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Title: Introduction to neuroscience


1
Introduction to neuroscience
  • By David William Harper
  • Illustrations adapted from Biological
    Physiology 7th edition by J. W. Kalat
  • Text adapted from notes by SJN

2
Recommended Text
  • J. W. Kalat, Biological Physiology, Edition 7,
    Wadsworth
  • Bear et al, neuroscience

3
The Major Issues
4
The mind-brain relationship
  • It is to try to understand what we think
  • understand human thinking
  • why we are different from robots
  • Its fundamental to understand the principle of
    how it works because it is the base of all
    physical biological methods
  • Our view will be biological

5
Biological explanation of behaviour
  • Most scholars agree that the mind does not exist
    independently of the brain
  • Biological physiology the study of the
    physiological, evolutionary and developmental
    mechanisms of behaviour and experience
  • Physiological explanation relates behaviour to
    the activity of the brain
  • Ontogenetic explanation influences of and
    interactions between genes, nutrition and
    experiences producing behaviour
  • Evolutionary explanation relates behaviour to
    the evolutionary history
  • Functional explanation describes why a
    behaviour involved in a particular way (different
    areas etc.)

6
The brain and conscious experience
  • What is the mind and how it affects body (brain)
  • philosophers tried to answer that question for
    many years
  • Its important to understand different positions
    and how our minds the affect our bodies

7
mind-body problem
  • what is the relationship between mind and the
    brain?
  • Potential views
  • Dualism
  • Body is one material
  • Mind is another material
  • They are not necessary to be extremely different
    but they are different properties
  • Not necessary to be completely different
  • Monism
  • The universe consists of only one type of
    existence, either
  • materialism - everything is physical
  • mentalism - only the mind really exists
  • identity position the mental processes are the
    same as some brain processes
  • Solipsism
  • the problem of other minds

8
Philosophers
  • Chalmers
  • He said that we can separate the problems into
    two classes
  • easy problems
  • Neuroscientist has a link between 3rd person and
    1st person
  • Collecting empirical information
  • New technology with advances
  • We can answer easy questions in a definite
    manner
  • Each function of easy can be indentified to a
    particular area/mechanism
  • hard problems
  • Cannot be resolved
  • Subjective - smell of roses
  • Different experiences between people
  • qualia, i.e. particular qualities, subjective
    experience cannot be explained in mechanistic
    terms
  • consciousness is a fundamental property of matter
    it cannot be explained

9
Philosophers
  • Dennett
  • no hard problems - once all the easy problems are
    explained then there will be nothing left
  • Disagreed with traditional carthage theatre of
    mind
  • Different processes compete with each other for
    the spot light or access to consciousness
  • multiple draft theory of mind
  • Multiple processes running like a river
  • Continually being revised/changed by joining
    river
  • Churchland
  • consciousness may have conventional physical
    explanation

10
Nature and Nurture
11
Genes
  • Genes
  • units of heredity maintaining their structural
    identity throughout generations.
  • fragments of DNA (deoxyribonucleic acid)
  • used by organisms to code for different proteins
  • they consist of a series of so called nucleotides
    and have generally a similar general structure
  • They come in pairs because they are parts of
    chromosomes.
  • Chromosome
  • A double-strand of DNA (i.e. strands of genes)
  • Contained in the cell nucleus
  • In the process of transcription, information it
    encodes is used to form RNA.
  • Chromosomes also come in pairs
  • RNA (ribonucleic acid)
  • transported from nucleus to the ribosomes
    ("protein factories") where it's "programme" is
    translated into a sequence of aminoacids - a
    protein.
  • DNA gt RNA gt protein synthesis
  • (structural proteins and enzymes biological
    catalyst regulating body biochemical reactions)

12
Genes
  • Homozygous individual
  • one which has a pair of identical pair of genes
    on the two chromosomes
  • Heterozygous individual
  • has unmatched pair of genes
  • Genes can be either
  • Dominant
  • shows a strong effect in both hetero- and
    homozygous condition
  • Recessive
  • shows effects only in the heterozygous condition

13
Behaviour
  • Genes influence behaviour directly by changing
    chemicals in the brain and indirectly by
    affecting the body
  • However Genes are not the full picture because we
    are continuing to develop subject to external
    circumstances / environment

14
History of Neuroscience
15
A history of neuroscience
  • Before Hippocrates, people associated different
    organs with thoughts/feelings
  • Theories are reflective of their time and the
    knowledge available then.
  • Egyptians did not consider that the brain
    mattered and did not preserve it with other
    organs for mummification

16
A history of neuroscience
  • Hippocrates
  • first to ascribe sensation and intelligence to
    the brain (before him people thought that the
    mind was located in a hart or in the gut)
  • Galen
  • humors (fluids) theory
  • personality traits are due to different
    concentration of humours
  • Descartes
  • tried to explain the relationship between mind
    and the body
  • The brain was material and fluid-mechanical (like
    a hydraulic system of pipes) and supposed to
    communicate with the mind via pineal gland, the
    only asymmetrical organ in the brain
  • pure reason - emotions and the logical mind are
    separate from each other
  • Galvani
  • considered nerves as wires
  • discovered that brain is using electricity for
    communication

17
A history of neuroscience
  • Gall
  • invented phrenology (the study of correlating the
    structure of the head with the personality
    traits)
  • Broca
  • proposed localisation of function
  • each cognitive function had a corresponding
    region in the brain that generated it
  • Golgi
  • proposed nerve net view of the brain
  • considered all the neurons to form a single
    connected system
  • Discovered method of staining neurons to see the
    structure which helped to establish Gajals ideas
  • R. Y Cajal
  • discovered that neurons act as functional units
    and are actually disconnected from each other

18
Neuroscience Today
  • Levels of Analysis
  • molecular neuroscience
  • cellular neuroscience
  • systems neuroscience
  • behavioural neuroscience
  • cognitive neuroscience
  • Research methodology
  • predominantly experimental
  • some mathematical and computational modelling

19
Neural System Disorders
  • Alzheimers disease (degenerative disorder)
  • Cerebral palsy (motor disorder)
  • Depression (mood disorder)
  • Epilepsy
  • Multiple sclerosis
  • Parkinsons disease
  • Schizophrenia
  • Stroke

20
Animal Testing
  • Animal nervous systems similar to Human
  • Many neuroscience discoveries were found using
    animal nervous systems.
  • Question are experiments on animals necessary?
  • Question Is it right?

