CNS & motor output away form the CNS

12. Composition of Nervous System • The Nervous System contains two types of cells: • 1) Neurons – cells specialized for transmitting chemical & electrical signals form one location to another • 2) Glia or supporting cells – structurally reinforce, protect, insulate, & generally assist neurons

13. Neurons • Possess a large cell body located either in the CNS or a ganglion • Possess two fingerlike extensions (processes) that conduct messages: • 1) Dendrites – convey signals to the neurons cell body Numerous, short, extensively branched to increase surface area • 2) Axons – conduct impulses away from the cell body. Single, long process

14. Fig. 48.2 Structure of Vertebrate Neuron

15. AXONS • Vertebrate axons in the PNS are wrapped in concentric layers of Schwann cells, which form an insulating myelin sheath. • In the CNS, the myelin sheath is formed by ogliodendrites. • Extend from the neuron cell body to many branches (arborization of the axon) which are tipped with synaptic terminals that release neurotransmitters.

16. Transmission of the impulse • Must cross the synapse, the gap between a synaptic terminal and a target cell (either another neuron or an effector cell). • Neurotransmitters are chemicals that cross the synapse to relay the impulse • Table 48.1:acetylcholine, norepinephrine, dopamine, serotonin, neuropeptides: endorphines

17. Neurons are arranged in circuits • Simple circuit: synapse between sensory neurons & motor neurons, resulting in a simple reflex. • Complex circuit: such as those associated with most behaviors, involve integration by interneurons in the CNS • Convergent circuits • Divergent circuits • Reverberating circuits (memory storage)

18. Figure 48.3The knee-jerk reflex

19. Coordination by cluster • Nerve cell bodies are often arranged into clusters; these clusters allow coordination of activities by only a art of the nervous system • A nucleus is a cluster of nerve cell bodies within the brain • A ganglion is a cluster of nerve cell bodies in the peripheral nervous system

20. Supporting cells • Do not conduct impulses • Outnumber neurons by 10- 50- fold • Several types of glia cells: • 1) astrocytes – encircle capillaries of the brain • 2)oligodendrocytes form the myelin sheaths that insulate the CNS nerve processes • 3) Schwann cells form the insulating myelin sheath around axons in the PNS

21. Myelination of neurons • Occurs when Schwann cells or oligodenrocytes grow around an axon so their plasma membranes form concentric layers • Provides electrical insulation • Increases speed of nerve impulse propagation • In MS, myelin sheaths deteriorate causing a disruption of nerve impulse transmission & consequent loss of coordination

22. Nature of Neural Signals • Signal transmission along a neuron depends on voltages created by ionic fluxes across neuron plasma membranes. • Membrane potentials arise from differences in ion concentrations between a cell’s contents and the extracellular fluid. • All cells have an electrical potential or voltage across their plasma membrane. • The charge outside is designated as zero, so the minus sign indicates that the cytoplasm inside is negatively charged compared to the extracellular fluid.

23. Nature of Neural Signals • Ion channel = integral transmembrane protein that allows a specific ion to cross the membrane. • May be passive all the time or it may be gated, requiring stimulus to change into an open conformation. • Is selective for a specific ion, such as Na+, K+, and Cl- • A shift in ionic gradients is prevented by sodium-potassium pumps which maintain the concentration gradient.

24. Action Potential • A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered selective opening & closing of voltage-gated ion channels. • There are four stages.

25. Four stages of an Action Potential • 1) resting stage; no channels open. • 2) depolarizing phase – membrane reverses polarity (cell interior becomes + relative to exterior); the Na+ activation gates open, Na+ rushes in, potassium gates remain closed. • 3) repolarizing phase - returns the membrane potential to resting level; inactivation gates close Na+ channels & K+ channels open. • 4) undershoot phase – membrane potential is temporarily more negative than the resting stage (hyperpolarized); Na+ channels remain closed but K+ channels remain open since the inactivation gates have not had time to respond to repolarization of the membrane.

