Nervous Coordination

1. NervousCoordination Chapter 33

2. Irritability • The ability to respond to environmental stimuli is a fundamental property of life. • Single celled organisms respond in a simple way – e.g. avoiding a noxious substance. • The evolution of multicellularity required more complex mechanisms for communication between cells. • Neural mechanisms – rapid, brief • Hormonal mechanisms – slower, long term

3. CNS & PNS • Central Nervous System (CNS) – includes the brain and spinal cord. • Peripheral Nervous System (PNS) – includes motor and sensory neurons.

4. Neurons • A neuron (nerve cell) is the functional unit of the nervous system. • Sensory (afferent) neurons carry impulses from sensory receptors to the CNS. • Motor (efferent) neurons carry impulses away from the CNS to effectors (muscles and glands). • Interneurons connect neurons together.

5. Neurons • Two types of cytoplasmic processes extend from the cell body. • Dendrites bring signals in to the cell body. • Often highly branched. • Axons carry signals away from the cell body.

6. Nerves • Nerve processes (usually axons) are often bundled together, surrounded by connective tissue, forming a nerve. • Cell bodies are located in the CNS or in ganglia (bundles of cell bodies outside the CNS).

7. Glial Cells • Non-neural cells that work with neurons are called glial cells. • Astrocytes – star-shaped cells that serve as nutrient and ion reservoirs for neurons.

8. Glial Cells • The axon is covered with an insulating layer of lipid-containing myelin, which speeds up signal propagation. • Concentric rings of myelin are formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

9. Action Potential • A nerve signal or action potential is an electrochemical message of neurons. • An all-or-none phenomenon – either the fiber is conducting an action potential or it is not. • The signal is varied by changing the frequency of signal conduction.

10. The Nerve Impulse • Across its plasma membrane, every cell has a voltage called a membrane potential. • The inside of a cell is negative relative to the outside.

11. The Nerve Impulse • Neuron at rest – active transport channels in the neuron’s plasma membrane pump: • Sodium ions (Na+) out of the cell. • Potassium ions (K+) into the cell. • More sodium is moved out; less potassium is moved in. • Result is a negative charge inside the cell. • Cell membrane is now polarized.

12. Sodium-Potassium Exchange Pump • Na+ flows into the cell during an action potential, it must be pumped out using sodium pumps so that the action potential will continue. potassium

13. The Nerve Impulse • Resting potential – the charge that exists across a neuron’s membrane while at rest. • -70 mV. • This is the starting point for an action potential.

14. The Nerve Impulse • A nerve impulsestarts when pressure or other sensory inputs disturb a neuron’s plasma membrane, causing sodium channels on a dendrite to open. • Sodium ions flood into the neuron and the membrane is depolarized – more positive inside than outside.

15. The Nerve Impulse • The nerve impulse travels along the axon or dendrites as an electrical current gathered by ions moving in and out of the neuron through voltage-gated channels. • Voltage-gated channels – protein channels in the membrane that open & close in response to an electrical charge.

16. The Nerve Impulse • This moving local reversal of voltage is called an action potential. • A very rapid and brief depolarization of the cell membrane. • Membrane potential changes from -70 mV to +35 mV. • After the action potential has passed, the voltage gated channels snap closed and the resting potential is restored. • The membrane potential quickly returns to -70 mV during the repolarization phase. • An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane. • Carries information along axons. • An action potential is self-propagating – once started it continues to the end.

17. High Speed Conduction • Speed is related to the diameter of the axon. • Larger axons conduct faster. • A squid’s giant axon can carry impulses 10x faster than their normal axons. • Used for powerful swimming.

18. High Speed Conduction • Vertebrates do not have giant axons. • Instead, they achieve high speed conduction by a cooperative relationship between axons and layers of myelin.

19. High Speed Conduction • Insulating layers of the myelin sheath are interrupted by nodes of Ranvier where the surface of the axon is exposed to interstitial fluid. • Action potentials depolarize the membrane only at the nodes. • This is saltatory conduction, where the action potential jumps from node to node.

20. Synapses: Junctions Between Nerves • Eventually, the impulse reaches the end of the axon. • Neurons do not make direct contact with each other – there is a small gap between the axon of one neuron and the dendrite of the next. • This junction between a neuron & another cell is called a synapse.

21. Synapses: Junctions Between Nerves • Thousands of synaptic knobs may rest on a single nerve cell body and its dendrites. • Two types of synapses: • Electrical synapses • Chemical Synapses

22. Electrical Synapse • Electrical synapses are points where ionic currents flow directly across a narrow gap junction from one neuron to another. • No time lag – important in escape reactions.

23. Chemical Synapse • Presynaptic neurons bring action potentials toward the synapse. • Postsynaptic neurons carry action potentials away from the synapse. • A synaptic cleft is the small gap between the two neurons.

24. Neurotransmitters • Chemical messengers called neurotransmitters carry the message of the nerve impulse across the synapse.