21
Nerve cells, Action Potentials and Synaptic
Transmission
22
Nervous system
  • Nervous system consists of a vast quantity of
    interconnected cells organised in various
    structures.
  • Major part in information processing in the
    nervous system play neurons, cells receiving
    information and transmitting it to other cells.
  • Cannot separate the neurons metabolic process
    from the process of information processing,
    functional processes

23
Distribution of neurons
Cerebral cortex 12-15x109
Cerebellum 70x109
Spinal cord 1x109
24
The neuron structure
  • Neurons share many features of other cells but
    also have some distinct features
  • Cell body (soma)
  • Contains cytosol fluid in which float cell
    organelles
  • Membrane
  • Phospholipid bilayer actively regulating
    cell-environment interaction
  • Dendrites
  • Input
  • Axons
  • output

25
Organelles
  • Nucleus
  • Contains chromosomes (DNA)
  • Involved in hereditary control
  • Blueprint
  • Rough endoplasmic reticulum
  • Contains ribosomes
  • Responsible for synthesis, isolation,
    modification, and transportation of proteins
  • Smooth endoplasmic reticulum
  • Protein 3D folding
  • Regulations internal concentration of calcium
  • Golgi apparatus
  • Protein sorting
  • Mitochondria
  • Metabolic activities
  • Energy production

26
Membrane
  • Gives identity to the structure which we call a
    cell
  • Isolation from external environment
  • Protects the cell
  • Controlled communication
  • Gets rid of waste
  • Allows input
  • Ion channels
  • Open and close selectively to control flow
  • Sodium and potassium channels affect membrane
    potential

27
Ion channel
Protein ion channel
Phospholipid molecules
28
Axon
  • Main information sender
  • Thin fibre constant diameter
  • Extend for long distances
  • Axon collaterals
  • Branches to intervene with several other cells
  • Presynaptic terminals or boutons
  • At end of axon collaterals
  • Junction for communication
  • Axoplasmic (anterograde) transport
  • Delivery of neurotransmitters to presynaptic
    terminals
  • Makes possible the interneuron communication

29
Dendrites
  • Main information receivers of the neuron
  • Branching fibres
  • Taper away from cell body

30
Cytoskeleton
  • The cytoskeleton is a cellular "scaffolding" or
    "skeleton" contained within the cell
  • Microtubials
  • transport of organelles within the cell
  • Neurofillaments
  • Microfilaments
  • resists tension and maintains cellular shape

31
Neuron Classification
  • Main neuron classification
  • Morphological
  • Dendrites
  • Connections
  • Axon length
  • Chemical
  • Neurotransmitter type

32
Glia
  • Important cells of the nervous system
  • They do not convey information over long
    distances
  • Exchange chemicals with neighbouring neurons

33
Astrocytes
  • Astrocytes pass chemicals back and forth between
    neurons and blood and among various neurons in
    an area

Capillary
Astrocyte
34
Schwann cell
  • Axon isolation in periphery nervous system
  • Myelin sheaths

Axon
Schwann Cell
35
Oligodendrocytes
Axon
Oligodendrocyte
  • Axon isolation in central nervous system

Myelin sheath
36
Myelin sheaths
Axon
Myelin sheath
Axon
Node of Ranvier
37
Resting potential
  • The membrane maintains readiness for response
  • Creates slightly negative resting potential
    (-70mV)
  • Difference in voltage across membrane between
    inside and outside of neuron
  • Negative due to negatively charged proteins
  • Neuron membranes ion channels allow selective
    ions to pass
  • Selective permeability maintains electrical
    gradient
  • At rest sodium channels tightly shut and
    potassium channels open

38
Resting potential
  • Sodium potassium pump
  • Active transport system
  • Uses energy to push 3 sodium ions out and 2
    potassium ions into cell
  • Differential concentration gradient
  • Na insideoutside 110
  • K insideoutside 201
  •  
  • Electric gradient pushes potassium ions inside
    the cell, whereas the concentration gradient
    pushes them out of the cell.
  •  
  • Resting potential results as a dynamical balance
    between concentration gradient, electric gradient
    and active transport by the sodium-potassium pump

39
Resting potential
Movement of ions
Distribution of ions
Na
Na
Sodium potassium pump
Na
K leaves cell because of concentration gradient
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
K
Na
Na
K
K
K
K
Na
K
K enters cell because of electrical gradient
K
Na
K
K
K
K
Na
Na
Na
Na
Na
Na
Na
Na
K
Na
Na
Na
Na
K
Na
Na
Na
Na
40
Net Results
  • Steady state 70mV
  • Dynamic equilibrium lots of activity to achieve
    this
  • erratic stochastic random but a bias a
    propensity for one state sodium closed,
    potassium open

State
Open
Closed
41
Action Potential
  • If membrane potential is slightly perturbed it
    quickly returns to the resting potential.
  • However if the disturbance (depolarisation) is
    sufficiently large (reaches threshold) an action
    potential starts a massive and rapid
    depolarisation followed by a slight reversal of
    the polarisation.

