26-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...
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26-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Chapter 26: Nervous systems
26-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 26.1a: Generalised neuron
26-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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)
26-6Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-7Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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)
26-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
26-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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 post-synaptic membrane
• Excites or inhibits post-synaptic neuron
26-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Synaptic potentials
• Neurotransmitter changes permeability of post-synaptic membrane potential
• Potential becomes more negative– hyperpolarised– inhibitory post-synaptic potential (ipsp)
• Potential becomes less negative– depolarised– excitatory post-synaptic potential (epsp)
26-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-18Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-21Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-22Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 26.15: Mechanisms of visual detection
26-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Mechanoreception
• External and internal mechanical stimuli• External
– mechanical stress in body walls– deflection of hairs– hearing
• Internal– position of limbs– tension of visceral walls
26-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-27Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 26.16: Sound detection in mammalian ear(a) Structure of the human ear
26-28Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 26.16: Sound detection in mammalian ear(b) The cochlea in section
26-29Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-30Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
26-31Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
26-32Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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