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Chapter 11

Chapter 11. Nervous System . Nervous System. Complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. Nervous System . Neurons. Specialized nerve cells Functional unit of nervous system 3 parts Cell body Dendrites

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Chapter 11

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  1. Chapter 11 Nervous System

  2. Nervous System • Complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body

  3. Nervous System

  4. Neurons • Specialized nerve cells • Functional unit of nervous system • 3 parts • Cell body • Dendrites • axon

  5. Neuron • Cell Body • contains nucleus and most organelles in cell • Synthesizes proteins, carbohydrates, lipids • Dendrites • Tree like branches that send signals towards the cell body • Axons • Single long, thin extension from cell body • Carry signals away from cell body • Contains axon terminals at tip of axon that enable signals to be transmitted from one neuron to another

  6. Neurons • 3 classes • Afferent neurons (sensory) – transmit stimuli collected by sensory receptors • Interneurons – integrate information, formulate a response • Efferent neurons (motor) – carry response signal to effectors (muscle, glands)

  7. Neural Signalling • Communication by neurons • Response to stimuli • 4 components • Reception – detection of stimulus (eyes, skin) • Transmission – movement of message along a neuron • Integration – interpretation of message • Response – output or action

  8. Neuronal Circuit/Reflex Arc • Connections between axon terminals of one neuron and the dendrites or cell body of a second neuron • Receptor → afferent neuron → one or more interneurons → efferent neuron → effector

  9. Reflex Arc • Simplest of neural circuits which does not require coordination of brain

  10. Neuron Support System • Speed rate at which electrical impulses move along axons • Glial cells • Provide nutrition and support to neurons • E.g. Schwann cells – form tightly wrapped layers of plasma membrane around axons – myelin sheaths • Myelin sheaths – electrical insulators (high lipid content) • Nodes of Ranvier • Gaps between Schwann cells • Expose axon membrane directly to extracellular fluid

  11. Nerve Signals • Use internal cellular energy to generate current (ATP) • Communicate across a synapse • Site where neuron makes a connection with another neuron or an effector • Two sides to a synapse • Pre-synaptic cleft – axon terminal • Post-synaptic cleft – dendrite or cell body • Communication occurs in 2 ways • Chemically • Electrically

  12. Chemical Synapse • Pre-synaptic cleft and post-synaptic cleft are separated by a gap (25nm) – synaptic cleft • Uses neurotransmitters to communicate between neurons

  13. Electrical Synapse • Pre-synaptic cleft and post-synaptic cleft are in direct contact • Current flows directly through nerurons • Gap junctions allow ions to flow • Provides rapid/synchronous transmission between neurons

  14. Conduction of Electrical Signals by Neurons • Membrane potential – difference in charge across the plasma membrane (K⁺ Na⁺) • Resting membrane potential/action potential • Sudden flow of ions across the plasma membrane via ion channels (Na⁺/K⁺ active transport pump) causes nerve impulses

  15. Na⁺/K⁺ Pump • Pumping of 3 Na⁺ ions out of the cell for every 2 K⁺ pumped into the cell • Net positive charge outside of cell

  16. Resting Membrane Potential • Neuron is not conducting a nerve impulse • Steady negative membrane potential (-70mV) • Cell is polarized

  17. Action Potential • Neuron conducts an electrical impulse • Temporary change in membrane potential • Positive charges flow inside the cell • 6 phases • Action potential is produced only if the stimulus is strong enough to cause depolarization to reach threshold – all or nothing principle

  18. Refractory Period • Threshold elevated to ensure a one way direction in neuron and gives channels time to reset themselves (resting period)

  19. Action Potential • Magnitude remains the same as it travels along the axis • The greater the stimulus, the faster the action potential is • Rate of conduction increases with diameter of axon

  20. Myelinated Axons • Saltatory conduction • Hopping of action potentials over myelin onto nodes of Ranvier • Na⁺ and K⁺ channels are crowded into nodes allowing for action potentials to develop • Speeds of up to 130 m/s compared to 1 m/s in unmyelinated • Allows for smaller sized and more tightly packed axons

  21. Conduction Across Chemical Synapses • Action potentials cannot jump across synapses • Use of neurotransmitters • Transmission becomes delayed allowing neurons to receive hundreds to thousands of axon terminals at the same time

  22. Conduction Across Chemical Synapses • Neurotransmitters are stored in synaptic vesicles in the cytosol of an axon terminal • Action potential stimulates the release of Ca²⁺ into the cytosol • Triggers a protein which allows vesicle to fuse with the plasma membrane releasing neurotransmitters into the synaptic cleft by exocytosis

  23. Role of Neurotransmitters • Diffuse across the synaptic cleft and bind to receptors located on the post synaptic cell • Binding opens gated ion channels (Na⁺, K⁺, Cl⁻) • Causes stimulatory/inhibitory effects