42
Action Potential
  • Action potentials constitute a basis for the
    information encoding by single cells.
  • Generation of action potentials or spikes has
    molecular basis, it depends on voltage gated
    channels.
  • Action potential is binary all or none
  • amplitude does not depend on the level of the
    initial depolarisation which triggered the spike.
  • Synaptic responses (axon dendrite join) is not
    binary

43
Action potential
  • When the depolarisation reaches a threshold level
  • sodium channels start to open very rapidly
  • sodium current flows into the cell leading to
    quick further depolarisation.
  • At the peak of depolarisation sodium channels
    close
  • Making it impossible for sodium to flow into
    cell.
  • Meanwhile the potassium channels open more slowly
    and reach peak opening when sodium channels are
    already closing.
  • The flow of potassium is in the opposite
    direction and drives the membrane potential down
    (hyperpolarizes it).
  • As potassium channels close slower than sodium
    channels, the imbalance of the ion flows leads to
    membrane hyperpolarisation.

44
Action potential neuron action
Overshoot
Rate of sodium into neuron
0
Rising phase
Rate of exit of potassium from neuron
Falling phase
-70mV
undershoot
1 msec
1 msec
After hyperpolarisation state refractory period
45
Ion channel states
  • main factor affecting state of channels is the
    membrane potential the sodium channel will be
    affected by the membrane potential
  • potassium channels 2 states, open and closed
  • sodium channels 3 states, open, inactive and
    closed
  • (underlining above indicates no ion flow)
  • Potassium at rest is slightly open

Sodium
Potassium
Closed
No flow
Inactive
Closed
Open
Open
46
Cybernetic analogy
  • Feedback
  • Closed loop system
  • Common variable is membrane potential
  • 2 systems sodium and potassium

Opening of ion channel
Flow of ions
47
Refractory period
  • When things calm down, the Sodium/potassium pump
    (working in the background) restores the
    concentrations and therefore the membrane
    potential to resting state.
  • This is the refractory period.
  • Absolute not going to see any reaction (most of
    sodium channels in a particular state inactive)
    shortly after peak of spike
  • Relative depends on the amount of stimulus you
    receive (re sodium ion channels are closed but
    could be opened if action exceeds / undershoots)
    requires higher stimulus due to
    hyperpolarisation

48
Propagation of Action Potentials
  • Membrane of a neuron is an active medium - It may
    sustain and propagate the initial membrane
    potential
  • Chain reaction
  • In an extended segment of the axon
  • Nearby action potential related sodium entry
    depolarizes membrane above threshold
  • Chain reaction
  • Leads to generation of an action potential in a
    neighboring patch

49
Propagation of Action Potentials
  • Spikes
  • Unidirectional propagation due to refractory
    period
  • Depolarisation spreads bidirectionally
  • Due to membrane behind current spike being in the
    absolute refractory period, only the membrane in
    front can generate another action potential

50
Propagation of Action Potentials
a)
b)
51
Propagation of Action Potentials
  • Action potential is slow in unmyelineated axons
    because it depends on diffusion of sodium ions
    (up to 10 m/s)
  • In axons with myelin sheath
  • Myelin sheath acts like an insulated
  • Saltatory conduction between nodes of Ranvier
    (gaps in myelin) speed up propagation up to 120
    m/s
  • This is due to there being no sodium channels
    under myelin and so propagation can occur without
    continuous spike regeneration

52
Propagation of Action Potentials
53
Synapse
  • Point of communication between two neurons
  • Pre-synaptic and postsynaptic neurons do not
    actually touch messages must transmit across
    narrow gap
  • Gap between pre- and postsynaptic membrane is
    called a synaptic cleft
  • Synaptic transmission takes the form of chemical
    diffusion of neurotransmitters

54
Synapse transmission
  • A single stimulation at a synapse produces graded
    potential at the postsynaptic cell rather than a
    full action potential
  • Excitatory synapse
  • excitatory postsynaptic potentials (EPSP)
    produced
  • Sodium gates are opened
  • Inhibitory synapse
  • Inhibitory postsynaptic potential (IPSP) produced
  • Potassium or chloride gates are opened

55
Synapse Transmission
  • Synaptic stimulation (graded potentials) on the
    dendritic tree can be integrated in two ways
  • Temporal Integration summation of stimuli at
    different times
  • Spatial Integration summation of different
    locations

56
Synapse transmission
Synthesis of neurotransmitter and formation of
vesicles
1
Transport of neurotransmitter down axon
2
3
Action potential travels down the axon
Action potential causes calcium to enter, evoking
release of neurotransmitter
4
7
Reuptake of neurotransmitter to be recycled
5
8
Vesicles without transmitter transported back to
cell body
Neurotransmitter attaches to receptor, exciting
or inhibiting postsynaptic neuron
6
Separation of neurotransmitter molecules from
receptor
57
Neurotransmitters
  • Some neurotransmitters are produced in cell body
    and are actively transported to the presynaptic
    terminals
  • Transport is relatively slow due to neurons
    needing time to replenish larger neurotransmitters

58
Neurotransmitters
  • Synaptic transmission is triggered by an action
    potential arriving along the axon terminal
    collateral towards the bouton
  • Causes depolarisation of bouton membrane leading
    to opening of the voltage-gated calcium channels
  • Calcium flows into presynaptic terminal and
    triggers exocytosis
  • Neurotransmitters released into the synaptic
    cleft
  • Neurotransmitters diffuse across cleft and are
    attached to receptors on the postsynaptic
    membrane, triggering a response

