Chapter 48/49 Nervous Systems CNS injuries and diseases

1
Chapter 48/49

• Nervous Systems

2

• The human brain contains an estimated 100 billion
  nerve cells, or neurons
• Each neuron may communicate with thousands of
  other neurons
• Complex information processing network is at work

3
Different neurons do different things

• Functional magnetic resonance imaging (fMRI)
• can reconstruct a three-dimensional map of brain
  activity
• The results of brain imaging and other research
  methods revealed that groups of neurons function
  in specialized circuits dedicated to different
  tasks

4
Nervous systems consist of circuits of neurons
and supporting cells

• All animals except sponges have some type of
  nervous system
• What distinguishes the nervous systems of
  different animal groups is how the neurons are
  organized into circuits

5
Organization of Nervous Systems

• The simplest animals with nervous systems, the
  cnidarians
• They have neurons arranged in nerve nets

6
Sea Stars

• Sea stars have a nerve net in each arm connected
  by radial nerves to a central nerve ring

7
Flatworms

• In relatively simple cephalized animals, such as
  flatworms a central nervous system (CNS) is
  evident

8
Annelids and arthropods

• Have segmentally arranged clusters of neurons
  called ganglia
• These ganglia connect to the CNS and make up a
  peripheral nervous system (PNS)

9
Molluscs

• Nervous systems in molluscs correlate with the
  animals lifestyles
• Sessile molluscs have simple systems
• More complex molluscs have more sophisticated
  systems

10
Vertebrates

• The central nervous system consists of a brain
  and dorsal spinal cord
• The PNS connects to the CNS via nerves

11
Information Processing

• Nervous systems process information in three
  stages Sensory input, integration, and motor
  output

12

• Sensory neurons transmit information from sensors
  (receptors) that detect external stimuli and
  internal conditions
• Sensory information is sent to the CNS
• Where interneurons integrate the information
• Motor output leaves the CNS via motor neurons
• Which communicate with effector cells

13
Cells of the Nervous System
14
Neuron Structure

• Cell body
• Dendrites
• Axons
• Myelin Sheath

• Schwann cells
• Synaptic Terminals
• Synapse
• Neurotransmitters

15
Types of neurons

• Sensory Neurons
• Interneurons
• Motor Neurons

16
Supporting Cells (Glia)

• Glia are supporting cells
• They are essential for the structural integrity
  of the nervous system and for the normal
  functioning of neurons

17
Ion pumps and ion channels maintain the resting
potential of a neuron

• Across its plasma membrane, every cell has a
  voltage called a membrane potential
• Resting Membrane Potential membrane potential of
  a neuron that is not transmitting signals
• The inside of a cell is negative relative to the
  outside

18

• The concentration of Na is higher in the
  extracellular fluid than in the cytosol
• While the opposite is true for K

19

• A neuron that is not transmitting signals
• Contains many open K channels and fewer open Na
  channels in its plasma membrane
• The diffusion of K and Na through these
  channels
• Leads to a separation of charges across the
  membrane, producing the resting potential

20
Gated Ion Channels

• Gated ion channels open or close
• In response to membrane stretch or the binding of
  a specific ligand
• In response to a change in the membrane potential

21
Action Potential

• Action potentials are the signals conducted by
  nerve fibers
• If a cell has gated ion channels
• Its membrane potential may change in response to
  stimuli that open or close those channels

22
Hyperpolarization

• Stimuli may trigger an increase in the magnitude
  of the membrane potential

23
Depolarization

• stimuli may trigger a reduction in the magnitude
  of the membrane potential

24

• Hyperpolarization and depolarization
• Are both called graded potentials because the
  magnitude of the change in membrane potential
  varies with the strength of the stimulus

25
Production of Action Potentials

• In most neurons, depolarizations are graded only
  up to a certain membrane voltage, called the
  threshold
• A stimulus strong enough to produce a
  depolarization that reaches the threshold
  triggers a different type of response, called an
  action potential

