Download The vertebrate nervous system is regionally specialized

Nervous system
1) Nervous systems consist of circuits of neurons and supporting
cells
2) Ion pumps and ion channels maintain the resting potential of a
neuron
3) Action potentials are the signals conducted by axons
4) Neurons communicate with other cells at synapses
5) The vertebrate nervous system is regionally specialized
6) The cerebral cortex controls voluntary movement and cognitive functions
Nervous systems consist of circuits of neurons and supporting cells
Human brain - 100 billion neurons
Complex information-processing circuits, networks
Functional magnetic resonance imaging (fMRI) – view inside
A functional magnetic resonance
Record of increased blood flow
Circuits for different tasks
Cambrian explosion (500 mil.)
– neurons in their present form
Nervous systems consist of circuits of neurons and supporting cells
All animals except the sponges
No differences in units – neurons
Differences – in organization of circuits/networks – nets, nerves, brains
Nervous systems consist of circuits of neurons and supporting cells
Information processing
three stages: sensory input, integration, and motor output
Sensory neurons – external
and internal stimuli
Interneurons – analysis
(context, past), integration
Motor neurons => effector
organs, tissues
Nervous systems consist of circuits of neurons and supporting cells
Simple nerve circuit => reflex
Automatic responses to stimuli
Nervous systems consist of circuits of neurons and supporting cells
Neuron structure
Structure: cell body, dendrites, axon, axon hillock, myelin sheath, synaptic
terminal, synapse, neurotransmitter
Nervous systems consist of circuits of neurons and supporting cells
Neuron structure
Up to 100,000 synapses on one neuron
Nervous systems consist of circuits of neurons and supporting cells
Supporting cells
Glia – 10-50 times more than neurons
Types: astrocytes, radial glia, oligodendrocytes, Schwann cells
Structural support, regulate extracellular concentrations of ions, transmitters
Facilitating information transfer – maybe part of learning and memory
Increase blood flow
Astrocytes =>
tight junctions
between cells
around capillaries
=> blood brain
barrier
Nervous systems consist of circuits of neurons and supporting cells
In embryo, radial glia => tracks for neurons migration
Radial glia, astrocytes - stem cells
Oligodendrocytes (in CNS), Schwann cells (PNS) => electrical insulation
Nervous systems consist of circuits of neurons and supporting cells summary
Organization of nervous systems
Invertebrate nervous systems range in complexity from simple nerve nets to highly centralized
nervous systems having complicated brains and ventral nerve cords. In vertebrates, the
central nervous system (CNS) consists of the brain and the spinal cord, which is located
dorsally.
Information processing
Nervous systems process information in three stages: sensory input, integration, and motor
output to effector cells. The CNS integrates information, while the nerves of the peripheral
nervous system (PNS) transmit sensory and motor signals between the CNS and the rest of
the body. The three stages are illustrated in the knee-jerk reflex.
Neuron structure
Most neurons have highly branched dendrites that receive signals from other neurons. They
also typically have a single axon that transmits signals to other cells at synapses. Neurons
have a wide variety of shapes that reflect their input and output interactions.
Supporting cells
Glia perform a number of functions, including providing structural support for neurons,
regulating the extracellular concentrations of certain substances, guiding the migration of
developing neurons, and forming myelin, which electrically insulates axons.
Ion pumps and ion channels maintain the resting potential of a neuron
In all cells - across plasma membrane = membrane potential
In neurons = between – 60 and – 80 mV
Ion pumps and ion channels maintain the resting potential of a neuron
The resting potential
Na+ gradient = 150/15 = 10
K+
= 5/150 = 1/30 maintained by sodium-potassium pump
Ion pumps and ion channels maintain the resting potential of a neuron
The resting potential
Two model membranes - ion channels selective for K+ only or for Na+ only
Start =
150 : 5 mM KCl
15 : 150 mM NaCl
Equilibrium potential (Eion) - given by Nernst equation
For K+ = - 92 mV (at 37°C)
For Na+ = + 62 mV
real neuron - many channels open for K
few for Na → closer to K value than to Na
Ion pumps and ion channels maintain the resting potential of a neuron
Gated ion channels
Open channels – ungated
Gated channels – open or close in response to stimuli
1) Stretch-gated – membrane mechanically deformed
2) Ligand-gated – neurotransmitter binds to channel (synapses)
3) Voltage-gated – by potential changes (axons)
Ion pumps and ion channels maintain the resting potential
of a neuron - summary
Every cell has a voltage across its plasma membrane called a membrane
potential. The inside of the cell is negative relative to the outside.