59
Neurotransmitters
  • Each neuron releases the same (limited)
    combination of neuron transmitters from al the
    axon branches
  • This increases message complexity
  • Neurons can respond to different
    neurotransmitters at different synapses
  • There are two types of neurotransmitters
    (dependant on the results of attachment to
    postsynaptic receptors)
  • Ionotropic
  • Metabotropic

60
Ionotropic
  • Ionotropic neurotransmitters attach to receptors
    resulting in opening of certain ion channels
  • Effects are rapid and short
  • Action localised to immediate vicinity of the
    membrane patch with the receptors
  • Examples
  • glutamate opens sodium channels (excitatory)
  • GABA opens chloride channels (inhibitory)
  • Acetylcholine allows sodium ions in (excitatory)

61
Metabotropic
  • Metabotropic effects initiate a sequence of
    metabolic reactions
  • Slow and longer lasting
  • Use second messenger systems
  • E.g. cAMP
  • Carry messages to areas within a cell
  • Open/close ion channels, alter protein synthesis
    or activate a portion of the chromosome
  • A metabotropic synapse may affect activity in the
    entire postsynaptic cell (nonlocal effect)

62
Anatomy of the nervous system
63
Anatomy of nervous system
  • Brain
  • Spinal cord
  • Hindbrain
  • Midbrain
  • Forebrain
  • Peripheral nervous system
  • Somatic conveys information from sense organs
    to CNS and controls voluntary muscles
  • Autonomic nervous system organs and involuntary
    muscles
  • Sympathic expenditure of energy, flight or
    fight
  • Parasympathic conserving energy and vegative
    functioning

64
Nervous system
Brain
Corpus Callosum
Cerebral Cortext
Spinal cord
Thalamus
Hypothalamus
Pituitary gland
Pons
Cerebellum
Medulla
Peripheral nervous system Somatic
(blue) Autonomic (red)
65
Autonomic nervous system
66
Anterior plane
Sagittal plane
Coronal plane
67
Anatomical directions
68
Spinal cord
  • Part of the CNS within spinal column
  • Responsible for communication with muscles and
    sensory organs below head
  • Each segment has a pair of nerves on both sides
  • Dorsal roots enter spinal cord carrying sensory
    information. Sensory neurons bodies are located
    in the dorsal root ganglia.
  • Ventral roots exit spinal cord and carry motor
    information to the muscles
  • Grey matter consists of densely packed cell
    bodies and dendrites
  • White matter consists of myelineated axons

69
Spinal Cord
White matter
Grey matter
Sensory nerve
Central canal
Dorsal root ganglion
Dorsal
Motor Nerve
Ventral
70
Brain Stem
Midbrain
Forebrain
Olfactory bulb
Hindbrain
Optic nerve
Figure 4.7 three major divisions of the
vertebrate brain In a fish brain, as shown here,
the forebrain, midbrain, and hind brain are
clearly visible as separate bulges. In adult
mammals the forebrain grows so large that it
surrounds the entire midbrain and part of the
hindbrain
71
Brain Stem
  • Consisting of parts of
  • Hindbrain
  • Midbrain
  • Some structures of forebrain
  • Hindbrain
  • Medula
  • Extension of spinal cord involuntary reflexes
  • Pons
  • Reticular formation motor areas of spinal cord
    and controls arousal and attention by projecting
    output to the cerebral cortex
  • Raphe system projects to the forebrain and
    modulates the brains response readiness to
    stimuli.
  • Cerebellum
  • Lots of neurons, more than forebrain
  • Control of movement, balance and coordination,
    shifting attention between auditory and visual
    stimuli, timing

72
Brain stem
  • Midbrain contains pathways between the forebrain
    and the hindbrain or the spinal cord
  • Consists of
  • Tactum
  • Tegmentum
  • Supperior colliculus
  • Substantia nigra (dopaminergic system)

73
Brain stem
  • Forebrain largest part of the brain in mammals
  • It is divided into two hemispheres connected by
    corpus callosum and anteriror commisure
  • Contains
  • Cerebral cortex
  • Thalamus
  • Limbic system
  • Basal ganglia

74
Forebrain
  • Limbic system set of structures around the
    brain stem
  • Olfactory bulb
  • Hypothalamus
  • Motivated behaviours (e.g. feeding, drinking,
    temperature regulation, sexual behaviour or
    fighting)
  • Pituitary gland
  • Endocrine gland responsible for the synthesis and
    release of hormones
  • Hippocampus
  • Stores certain kinds of memories and involved in
    memory consolidation
  • Amygdala
  • Expression of fear as well as aggressive behavoir
  • Cingulate gyrus

75
Forebrain
Thalamus
Hypothalamus
76
Forebrain
  • Thalamus
  • Relay station of sensory information between
    sensorium and cerebral cortex
  • Olfactory system not handled by Thalamus
    olfactory bulbs
  • Consists of many nuclei group of neurons to
    achieve some function
  • Some receive input from one sensory system (e.g.
    vision) and project to a single cortical area
  • Others have multiple connections with several
    sensory systems, other nuclei and many cortical
    areas

77
Forebrain
78
Forebrain
  • Basal ganglia
  • Makes connections to the frontal cortex
  • Involved in several functions
  • Contains
  • Caudate nucleus
  • Putamen
  • Globus pallidus
  • Deterioration causes
  • Impaired movement
  • Depression
  • Memory and reasoning deficits
  • Attentional disorders

79
Forebrain
80
Forebrain
81
Forebrain
82
Cerebral cortex
  • External layer of grey matter
  • Contains neuron bodies
  • Neurons communicate with other cortical neurons
  • Sending axons forming white matter of the
    forebrain underneath the cortex
  • Laminar structure
  • 6 layers of neurons
  • Neurons in cortex are arranged in columns
  • Cells in the same column have similar properties
  • Most of cortical areas are involved in many
    different functions but to varying degrees of
    freedom.

