Transcript Chapter 26

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