The resting potential
The membrane potential depends on ionic gradients across the plasma
membrane: The concentration of Na+ is higher in the extracellular fluid than
in the cytosol, while the reverse is true for K+. 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 the separation of charges across the membrane,
producing the resting potential.
Gated Ion channels
Gated ion channels open or close in response to membrane stretch, the
binding of a specific ligand, or a change in the membrane potential.
Action potentials are the signals conducted by axons
Stimuli influence the gated channels
Hyperpolarization – opening of gated K+ channels → near to EK = – 92 mV
Depolarization – opening of gated Na+ channels → near to ENa = 62 mV
Graded
potential –
correlation to
the stimulus
Graded up to
threshold →
Action
potential =
all-or-none
phenomenon
(1-2 msec)
Action potentials are the signals conducted by axons
Action potential – voltage-gated channels opened - Na+ channels before K+
Na+ channels before K+
1) Resting state –
activation gates closed
2) Depolarization –
stimulus opens Na+
channels
3) Rising phase – most
Na+ channels open
4) Falling phase – Na+
channels closed, K+
opened
5) Undershoot – K+
channel opened, then
closed => resting state
4,5 phase – second
depolarization unable –
refractory period
Action potentials are the signals conducted by axons
Conduction of action potentials
Regenerating itself along axon
Depolarization => repolarization => next segment depolarization => one
direction movement
Action potentials are the signals conducted by axons
Several factors affect speed of AP conduction
The larger the axon’s diameter, the faster the conduction – solution in
invertebrates (squids, some arthropods) - from several cm to 100 m/sec
(giant axons)
In vertebrates – insulation by myelin sheath => AP jumps from node to node
= saltatory conduction (up to 120 m/sec)
Conduction – myelinated, 20 µm in diameter = 1 mm giant axon
Action potentials are the signals conducted by axons - summary
An increase in the magnitude of the membrane potential is called a hyperpolarization;
a decrease in magnitude is called a depolarization. Changes in membrane potential
that vary with the strength of a stimulus are known as graded potentials
Production of action potentials
An action potential is a brief, all-or-none depolarization of a neuron’s plasma
membrane. When a graded depolarization brings the membrane potential to the
threshold, many voltage-gated Na+ channels open, triggering an influx of Na+ that
rapidly brings the membrane potential to a positive value. The membrane potential is
restored to its normal resting value by the inactivation of Na+ channels and by the
opening of many voltage-gated K+ channels, which increases K+ efflux. A refractory
period follows the action potential, corresponding to the interval when the Na+
channels are inactivated.
Conduction of action potentials
An action potential travels from the axon hillock to the synaptic terminals by
regenerating itself along the axon. The speed of conduction of an action potential
increases with the diameter of the axon and, in many vertebrate axons, with
myelination. Action potentials in myelinated axons jump between the nodes of
Ranvier, a process called saltatory conduction.
Neurons communicate with other cells at synapses
In most cases – AP is not transmitted, only in electrical synapses - gap junctions
In rapid stereotypical behaviors (in lobsters escape reaction)
Vast majority - chemical synapses – neurotransmitter released
Synaptic vesicles in synaptic terminals
Neurons communicate with other cells at synapses
AP at synaptic terminal depolarizes membrane => opens voltage-gated calcium
channels => Ca2+ inside causes fuse of vesicles and membrane => exocytosis into
synaptic cleft - adaptable connection
Direct synaptic transmission (indirect - over second messenger – slower, long-lasting)
Neurotransmitter to
ligand-gated ion
channels =>
postsynaptic potential
(PSP)
Na+ and K+ channels
=> depol.