83
Cerebral Cortex
84
Cerebral Cortex
85
Cerebral Cortex
86
Cerebral Cortex
  • 4 lobes
  • Occipital
  • Located at back main target for thalamic
    projects with visual information
  • Contains primary visual cortex perception and
    imagery
  • Parietal
  • Contains somatosensory cortex somatosensory
    representation of the body (damage leads to
    neglect cannot control or sense a particular
    body part)
  • Temporal
  • Auditory information involved in hearing and
    spoken language understanding as well as vision,
    emotional and motivational behaviours
  • Frontal
  • Primary motor cortex fine movement control
  • Prefrontal cortex working memory, memories of
    current stimulus, movement planning, regulation
    of emotional expression

87
Cerebral Cortex
88
Binding problem
  • The brain is neither
  • a collection of completely independent subsystems
    each with a very specialised function
  • or a homogenous system with all its parts
    contributing equally to the functionality of the
    whole
  • If there is some degree of specialisation and
    different sensory information is processed in
    different brain regions then how is it put back
    together into a coherent unitary experience of
    things?

89
Binding problem
  • Fundamental problem with deep philosophical
    implications
  • There is no homunculus which would collect and
    interpret all the information processed by
    different regions
  • There is no single area in the brain where it
    all comes together

90
Binding Problem
  • Proposed solutions
  • Synchrony of activity in different brain areas
    (gamma waves) may be responsible for binding of
    information
  • Information is encoded (mean rate of neuron
    firing) in the temporal structure of massages and
    this richer information may allow for binding.

91
Plasticity of the Brain
  • Development and change

92
Development of the vertebrate Brain
  • In vertebrate embryos the CNS starts as a fluid
    filled tube
  • Fluid is cerebrospinal fluid (CSF)
  • Proliferation of neurons
  • Cells lining the ventricles divide
  • Cells that become neurons glia migrate to their
    target regions in the brain
  • Initially they are just like other stem cells,
    but they begin to differentiate and develop first
    axon and later dendrites
  • Location of neurons affects their shape and
    properties
  • The glia cells start to wrap around axons of some
    neurons create myelin sheath around them
    (myelination)
  • Myelin forms in humans in the spinal cord, then
    the hindbrain, midbrain, and in the forebrain
  • Maturation of the brain is accompanied by both
    maturation of neurons as well as by the
    development of brain areas.

93
Development of the vertebrate Brain
94
Development of the vertebrate Brain
95
Development of the vertebrate Brain
  • Initially the number of neurons is much greater
    than in later stages.
  • Neurons make contacts with neurons sending nerve
    growth factor (NGF) and other neurotophins
  • Only neurons receiving neurotrophins survive
  • If insufficient neurotrophins are received, its
    axon degenerates and the cell dies (apoptosis
    programmed cell death)

96
Development of the vertebrate Brain
97
Axon Pathfinding
  • During connectivity development, axons navigate
    to specific targets
  • Pathfinding is very precise and ensures the
    nervous system can send signals to the right
    targets
  • Chemical navigation
  • Axons follow different chemicals over different
    parts of their pathway
  • Become sensitive and insensitive to certain
    chemicals as they pass through the regions
  • In the target area the amount of neurotrophins
    determines the extent of axonal branching
  • Attach to target area by arraying over chemical
    gradients

98
Axon Pathfinding
99
Axon Pathfinding
  • When the target is reached, the axon form many
    synapses on the target neuron
  • With time some of the synapses are removed
  • General principle axonal competition
  • Neural Darwinism natural selection where more
    successful synapses are retained at the expense
    of less successful ones

100
Experience and dendritic branching
  • Development of connectivity in the brain is
    dependent on the experience of the individual
  • Enriched environments enhance axonal sprouting
    and dendrite branching
  • In humans, the extent of education correlates
    positively with the increased dendritic branching

101
Generation of new neurons
  • Traditionally believed that neurons do not
    regenerate in the adult brain
  • Recent evidence suggest that this is not entirely
    correct
  • Immature cells in the nose divide and replace
    olfactory receptors
  • Stem cells (undifferentiated cells) lining the
    interior of ventricles generate cells migrating
    to the olfactory bulb replacing the neurons and
    glia
  • Also new cells develop in the hippocampus and
    some parts of the cortex, although their
    functional role is unknown

102
Role of action potential in development
103
Brain development
  • Although there are many factors affecting the
    brain development they seem to affect equally
    different brain areas.
  • In mammals the size of the brain correlates very
    well with the size of its major components

104
Impediment of Brain Development
  • Genetic abnormalities
  • Malnutrition
  • Chemical environment
  • Foetal alcohol syndrome
  • Exposure to alcohol during prenatal development
  • May result in
  • Decreased alertness
  • Hyperactivity
  • Mental retardation
  • Motor problems
  • Hearing defects
  • Facial abnormalities
  • Neurons in affected individuals have shorter
    dendrites with few branches. In adulthood, they
    are more susceptible to alcoholism, drug
    addiction, depression

105
Brain damage
  • Causes
  • Strong or repeated blows to the head (e.g.
    boxing)
  • Strokes or cerebrovascular events (occurs mostly
    in older brains and kills neurons by
    overstimulation)

