28-1 copyright 2010 mcgraw-hill australia pty ltd powerpoint slides to accompany biology: an...

28-1Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and SaintSlides prepared by Karen Burke da Silva, Flinders University

Chapter 28: Nervous systems


Neurons: the functional units of nervous systems• 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. 28.1a: Generalised neuron


Glial cells• Are associated with neurons in nervous systems• Functions

– mechanical support– electrical insulation– maintenance of extracellular environment– guiding neuron development and repair


What types of neurons are there?• 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)


Neurons transfer information as electrical signals• 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 depend 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– about 100 impulses/sec


How are action potentials conducted?• 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)

Fig. 28.3: Fast-conducting axons in vertebrates



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


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

Fig. 28.5: Chemical synapse



What are 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)


How do neurons integrate 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

Question 1:

Difference in intracellular and extracellular concentrations of ions would be most likely due to:

a) the permeability characteristics of the membrane

b) the presence of negatively charged proteins and other anions within the cell

c) the activity of the sodium-potassium pump

d) all of the above

e) a and c only



The 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

Fig. 28.7: Invertebrate nervous systems



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


The 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

Fig. 28.8: Structural divisions of the mammalian brain


Fig. 28.9: Structural differences of mammalian brains



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. 28.15: 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. 28.16: Sound detection in mammalian ear(a) Structure of the human ear


Fig. 28.16: Sound detection in mammalian ear (cont.)(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


The vertebrate autonomic nervous 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


The vertebrate autonomic nervous 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. 28.17: Autonomic nervous system

Summary• Nervous systems are comprised of networks of

neurons and are capable of providing precise and rapid co-ordination of cellular function, movement and behaviour

• Various forms of energy can be converted to electrical signals by sensory neurons

• Transmission from neurons to other cells is usually chemical, but in some cases is electrical

• Complex behaviours require complex nervous systems


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