  24. Neurotransmitters

  25. Central Nervous System • Comprised of the • Brain, spinal cord • Manages body activities by integrating incoming sensory information from the PNS into effective responses • Control centre of the body

  26. Protective Connective Tissue • Meninges • 3 layers of connective tissue that surround and protect the brain and spinal cord • Cerebrospinal Fluid • Cushions the brain and spinal cord, nourishes and protects from toxic substances

  27. Spinal Cord • Carries impulses between brain and PNS • Contains interneuron circuits that control motor reflexes • Structures • Grey matter – consists of nerve cell bodies and dendrites • White matter – consists of myelinated axons • Dorsal root – incoming afferent neurons • Ventral root – outgoing efferent neurons • 31 pairs of spinal nerves • Caudaequina – collection of spinal nerves that leave inferior end of spinal cord

  28. Anatomy of Spinal Cord

  29. Cross Section

  30. Brain • Receives, integrates, stores, retrieves information • Interneurons generate responses that provide the basis for • Voluntary movements, consciousness, behaviour, emotions, learning, reasoning, language, and memory • Contains • Grey matter, white matter, meninges, cerebrospinal fluid • Broken into • Forebrain, midbrain, hindbrain

  31. Structures of Brain • Medulla oblongata • Involuntary behaviours – breathing, digestion, heart rate, blood pressure • Cerebellum • Voluntary behaviours – muscle contraction, balance, fine motor control • Pons – mass of fibres that connects cerebellum to higher centres of brain • Brain stem – pons and medulla • Connects forebrain to spinal cord

  32. Cerebrum • Controls most of the sensory and motor activities • Makes up most of the brain • Cerebral Cortex • Surface layer of cerebrum • Thin layer of grey matter – unmyelinated neurons • Carries out higher brain functions • Divided into left and right hemispheres – capability of functioning separately • Divided into parietal, frontal, temporal, occipital lobes

  33. Left and Right Hemispheres • Corpus Callosum • Thick axon bundles • Connect two hemispheres together and coordinates function • Recognizing faces, sense of time, recognizing emotions

  34. Sensory Regions of Cerebral Cortex • Frontal Lobe • Reasoning, motor skills, higher level cognition, expressive language • Parietal Lobe • Processing somatosensory area - touch, pain, temperature, pressure • Temporal Lobe • Primary auditory cortex, interpreting sounds and language • Hippocampus – memory • Occipital Lobe • Interpreting visual stimuli and information

  35. Somatosensory/Motor Cortex • Regions of the cerebral cortex that are involved with different functions • Form bands across the top of the brain

  36. Thalamus, Hypothalamus, Basal Nuceli • Thalamus • Receives sensory information and relays to appropriate regions of cerebral cortex • Waking and inducing drowsiness or sleep • Hypothalamus • Regulate homeostatic functions of body • Basal Nuclei/ganglia • Grey-matter centres that surround thalamus • Moderate voluntary movements directed by motor centres in cerebrum • Parkinsons disease

  37. Blood Brain Barrier • Tight junctions (impermeable membrane between two cells) that prevent most substances dissolved in blood from entering cerebrospinal fluid • Astrocytes protect from viruses, bacteria, toxic substances

  38. Peripheral Nervous System • Regulates both movement and internal environment of the body

  39. Efferent System • Made up of axons of neurons • Carries signals to muscle glands which act as effectors • Divided into • Somatic system – communicates with skeletal muscles • Autonomic system – communicates with smooth muscles and glands

  40. Somatic System • Conscious and voluntary • Controls body movements • Carries signals from CNS to skeletal muscles • 31 pairs of spinal nerves • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral,1 coccygeal • Somatic nerves consist only of axons • Some Involuntary contractions • Reflexes, shivering, balance posture

  41. Autonomic System • Works with endocrine system to regulate the body in response to change • Uses motor nerves • Controls involuntary processes • Digestion, secretion, circulation, reproduction, excretory, contraction of smooth muscle, breathing • Broken into • Sympathetic – associated with nerves of chest and abdomen • Parasympathetic – associated with brain

  42. Sympathetic/Parasympathetic • Always active • Have opposing effects on organs they affect (precise control) one stimulates, the other inhibits • Uses two neurons • Dendrites and cell body in CNS • Ganglion outside CNS – enlargement of nerve where cell bodies of neurons are located • Sympathetic predominates during situations of stress, excitement, strenuous physical activity • Parasympathetic predominates during situations that are quiet, low stress

  43. Sympathetic/Parasympathetic

  44. Pain/Painkiller • Interpretation by the brain of sensory input received by specialized cells called substantiagelatinosa (SG) • Forms a band in the dorsal horn of grey matter in spinal cord • These cells are unmyelinated • SG cells are stimulated by an afferent nerve of PNS (stub your toe) • SG sends signal to brain to release endorphins and enkephalins (opioids) • Attach to receptors of SG and prevent communication

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