106
Strokes
  • Neurons die in two stages
  • In the immediate vicinity of the stroke, the
    neurons die quickly
  • In the penumbra (region surrounding immediate
    damage) may did in next few days/weeks after the
    stroke
  • Two stroke types
  • Ischemia results from an obstruction of an
    artery by a blood clot. Afterwards the penumbra
    dies by lack of oxygen and glucose
  • Haemorrhage rupture of an artery. Penumbra is
    flooded by an excess of oxygen, calcium etc

107
Strokes
  • Both stroke types share a number of similar
    mechanisms leading to extensive damage.
  • The waste products from dead and dying cells
    flood penumbra. Due to breaking of bloodbrain
    barrier forms oedema (accumulation of fluid).
  • Lower levels of energy lead to slowing down of
    sodium-potassium pump and consequently to
    accumulation of potassium outside neurons.
  • This causes glia to dump their stores of
    neurotransmitters including glutamate
    (excitatory) causing over-excitation of neurons.
  • This leads to accumulation of positive ions (Na,
    Ca, Zn) in neurons, which is likely to
    trigger their death. In the final stages glia
    cells remove waste and dead neurons.

108
Stroke
109
Recovery
  • Recovery depends on
  • learned changes in behaviour using remaining
    skills
  • Increase of activity in neurons remote from the
    site of injury, which became less active due to
    decreased input (diaschisis) caused by the damage
  • Diaschisis treaded with
  • Stimulant drugs e.g. amphetamine, combined with
    the physical therapy
  • Physical therapy

110
Recovery
  • Recovery Mechanisms in the nervous system
  • Axonal regrowth in mammals axon do not
    regenerate far due to growth-inhibiting chemicals
    produced by central myelin
  • Sprouting axons take over vacant synapses after
    death of a neighbouring axon. Neurons may become
    responsive to other axons if axons innervating it
    become inactive or die.

111
Recovery
112
Recovery
  • Supersensitivity
  • Denervation supersenitivity heightened
    sensitivity of postsynaptic neuron to a
    neurotransmitter after destruction of an incoming
    axon
  • Disuse supersensitivity increased sensitivity
    in response to inaction by an incomming action
  • Supersensitivity results from increased number of
    receptors on the postsynaptic neuron and from
    their increased effectiveness

113
Sensory changes
  • The brain is able to reorganise its sensory
    representations in response to permanent changes
    in incoming information
  • Example limb amputation
  • Sensory neurons from limb no longer contact brain
  • Areas reorganise
  • Areas representing parts/limbs are not static but
    are in dynamic equilibrium due to cell
    competition and constant influx of information.
  • Amputation terminates influx of sensory
    information leading to reorganisation of
    somatosensory cortical areas, where neighbouring
    areas invade the area correspond to the amputated
    limb.
  • Phantom limb areas responsible for limb are
    reassigned

114
Sensory Changes
115
Sensory changes
116
Vision
117
From senses to sensations
  • Senses provide us with information about the
    environment
  • Exchange of information involves the exchange of
    energy between the environment and the organism
  • Three stages for sensation to occur
  • Reception absorption of energy by receptors
  • Transduction conversion of physical energy into
    electrophysical pattern in neurons
  • Coding creation of correspondence between the
    stimulus and some brain activity (e.g. mean rate
    coding, spike interval coding)
  • One of the fundamental and as yet unresolved
    problems in neuroscience is the labelled lines
    problem or colours of waves
  • We do not now how the action potentials
    propagated along particular nerves somehow code
    the same kind of information to the brain

118
The Eye
  • Reception and transduction of visual information
  • Oval shape
  • Light progression
  • Enters through the pupil
  • Concentrated by the lenses
  • Travels through the vitreous humour
  • Hits the Retina

119
Retina
  • Layer of photoreceptors
  • Macula
  • Area of heightened acuity
  • 3-5mm area
  • Fovea
  • central part of macula and has highest density of
    photoreceptors
  • Receptors have an almost one to one connection
    pattern with postsynaptic neurons (bipolar cells)
  • Periphery
  • Each receptor has higher coverage
  • Sums the inputs lower acuity but higher
    sensitivity to week stimulus (e.g. faint light)

120
Retina
  • inside out organisation
  • Light has to pas through layers of axons, cells
    and blood vessels in order to reach the
    photoreceptors
  • The receptors make connections onto the bipolar
    cells, which are connected to the ganglion
  • Axons of the ganglion cells bundle together and
    form an optic nerve
  • The blind spot does not contain any receptors and
    is completely insensitive to light. It is the
    place where the optic nerve leaves

121
The Eye
122
The Eye
123
Receptors
  • There are two types of receptors
  • Rods
  • 120 million
  • Increases towards periphery
  • Very sensitive responds to faint light (even to
    single photons)
  • Cones
  • 6 million
  • Concentrated mainly in the fovea and macula
  • Different types of cones each broadly tuned to
    different wave lengths of light
  • Involved in colour vision

124
Receptors
  • Photopigments
  • Chemicals contain in photoreceptors
  • Absorb photons and release energy
  • Stimulation of photopigments leads to activation
    of 2nd massagers responsible for closing of
    sodium channels resulting in membrane
    hyperpolarisation
  • Inhibition of photoreceptors causes inhibition of
    synapses onto bipolar cells
  • This results in excitation of bipolar cells
  • Light stimulation of receptors leads to
    activation of neurons and sending electrical
    signals down the optic nerve

125
Receptors
126
Colour Vision
  • Complex patterns of responses by many neurons and
    comparison of responses across different cone
    types
  • Although most vertebrates have some cones in
    their retina, only those with different cone
    types can discriminate colours (e.g. rats cannot
    they only have one cone type)