25. Neurotransmitters • Neurotransmitters are released into the synapse and bind with receptors on the postsynaptic cell membrane, which cause ion channels to open in the new cell.

26. Acetylcholine – Example Neurotransmitter

27. Kinds of Synapses • There are many types of neurotransmitters, each recognized by certain receptor proteins. • Excitatory synapse – the receptor protein is a chemically gated sodium channel (it is opened by a neurotransmitter). • When opened, sodium rushes in and an action potential begins in the new neuron.

28. Kinds of Synapses • Inhibitory synapse – the receptor protein is a chemically gated potassium channel. • When opened, potassium ions leave the cell which increases the negative charge and inhibits the start of an action potential.

29. Kinds of Synapses • An individual nerve cell can have both types of receptors. • Sometimes both excitatory and inhibitory neurotransmitters arrive at the synapse. • Integration is the process where the various neurotransmitters cancel out or reinforce each other.

30. Evolution of Nervous Systems • Metazoan phyla show a progressive increase in the complexity of their nervous systems. • Reflects stages of evolution.

31. Evolution of Nervous Systems • The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets.

32. Evolution of Nervous Systems • In relatively simple cephalized animals, such as flatworms, a central nervous system (CNS) is evident.

33. Evolution of Nervous Systems • Annelids have a bilobed brain, a double nerve cord with segmental ganglia (clusters of neurons) and distinctive sensory and motor neurons. • These ganglia connect to the CNS and make up a peripheral nervous system (PNS).

34. Evolution of Nervous Systems • Molluscs generally have three pairs of well-defined ganglia. • In cephalopods, these ganglia have developed into complex nervous centers with highly developed sense organs.

35. Evolution of Nervous Systems • The arthropod plan resembles that of annelids, but ganglia are larger and sense organs are better developed. • Often elaborate social behavior.

36. Radialnerve Nervering (b) Sea star (echinoderm) Evolution of Nervous Systems • Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring.

37. Brain Sensoryganglion Spinalcord (dorsalnerve cord) (h) Salamander (chordate) Evolution of Nervous Systems • In vertebrates, the central nervous system consists of a brain and dorsal spinal cord. • The PNS connects to the CNS.

38. Vertebrate Nervous System • Vertebrates have a hollow,dorsal nerve cord terminating anteriorly in a large ganglionic mass – the brain. • Invertebrate nerve cords are solid and ventral. • Encephalization – the elaboration of size, configuration, and functional capacity of the brain.

39. Spinal Cord • The spinal cord begins as an ectodermal neural groove, which becomes a hollow neural tube. • The spinal cord is protected by the vertebrae (derived from the notochord). • White, myelinated sheath of axons & dendrites surround the gray matter containing cell bodies.

40. Reflex Arc • A simple reflex produces a very fast motor response to a stimulus because the sensory neuron bringing information about the stimulus passes the information directly to the motor neuron.

41. Reflex Arc • Usually, there are interneurons between sensory and motor neurons. • An interneuron may connect two neurons on the same side of the spinal cord, or on opposite sides.

42. Brain • The vertebrate brain has changed dramatically from the primitive linear brain of fishes and amphibians. • It has expanded to form the deeply fissured, intricate brain of mammals.

43. The Vertebrate Brain • The vertebrate brain has three parts: • Hindbrain – extension spinal cord responsible for hearing, balance, and coordinating motor reflexes. • Midbrain – contains optic lobes and processes visual information. • Forebrain – process olfactory information.

44. The Hindbrain • The hindbrain consists of the medulla oblongata, the pons, and the cerebellum. • The medulla oblongata, is really a continuation of the spinal cord. • The pons carries impulses from one side of the cerebellum to the other and connects the medulla and cerebellum to other brain regions.

45. Cerebellum • The cerebellum controls balance posture, and muscle coordination. • Birds have a highly developed cerebellum because flying is complicated.

46. Brain Stem • The brain stem includes the midbrain, pons, and medulla oblongata. • It connects the rest of the brain to the spinal cord. • Controls breathing, swallowing, digestive processes, heartbeat, and diameter of blood vessels.

47. Midbrain • The midbrain consists of the tectum, including optic lobes, which contain nuclei that serve as centers for visual and auditory reflexes.

48. Forebrain • Vertebrates other than fishes have a complex forebrain: • Diencephalon contains: • Thalamus – relay center between cerebrum & sensory nerves. • Hypothalamus – participates in basic drives & emotions. Also controls pituitary gland. • Telencephalon (cerebrum in mammals) is devoted to associative activity.

49. Thalamus • The thalamus is the major site of sensory processing. • Sensory information is received from the sensory nerves processed in the thalamus and sent on to the cerebral cortex. • The thalamus also controls balance.

50. Hypothalamus • The hypothalamus integrates internal activities, regulating processes such as: • Body temperature • Blood pressure • Respiration • Heartbeat • The hypothalamus also controls the pituitary – a major hormone producing gland.