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Chapter 26: Nervous systems Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-1 Neurons • • Nervous systems transmit and integrate information through specialised cells called neurons Neurons have three structural regions – dendrites branching processes that receive signals from other cells – cell body or soma area containing nucleus, integrates signals – axon elongate process that carries output signal Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-2 Fig. 26.1a: Generalised neuron Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-3 Glial cells • • Glial cells are associated with neurons in nervous systems Functions of glial cells – – – – mechanical support electrical insulation maintain extracellular environment guide neuron development and repair Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-4 Types of neurons • Sensory (afferent) neurons – receive signals from sensory receptors (extero- and enteroreceptors) • Interneurons – integrate information from sensory neurons and pass output on to motor neurons • Motor (efferent) neurons – provide signals that control muscles and glands (effectors) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-5 Transfer of information • Information is transmitted as electrical impulses • When inactive, neurons maintain a difference in charge across the plasma membrane – negative charge inside membrane – positive charge outside membrane – membrane is polarised • Changes in membrane voltage pass along neurons Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-6 Neuronal membranes • Charge on inside of inactive neuron is resting potential – –70 to –80 mV • Maintained by ion pumps (transmembrane proteins) that use energy from ATP to – remove Na+ from cell – bring K+ into cell • But membrane is more permeable to K+ than Na+, so K+ leaks out of cell – leaves inside of membrane negative compared to outside Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-7 Active response • • When a neuron membrane is stimulated, the membrane becomes depolarised Once depolarisation has reached the threshold potential, the active response is triggered – protein channels open, increasing their permeability to Na+ – as the potential changes, other channels open allowing K+ to leave • Properties of active response depends on the properties of the membranes Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-8 Action potential • • Active responses fade with distance so cannot conduct impulses along lengthy axons Over long distances, information is transmitted by action potentials – action potentials do not diminish with distance • In membranes that generate action potentials, opening of Na+ channels creates a positive feedback loop along adjacent membrane – propagates wave of depolarisation along axon Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-9 Refractory period • After each action potential, the membrane cannot transmit another potential for a brief period – refractory period • Limits frequency with which impulses can be transmitted – c. 100 impulses/sec Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-10 Conduction • Conduction of action potentials along axon vary between 0.5 ms-1 and 120 ms-1 – speed affected by diameter and insulation • • • Fast-conducting vertebrate axons surrounded by myelin (formed by glial cells) Bare regions on axon between myelin are called nodes of Ranvier Impulse skips between nodes (saltatory conduction) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-11 Synapses • • • • Electrical information is transmitted to other neurons and muscles through synapses Activity in post-synaptic cells can be increased (excited) or decreased (inhibited) Signals are transmitted across chemical synapses by release of neurotransmitters In electrical synapses, electrical signals are transmitted directly (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-12 Synapses (cont.) • When stimulated by an action potential, presynaptic neuron releases neurotransmitter from synaptic vesicles • Synaptic vesicles fuse with presynaptic membrane and empty into synaptic gap • Neurotransmitter binds to receptors on postsynaptic membrane • Excites or inhibits post-synaptic neuron Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-13 Synaptic potentials • • Neurotransmitter changes permeability of postsynaptic membrane potential Potential becomes more negative – hyperpolarised – inhibitory post-synaptic potential (ipsp) • Potential becomes less negative – depolarised – excitatory post-synaptic potential (epsp) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-14 Integrating information • Role of each synaptic input depends on – activity of synapse inhibitory or excitatory – location of synapse on post-synaptic neuron dendrite, cell body or axon – timing of input activity relative to other inputs Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-15 Evolution of nervous systems • Basic properties of neurons are the same in all animals • Diffuse nerve nets in lower invertebrates • Increasing organisation of neurons into nerves and ganglia • Anterior aggregations of ganglions (encephalisation) associated with more complex behaviour Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-16 Vertebrate nervous systems • Vertebrate nervous systems composed of – central nervous system brain and spinal cord integrates information – peripheral nervous system nerves and ganglia transmits information between CNS and organs Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-17 Mammalian brain • The mammalian brain is a complex structure • Convoluted cerebral cortex is involved in control of movement and higher functions, including learned behaviours • Cerebellar cortex (cerebellum) is concerned with balance and movement • The brain stem (thalamus, hypothalamus, pons, medulla) controls basic functions Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-18 Controlling movement • • Motor or somatic control systems range in complexity Monosynaptic reflexes (single synapse) – a sensory neuron connected directly to a motor neuron • Coordination of conscious patterns of muscle movement – widely distributed neural interactions Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-19 Senses • Sensory receptors monitor the external world • Receptors are specific to stimulus type – example: photoreceptors detect light • Sensory receptors are aggregated into organs – example: photoreceptors form eyes • Receptors detecting internal states – visceral or enteroreceptors Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-20 Vision • Detection of patterns of light – stimulation of photosensitive pigments • • • Eyespots detect light and dark Pigment cups detect direction Simple eyes are image-forming – with lens (vertebrates) or without lens (Nautilus) • Compound eyes are image-forming – multiple repeated units Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-21 Fig. 26.15: Mechanisms of visual detection Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-22 Visual specialisations • Some birds and insects can see ultraviolet – important component of plant colour patterns – cannot be detected by species with different visual range • • Polarised light used in navigation by some species Light sensitivity increased by presence of reflective layer at back of eye – nocturnal or deep sea species • Acuity – high degree of image resolution for detecting prey Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-23 Chemoreception • • Detection of chemicals in environment Chemoreceptors often have high specificity – may be extremely sensitive – example: some organisms (e.g. silk moths) can detect one or a few molecules of target substance • Olfaction – airborne chemicals • Taste – contact chemicals Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-24 Mechanoreception • External and internal mechanical stimuli • External – mechanical stress in body walls – deflection of hairs – hearing • Internal – position of limbs – tension of visceral walls Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-25 Hearing • Type of mechanoreception – hearing receptors detect and amplify pressure waves of sound – activated by one frequency or a range of frequencies • Membrane (tympanum) vibrates like surface of drum – on legs, body or wing bases of insects – in ears of vertebrates • In vertebrate ears, vibrations are amplified by small bones and transmitted to fluid-filled cochlea where sensory hairs are stimulated Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-26 Fig. 26.16: Sound detection in mammalian ear (a) Structure of the human ear Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-27 Fig. 26.16: Sound detection in mammalian ear (b) The cochlea in section Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-28 Pain • Pain receptors mostly in skin surface – thought to be activated by chemicals released from damaged or irritated tissue • Mechanical pain receptors – cutting, mechanical damage • Heat pain receptors – when skin is heated above a threshold • Polymodal pain receptors – Mechanical, heat and chemical stimuli Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-29 Visceral control • Visceral organs are controlled by the autonomic nervous system – not under conscious control • Integrated with endocrine system – coordinates physiological functions – regulates internal environment • Examples of autonomic functions – rate and strength of heart beat – diameter of pupil – formation and release of hormones Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-30 Vertebrate autonomic system • Vertebrate autonomic nervous system divided into – central portion within brain stem and spinal cord – peripheral portion • ganglia and nerves Peripheral portion divided into – sympathetic division – parasympathetic division – enteric division (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-31 Vertebrate autonomic system (cont.) • Sympathetic division – thoracic and lumbar parts of spinal cord • Parasympathetic division – brain stem and sacral spinal cord • Enteric division – embedded in walls of digestive organs – complete reflex circuits – reflexes are modulated by sympathetic and parasympathetic inputs Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-32 Fig. 26.17: Autonomic nervous system Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 26-33