26
Action Potential

• An action potential
• Is a brief all-or-none depolarization of a
  neurons plasma membrane
• Is the type of signal that carries information
  along axons

27
Step 1Sodium gates

• Both voltage-gated Na channels and voltage-gated
  K channels
• Are involved in the production of an action
  potential
• When a stimulus depolarizes the membrane
• Na channels open, allowing Na to diffuse into
  the cell

28
Step 2Potassium gates

• As the action potential subsides
• K channels open, and K flows out of the cell
• A refractory period follows the action potential
• During which a second action potential cannot be
  initiated

29
Generation of an action potential
Na
Na
Na
Na
K
K
Rising phase of the action potential
Depolarization opens the activation gates on most
Na channels, while the K channels activation
gates remain closed. Na influx makes the inside
of the membrane positive with respect to the
outside.
The inactivation gates on most Na channels
close, blocking Na influx. The activation gates
on mostK channels open, permitting K
effluxwhich again makesthe inside of the cell
negative.
50
Actionpotential
Na
Na
0
Membrane potential (mV)
Threshold
Threshold
50
K
Resting potential
100
Time
A stimulus opens the activation gates on some
Na channels. Na influx through those channels
depolarizes the membrane. If the depolarization
reaches the threshold, it triggers an action
potential.
Na
Na
Na
              
  
  
K
              
  
  
Undershoot
Both gates of the Na channelsare closed, but
the activation gates on some K channels are
still open. As these gates close onmost K
channels, and the inactivation gates open on Na
channels, the membrane returns toits resting
state.
Cytosol
K
Sodiumchannel
The activation gates on the Na and K
channelsare closed, and the membranes resting
potential is maintained.
30
Conduction of Action Potentials

• An electrical current depolarizes the neighboring
  region of the axon membrane

31
Conduction Speed

• The speed of an action potential
• Increases with the diameter of an axon
• In vertebrates, axons are myelinated
• Also causing the speed of an action potential to
  increase

32
Saltatory Conduction

• Action potentials in myelinated axons
• Jump between the nodes of Ranvier in a process
  called saltatory conduction

33
Synapse

• Electrical synapse
• Electrical current flows directly from one cell
  to another via a gap junction
• Chemical synapse
• A presynaptic neuron releases chemical
  neurotransmitters, which are stored in the
  synaptic terminal

34
Neurotransmitter release

• When an action potential reaches a terminal the
  final result is the release of neurotransmitters
  into the synaptic cleft

35
Synaptic Transmission

• Neurotransmitters bind to ligand-gated ion
  channels
• Binding causes the ion channels to open,
  generating a postsynaptic potential and
  generation of action potential
• Fate of neurotransmitter in the cleft
• Diffuses out of the synaptic cleft
• Taken up by surrounding cells and degraded by
  enzymes
• Degraded in the cleft

36
Neurotransmitters

• Different neurons may release different
  neurotransmitters
• The same neurotransmitter can produce different
  effects in different types of cells

37
Neurotransmitters
38
Neurotransmitters

• Acetylcholine
• Is one of the most common neurotransmitters in
  both vertebrates and invertebrates
• Can be inhibitory or excitatory
• Biogenic amines
• Include epinephrine, norepinephrine, dopamine,
  and serotonin
• Are active in the CNS and PNS
• Amino acids and peptides
• Are active in the brain
• Gases such as nitric oxide and carbon monoxide
• Are local regulators in the PNS

39
Vertebrate Nervous System

• Regionally specialized
• In all vertebrates, the nervous system shows a
  high degree of cephalization and distinct CNS and
  PNS components

40
Central Nervous System

• The brain provides the integrative power
• That underlies the complex behavior of
  vertebrates
• The spinal cord integrates simple responses to
  certain kinds of stimuli
• And conveys information to and from the brain