Excitatory PSP (EPSP)
K+ channels => hyperp.
Inhibitory PSP (IPSP)
Acetylcholine degraded
by cholinesterase
Neurons communicate with other cells at synapses
Summation of postsynaptic potentials
PSP - graded
Single EPSP usually too small => temporal and spatial summation =>
AP at hillock
Neurons communicate with other cells at synapses
One substance - more receptors => different effects
Neurons communicate with other cells at synapses – summary
In an electrical synapse, electrical current flows directly from one cell to another via a gap
junction. In a chemical synapse, depolarization of the synaptic terminal causes synaptic vesicles
to fuse with the terminal membrane and to release neurotransmitter into the synaptic cleft.
Direct synaptic transmission
The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane,
producing an excitatory or inhibitory postsynaptic potential (EPSP or IPSP). After release, the
neurotransmitter diffuses out of the synaptic cleft, is taken up by surrounding cells, or is
degraded by enzymes. A single neuron has many synapses on its dendrites and cell body.
Whether it generates an action potential depends on the temporal and spatial summation of
EPSPs and IPSPs at the axon hillock.
Indirect synaptic transmission
The binding of neurotransmitter to some receptors activates signal transduction pathways, which
produce slowly developing but long-lasting effects in the postsynaptic cell.
Neurotransmitters
The same neurotransmitter can produce different effects on different types of cells. Major known
neurotransmitters include acetylcholine, biogenic amines (epinephrine, norepinephrine,
dopamine, and serotonin), various amino acids and peptides, and the gases nitric oxide and
carbon monoxide.
The vertebrate nervous system is regionally specialized
Vertebrates – cephalization
Distinct CNS a PNS components
Spinal cord
Simple responses – reflexes
Conveys information from
and to brain
Segmental ganglia outside
The vertebrate nervous system is regionally specialized
CNS – derived from dorsal embryonic nerve cord – hollow => in adult –
Central canal, four ventricles of brain, between two meninges – cerebrospinal fluid
Supply of nutrients and hormones, removal of wastes
White matter – myelinated axons
Gray matter – dendrites,
unmyelinated axons, cell bodies
The vertebrate nervous system is regionally specialized
Peripheral nervous system
Mammals – 12 pairs of cranial nerves, 31 pairs of spinal nerves
PNS – two functional components
1) Somatic nervous
system – signals to and
from skeletal muscles,
external stimuli
2)
Autonomic nervous
system – regulates
internal environment –
controlling smooth and
cardiac muscles,
internal organs, tissues
The vertebrate nervous system is regionally specialized
Autonomic nervous system:
Sympathetic, parasympathetic, enteric division
1) Sympathetic
Arousal + energy
generation
“fight-or-flight”
response
2) Parasympathetic
Self-maintenance
function
“rest and digest”
3) Enteric – semiindepend. to 1 and 2
network of neurons
secretion, peristalsis
The vertebrate nervous system is regionally specialized
Embryonic development of the brain
In all vertebrates - three anterior bulges of the neural tube become evident
The vertebrate nervous system is regionally specialized
The brainstem
Evolutionarily – old part of the brain
Medulla oblongata – centers control visceral (automatic, homeostatic)
functions – breathing, heart and blood vessel activity, swallowing, vomiting,
digestion
Pons – regulates breathing centers in medulla
Transmission of information - sensory axons to and motor from higher brain
Coordination of large-scale body movements
– walking, changing of sides of axons
Midbrain – centers of auditory and visual
systems
In mammals – visual reflexes
(e.g. turning head automatically)
The vertebrate nervous system is regionally specialized
The brainstem – arousal and sleep
Arousal – awareness of the external word; Sleep – receiving external stimuli
without awareness
Reticular formation (RF) – network of neurons in brainstem
Part of RF – reticular activating system (RAS) – regulates arousal x sleep,
Stimulation of centers in pons and
medulla => sleep neurotransmitter
- serotonin (consolidation of learning
and memory?)