127
Theories of Colour Vision
  • If we assume that neurons encode information in
    their firing frequency then only one property can
    be encoded at once (e.g. luminance or colour).
    Patterns or activity across different neurons
    must be responsible.
  • There are three main theories
  • Trichomatic theory (Young and Helmholtz)
  • Opponent process theory
  • Retinex theory (Land)

128
Theories of Colour Vision
  • Trichomatic theory (Young and Helmholtz)
  • A given wavelength of light stimulates a
    distinctive ratio of responses of 3 types of
    cones
  • Although stronger light results in stronger
    responses, the ratio remains largely the same
  • Can explain encoding of both brightness and
    colour at the same time
  • Predicted existence of difference cone types
  • It does have problems explaining negative colour
    afterimage

129
Theories of Colour Vision
  • Opponent process theory
  • Colours are perceived on continuous scales,
    red-green, yellow-blue, white-black
  • Emphasised involvement of other cells than
    receptors
  • Neurons other than receptors increase activity in
    response to one colour and decrease activity to
    indicate the opposite colour
  • Suggests specific neural connectivity between
    cells in retina
  • It cannot account for colour constancy the
    ability to recognise correctly colours in varying
    lighting conditions it requires comparison of
    colours across different areas.

130
Theories of Colour Vision
131
Theories of Colour Vision
  • Retinex theory (Land)
  • Can explain colour constancy
  • Cortex compares the responses of different parts
    of the retina to determine the brightness and
    colour of each area
  • Responsibility for colour perception is in the
    cortex

132
Mammalian visual system
  • Receptors make contacts onto the bipolar cells
  • Bipolar cells make contact with amacrine and
    ganglion cells
  • Ganglion cells axons form an optic nerve leaving
    eyes
  • Optic nerves from both eyes meet at the optic
    chiasm, where some of the axons from each eye
    cross to the other side of the brain (50 in
    humans)

133
Mammalian visual system
  • Majority of axons then enter lateral geniculate
    nucleus
  • Part of thalamus involved in processing of visual
    information
  • LGN makes projections to the visual cortex in the
    occipital lobe
  • Small number of the axons goes to another
    structure called suprachiasmatic colliculus and
    few to the part of hypothalamus responsible for
    the wake-sleep cycle.

134
Mammalian visual system
135
Mammalian visual system
136
Mechanisms of processing
  • Receptive field
  • Part of the visual field to which a neuron
    responds
  • The receptive field of a receptor is the area in
    space from which light arrives
  • The receptive field of a ganglion cell will be a
    combination of the receptive fields of receptors
    it is connected to.
  • The receptive field becomes more complex in
    consecutive stages of the visual system

137
Mechanisms of processing
  • Lateral inhibition
  • Contrast enhancement
  • Stimulation in one area of the retina inhibits
    the responses in the neighbouring areas
  • Visual pathways
  • Organisation of neurons
  • Starts at the level of retinal ganglion

138
Mechanisms of processing
  • Parvocelluar
  • Neurons concentrated around fovea
  • They are smaller and have smaller receptive
    fields
  • Involved in processing colour and fine detail
  • Magnocellular
  • Neurons evenly distributed
  • Respond to depth, movement and large patterns
  • Two neuron types make connection with the
    correspond types of neurons in the LGN. Thus both
    pathways remain distinct

139
Cerebral Cortex
  • LGN makes projects to the primary visual cortex
    (V1)
  • the first stage of processing
  • V1 sends projects to V2
  • Further stages of the visual processing
  • Makes feedback connections to V1

140
Cerebral cortex
  • The two pathways originating in the retina form
    three pathways in the cortex
  • The mostly magnocellular pathway branches into
    the ventral part involved in motion perception
    and the dorsal part involved in integration of
    vision and action
  • The mixed magnocellular and parvocellular pathway
    is responsive to colour and brightness
  • The parvocellular pathway is involved in fine
    shape analysis

141
Cerebral cortex
  • Ventral stream
  • Mixed m/p path mostly p path
  • Visual paths in the temporal cortex
  • what pathway
  • Object identification
  • Dorsal stream
  • Magnocelluar path entering parietal cortex
  • where pathway
  • Shape perception

142
Shape analysis pathway
  • Cells in V1 and V2 are classified according to
    their receptive field type
  • Simple cells
  • Mostly in the primary visual cortex
  • Fixed excitatory and inhibitory parts in their
    receptive fields
  • Complex cells
  • Found in V1 and V2
  • No fixed excitatory/inhibitory zones
  • Respond to preferred stimulus orientation
    regardless of the position within the receptive
    field
  • Cells in the visual cortex are grouped in columns
    in which cells with similar properties cluster
    together

143
Shape analysis pathway
  • V1 and V2 feature detectors or frequency filters?
  • Hubel and Wiesel classification of cells suggest
    that cells might be simply feature detectors
  • There is however evidence that they may be
    responsive to gratings rather than to bars
  • This would mean that cells are frequency filters
    tuned to different spatial frequencies and the
    visual system performs some form of Fourier
    analysis
  • This is still open to debate
  • Cells in higher order systems (e.g. inferior
    temporal cortex) have big receptor fields
  • Provides no additional positional information but
    seem able to respond to specific shapes (shape
    detectors)

144
Disorders of shape recognition
  • Certain damages to the brain results in
    impairments of visual perception
  • Visual agnosia
  • Inability to recognise objects
  • Prosopagnosia
  • Inability to recognise faces but able to
    recognise other objects

145
Colour perception pathway
  • Colour perception pathway involves the
    parvocellular path
  • Cells in V1 (sensitive to the colour) form blobs
  • Neurons from V1 blobs innervate V2, V4, and
    posterior inferior temporal cortex
  • Particularly V4 was implicated in colour
    constancy perception