41
Peripheral Nervous System

• The PNS transmits information to and from the CNS
• And plays a large role in regulating a
  vertebrates movement and internal environment
• The cranial nerves originate in the brain
• And terminate mostly in organs of the head and
  upper body
• The spinal nerves originate in the spinal cord
• And extend to parts of the body below the head

42
PNS components

• The somatic nervous system
• The autonomic nervous system

43
Somatic and Autonomic

• The somatic nervous system
• Carries signals to skeletal muscles
• The autonomic nervous system
• Regulates the internal environment, in an
  involuntary manner
• Is divided into the sympathetic, parasympathetic,
  and enteric divisions

44
Sympathetic and parasympathetic divisions

• They have antagonistic effects on target organs

45

• The sympathetic division
• Correlates with the fight-or-flight response
• The parasympathetic division
• Promotes a return to self-maintenance functions
• The enteric division
• Controls the activity of the digestive tract,
  pancreas, and gallbladder

46
The Brainstem

• The brainstem consists of three parts the
  medulla oblongata, the pons, and the midbrain

47
Brainstem

• The medulla oblongata
• Contains centers that control several visceral
  functions
• The pons
• Also participates in visceral functions
• The midbrain
• Contains centers for the receipt and integration
  of several types of sensory information
• All three areas are center of reflex actions

48
Arousal and Sleep

• A diffuse network of neurons called the reticular
  formation is present in the core of the brainstem
• A part of the reticular formation, the reticular
  activating system (RAS) regulates sleep and
  arousal

49
The Cerebellum

• Important for coordination and error checking
  during motor, perceptual, and cognitive functions
• Also involved in learning and remembering motor
  skills

50
The Diencephalon

• The embryonic diencephalon develops into three
  adult brain regions
• The epithalamus, thalamus, and hypothalamus

51

• The epithalamus
• Includes the pineal gland and the choroid plexus
• The thalamus
• Is the main input center for sensory information
  going to the cerebrum and the main output center
  for motor information leaving the cerebrum
• The hypothalamus regulates
• Homeostasis
• Basic survival behaviors such as feeding,
  fighting, fleeing, and reproducing

52
Circadian Rhythms

• The hypothalamus also regulates circadian rhythms
  (Such as the sleep/wake cycle)
• Animals usually have a biological clock
• Which is a pair of suprachiasmatic nuclei (SCN)
  found in the hypothalamus

53
The Cerebrum
54
Cerebral Hemispheres

• The cerebrum has right and left cerebral
  hemispheres that each consist of cerebral cortex
  overlying white matter and basal nuclei

55
Basal Nuclei

• The basal nuclei
• Are important centers for planning and learning
  movement sequences

56
Cerebral Cortex

• In mammals
• The cerebral cortex has a convoluted surface
  called the neocortex
• In humans its the largest and most complex part
  of the brain where sensory information is
  analyzed, motor commands are issued, and language
  is generated
• A thick band of axons, the corpus callosum
• Provides communication between the right and left
  cerebral cortices

57
Cerebral Cortex

• The cerebral cortex controls voluntary movement
  and cognitive functions
• Frontal, parietal, temporal, and occipital

58
Information Processing in the Cerebral Cortex

• Each of the lobes contains primary sensory areas
  and association areas
• Specific types of sensory input
• Enter the primary sensory areas
• Adjacent association areas
• Process particular features in the sensory input
  and integrate information from different sensory
  areas

59

• In the somatosensory cortex and motor cortex
  neurons are distributed according to the part of
  the body that generates sensory input or receives
  motor input

60
Lateralization of Cortical Function

• During brain development, in a process called
  lateralization
• Competing functions segregate and displace each
  other in the cortex of the left and right
  cerebral hemispheres
• The left hemisphere
• Becomes more adept at language, math, logical
  operations, and the processing of serial
  sequences
• The right hemisphere
• Is stronger at pattern recognition, nonverbal
  thinking, and emotional processing

