26-1 Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 26: Nervous systems

26-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 26: Nervous systems


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


Fig. 26.1a: Generalised neuron


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


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)


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


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


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


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


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


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)


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.)


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


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)


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


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


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


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


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


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


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


Fig. 26.15: Mechanisms of visual detection


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


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


Mechanoreception

• External and internal mechanical stimuli• External

– mechanical stress in body walls– deflection of hairs– hearing

• Internal– position of limbs– tension of visceral walls


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


Fig. 26.16: Sound detection in mammalian ear(a) Structure of the human ear


Fig. 26.16: Sound detection in mammalian ear(b) The cochlea in section


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


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


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.)


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


Fig. 26.17: Autonomic nervous system

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26-1 Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint Chapter 26: Nervous systems