26. Figure 48.9Role of gated ion channels in the action potential

27. Action Potential…. • Refractory Period occurs during the undershoot phase. • During this period, the neuron is insensitive to depolarizing stimuli. • Limits the maximum rate at which action potentials can be stimulated in a neuron. • Action potentials are all-or-none events. The nervous system distinguishes between strong & weak stimuli based on the frequency of action potentials generated.

28. Action potential “travel” • Action potentials “travel” along the axon because they are self-propagating. • A neuron is stimulated at its dendrites or cell body and the action potential travels along the axon. • The signal travels in a perpendicular direction along the axon regenerating the action potential.

29. Figure 48.7Propagation of the action potential

30. Action potential “travel” • Saltatory conduction – the action potential “jumps” from one node of Ranvier to the next, skipping myelinated regions of membranes • Figure 48.11

31. Figure 48.11 Saltatory Conduction

32. Communication • Communication between cells happens across the synapse • Synapse– tiny gap between a synaptic terminal of an axon & a signal- receiving portion of another neuron or effector cell • Presynaptic cell is the transmitting cell; postsynaptic cell is the receiving cell • There are two types of synapses: • 1) electrical • 2) chemical

33. Electrical Synapses • Allow action potentials to spread directly from pre- to postsynaptic cells via gap junctions (intercellular channels) • Allow impulse travel without delay or loss of signal strength • Less common than chemical synapses • Common in crustaceans

34. Chemical Synapses • Synaptic vesicles containing thousands of neurotransmitter molecules are present in the cytoplasm of the synaptic terminal of the presynaptic neuron • Chemical synapses allow transmission in one direction only • Receptors for neurotransmitters are located only on postsynaptic membranes

35. Figure 48.12 A Chemical Synapse

36. One neuron may receive information from thousands of synapses. Some synapses are excitatory, others are inhibitory (Fig. 48.11). • The same neurotransmitter can produce different effects on different types of cells • (Table 48.2)

37. Figure 48.13 Integration of multiple synaptic inputs

38. Numerous synaptic terminals communicating with a single postsynaptic cell (SEM).

39. Fig. 48.17 Functional hierarchy of the peripheral nervous system

40. Figure 48.18Parasympathetic – maintains normal activity- slows down- acetylcholineSympathetic-”flight or fight”- speeds up- norepinephrine

41. Integration & Control CNS&PNS AutonomicSomatic NS NS (internal organs- (muscles, skin, viscera) joints) ParasympatheticSympathetic (normal activity) (“flight or fight”)

42. CNS: Meninges • Meninges series of 3 membrane layers which surround brain & spinal cord: • 1) dura mater(outer) • 2) arachnoid membrane • 3) pia mater(inner)

43. CNS: Spinal Cord • connecting mechanism between the body & the brain • part of the CNS • enclosed in vertebral column • 31 pairs of spinal nerves • fluid-filled cavity – cerebrospinal fluid

44. Fig. 48.16The nervous system of a vertebrate:31 pairs of spinal nerves each containing one sensory neuron (afferent dorsal route) & one motor neuron (efferent ventral route)

46. Vertebrate Brain • 100 billion neurons in human brain • 10 billion in the cerebral cortex • weights about 3 lbs • composed of gray matter (cell bodies) & white matter (axons & dendrites) • developed from the 3 bulges at the anterior end of the dorsal tubular nerve cord: hind – mid–fore brain)

47. Fig.48.19Embryonic Development of the Vertebrate Brain

48. Fig.48.19 Embryonic Development of the brain

49. Major parts of the human brain.

50. Vertebrate Brain (Hindbrain) • Medulla–(upper spinal cord) center for respiratory, cardiac function; vomiting, sweating, gastric secretion, heartbeat • Cerebellum – regulate & controls bodily muscular contractions; coordination, balance, equilibrium • Pons – bridge between two halves of the cerebellum; carries fibers that coordinate activity of muscles on two sides of the body