61
Language and Speech

• Studies of brain activity have mapped specific
  areas of the brain responsible for language and
  speech

PET scans showing brain activity levels
62
Language and Speech

• Portions of the frontal lobe, Brocas area and
  Wernickes area are essential for the generation
  and understanding of language
• Brocas area damage comprehend speech but have
  impaired ability to mechanically form speech
• Wernickes area damage no language
  comprehension, cant understand either spoken or
  written language. Speak in rapid nonsensical
  manner

63
Emotions

• The limbic system is a ring of structures around
  the brainstem

64
Limbic system

• This limbic system includes three parts of the
  cerebral cortex The amygdala, hippocampus, and
  olfactory bulb. Also some parts of the thalamus
  and hypothalamus
• These structures interact with the neocortex to
  mediate primary emotions and attach emotional
  feelings to survival-related functions
  (reproduction, aggression, feeding)
• Structures of the limbic system form in early
  development and provide a foundation for
  emotional memory, associating emotions with
  particular events or experiences

65
Memory and Learning

• The frontal lobes
• Are a site of short-term memory
• Interact with the hippocampus and amygdala to
  consolidate long-term memory
• Many sensory and motor association areas of the
  cerebral cortex are involved in storing and
  retrieving words and images

66
CNS injuries and diseases

• Unlike the PNS, the mammalian CNS
• Cannot repair itself when damaged or diseased
• Research on nerve cell development and stem cells
• Hot area of research
• May one day make it possible for physicians to
  repair or replace damaged neurons

67
Neural Stem Cells

• The adult human brain contains stem cells that
  can differentiate into mature neurons
• The induction of stem cell differentiation and
  the transplantation of cultured stem cells
• Are potential methods for replacing neurons lost
  to trauma or disease

68
Diseases and Disorders of the Nervous System

• Mental illnesses and neurological disorders take
  a huge toll on society
• patients loss of a productive life
• high cost of long-term health care
• The direct costs of mental health services in the
  United States in 1996 totaled 69.0 billion. This
  figure represents 7.3 percent of total health
  spending. An additional 17.7 billion was spent
  on Alzheimers disease The indirect costs of
  mental illness were estimated in 1990 at 78.6
  billion
• Causes (genetic environmental) are not often
  clear at present
• Treatments that exist are not great and usually
  amount to control of symptoms and not cure for
  the underlying abnormality

69
Schizophrenia

• About 1 of the worlds population suffers from
  schizophrenia
• There are several forms each characterized by an
  inability to distinguish reality
• Symptoms
• Hallucinations, delusions, blunted emotions, and
  many other symptoms
• Treatments have focused on brain pathways that
  use dopamine as a neurotransmitter

70
Depression

• Two broad forms of depressive illness bipolar
  disorder and major depression
• Bipolar disorder is characterized by
• Manic (high-mood) and depressive (low-mood)
  phases
• In major depression
• Patients have a persistent low mood
• Treatments for these types of depression include
  a variety of drugs such as Prozac and lithium

71
Alzheimers Disease

• Alzheimers disease (AD)
• Is a mental deterioration characterized by
  confusion, memory loss, and other symptoms
• AD is caused by the formation of neurofibrillary
  tangles and senile plaques (amyloid protein) in
  the brain

72
Parkinsons Disease

• Parkinsons disease is a motor disorder
• Caused by the death of dopamine-secreting neurons
  in midbrain nucleus (substantia nigra)
• Characterized by difficulty in initiating
  movements, slowness of movement, and rigidity

73
Multiple sclerosis

• Caused by immune destruction of myelin sheaths
• Leads to improper nerve impulse conduction and
  associated loss of function

74
Nerve toxins

• Tetanus
• Blocks inhibitory synapses leading to spastic
  paralysis
• Botulism
• Prevents release of Acetylcholine leading to
  flaccid paralysis
• Curare
• Prevents binding of Acetylcholine to receptors
  leading to flaccid paralysis