146
Motion and depth pathways
  • Areas involved in motion perception
  • Magnocellular pathway
  • MT (V5)
  • MST (cells are sensitive to the motion velocity)
  • Patients with certain brain damage suffer from
    motion blindness
  • Intact object recognition
  • Cannot determine object movement or its speed and
    direction of movement

147
Hearing, Somatosensation, Chemical Senses
148
Hearing
  • Sound
  • Amplitude intensity perception loudness
  • Frequency perception pitch
  • Structure of the Ear
  • Pitch perception
  • Localisation of sounds
  • Vestibular system

149
Structure of the Ear
  • Outer ear
  • Pinna
  • External auditory canal
  • Middle ear
  • Eardrum
  • Hammer, anvil and stirrup
  • Inner ear
  • Oval window
  • Cochlea 3 fluid filled tunnels, hair cells lie
    between the basilar membrane and tectorial
    membrane

150
Structure of the Ear
  • When sound waves strike the tympanic membrane
  • They cause it vibrate three tiny bones the
    hammer, anvil and stirrup that convert the
    sound waves into stronger vibrations in the
    fluid-filled cochlea
  • Thos vibrates displace the hair cells along the
    basilar membrane in the cochlea
  • A cross section through the cochlear. The array
    of hair cells in the cochlea is known as the
    organ of Corti
  • Close-up of the hair cells

151
Auditory Cortex
  • Cells tuned to different tone frequencies
  • Cells responding to a given tone cluster together
    (tonoptic maps)
  • Ventral path (type of sound)
  • Dorsal path (localisation of sound)

152
Auditory Cortex
153
Pitch Perception
  • Frequency theory
  • Basilar membrane vibrates in sync with a sound
    causing the auditory nerve axons to produce
    action potentials with the same frequency
  • Place theory
  • Each area of basila membrane is tuned to a
    specific frequency and vibrates if that frequency
    is present

154
Pitch perception
  • Currant theory combination of the two
  • Low frequency sounds frequency of action
    potentials in the auditory system
  • Intermediate frequencies volleys of responses
    across many receptors can lead to the encoding of
    sounds of frequency of up to ca. 5000Hz in the
    who auditory nerve even though no individual axon
    can fire with that frequency
  • High frequency sounds area of greatest response
    along the basilar membrane due to location of
    peaks of the travelling wave in the basilar
    membrane

155
Pitch perception
156
Localisation of sounds
  • High frequency
  • Loudness difference between ears
  • Low frequency
  • Differences in phase

157
Vestibular System
  • Vestibular organ
  • adjacent to cochlea
  • Detects position and acceleration of the head
  • Consists of 2 otholit organs and three
    semicircular canals (in three planes) filled with
    a jellylike substance and lined with hair cells
  • Head tilts are detected by the otholit organs
    (otholits calcium carbonate particles push
    against the hair cells)
  • Acceleration is detected by the canals

158
Vestibular System
159
Somatosensation
  • Many senses in one
  • Depends on a number of receptors sensitive to
    different kinds of skin and internal tissue
    stimulation
  • Each receptor contributing in some degree to many
    somatosensory experiences

160
Somatosensation
161
Somatosensation
162
Pain
  • Function
  • Trasmitted by axons releasing glutamate for
    moderately painful stimuli and a combination of
    glutamate and substance P for strong pain
  • Many kinds of pain are dependent on unmyelinated
    or thinly myelinated axons carrying information
    to the spinal cord and releasing there a
    cotrasmitter substance P
  • A chemical capsaicin (jalapenos) produces a
    transient pain by enhancing release of substance
    P and stimulation of the moderate heat receptors.
    This pain is followed by the relative
    insensitivity due to need for restoring the
    neuronal supply of substance P and weakening of
    heat receptors.

163
Pain relief
  • A harmful stimulus may evoke varying feeling of
    pain depending on the other current and recent
    stimulus
  • Experience of pain is related to the activity of
    cingulate cortex (emotional response) and
    somatosensory cortex
  • Gate theory some areas of the spinal cord
    receive stimulus from pain receptors and from
    other receptors in the skin as well as axons
    descending from the brain these stimuli can
    close the gates and block the trasmission of
    pain.
  • Opioid Mechanisms opioids bind to the receptors
    concentrated in the same brain areas where
    substance P is concentrated they reduce pain
    because they attach to the endorphin receptors

164
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165
Pain relief
  • Analgesia some painful stimuli activates
    neurons releasing endorphins in the
    periaqueductal area
  • Axons from exited cells in medulla and the
    periqueductal area send messages to the spinal
    cord and block release of substance P
  • Endorphins
  • Act on small diameter pain fibres they relieve
    slow dull pain ineffective for sharp strong pain
  • Release of the endorphins (decrease of pain
    sensitivity) can be triggered by both pleasant
    and unpleasant experiences

166
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167
Pain relief
  • Both morphine and pain transiently impair the
    immune system
  • Pain impairs it more so the net effect of
    relieving pain with morphine is enhancement of
    the immune system
  • Tissue damage activates the immune system, which
    released chemicals (e.g. histamine) repairing the
    damage but also makes the pain receptors over
    responsive to further pain or even a mild stimulus

168
Taste
  • Sensory information can be coded using labelled
    lines or across fibre coding
  • Taste receptors
  • modified skin cells (life cycle of 10-14 days)
  • In taste buds located in papillae on the tongue
  • 5 kinds of taste receptors
  • Salty receptors detect sodium ions crossing the
    membrane
  • Sour receptors respond to the stimulus by
    blocking potassium channels
  • Sweet, bitter and umani receptors response
    similar to the metabotropic receptors,via a
    second messengers system

169
Taste
170
Taste
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