Midbrain – stimulation
=> arousal
Sensory filter – always same
stimuli can be ignored
The vertebrate nervous system is regionally specialized
The cerebellum – from part of metencephalon
Integrates sensory information - from organs of equilibrium, visual
system, motor commands, length of muscles
Coordinates movement and balance
Involved in learning and
remembering motor skills
The vertebrate nervous system is regionally specialized
The diencephalon – epithalamus, thalamus, hypothalamus
Epithalamus – pineal glad (melatonin) and choroid plexus (capillaries producing
cerebrospinal fluid from blood)
Thalamus – input center for sensory information, sorting, sending to cerebrum
output center for motor information from cerebrum
Hypothalamus – homeostatic regulation
neurosecretory cells,
body thermostat, centers regulating
hunger, thirst,
sexual, mating behavior,
centers for pleasure, rage
The vertebrate nervous system is regionally specialized
Circadian rhythms
– sleep/wake, hormone release, sensitivity, hunger
Biological clock - in mammals in suprachiasmatic nucleus (SCN), in Drosophila on
wings
In human – 24 hours 11 minutes
Synchronization with natural light/dark
cycles
The vertebrate nervous system is regionally specialized
The cerebrum – in vertebrate evolution a region for, above all, olfactory
and also auditory and visual information processing
Basal nuclei – centers for planning and learning movement sequences
Cortex – sensory, motor, association centers
In mammals – isocortex (neocortex)
evolved in ancestors (mammal like
reptiles)
In human – about 5 mm thick, 0.5 m2
Convolutions – allow by 6 parallel
layers of neurons a large surface
Large cortex in primates, cetaceans
Hemispheres – connected by corpus
callosum
The vertebrate nervous system is regionally specialized –
summary
The peripheral nervous system
The PNS consists of paired cranial and spinal nerves and associated
ganglia. Functionally, the PNS is divided into the somatic nervous system,
which carries signals to skeletal muscles, and the autonomic nervous
system, which regulates the primarily automatic, visceral functions of
smooth and cardiac muscles. The autonomic nervous system has three
divisions: the sympathetic and parasympathetic divisions, which usually
have antagonistic effects on target organs, and the enteric division, which
controls the activity of the digestive tract, pancreas, and gallbladder.
Embryonic development of the brain
The vertebrate brain develops from three embryonic regions: the forebrain,
the midbrain, and the hindbrain. In humans, the most expansive growth
occurs in the part of the forebrain that gives rise to the cerebrum.
The vertebrate nervous system is regionally specialized – summary
The brainstem
The medulla oblongata, pons, and midbrain make up the brainstem, which controls
homeostatic functions such as breathing rate, conducts sensory and motor signals
between the spinal cord and higher brain centers, and regulates arousal and sleep.
The cerebellum
The cerebellum helps coordinate motor, perceptual, and cognitive functions. It also is
involved in learning and remembering motor skills.
The diencephalons
The thalamus is the main center through which sensory and motor information passes
to and from the cerebrum. The hypothalamus regulates homeostasis; basic survival
behaviors such as feeding, fighting, fleeing, and reproduction; and circadian rhythms.
The cerebrum
The cerebrum has two hemispheres, each of which consists of cerebral cortex
overlying white matter and basal nuclei, which are important in planning and learning
movements. In mammals, the cerebral cortex has a convoluted surface called the
neocortex. A thick band of axons, the corpus callosum, provides communication
between the right and left cerebral cortices.
The cerebral cortex controls voluntary movement and cognitive functions
4 lobes
Association centers dominate in human
The cerebral cortex controls voluntary movement and cognitive functions
Information processing in the cerebral cortex
Via thalamus to primary sensory areas – processed parameters of objects
In association areas – integrated, processed information from sensory areas
(complex images, faces)
Based on information from
association areas
primary motor cortex generate
commands
Action potentials travel along
axons through brainstem, spinal
cord, to motor neurons – excite
skeletal muscle
Neuron number, distribution
according to the body part and
skills needed
The cerebral cortex controls voluntary movement and cognitive functions
Lateralization of cortical function
After birth – competing functions segregate and displace each other in the
cortex = lateralization of functions
Left hemisphere – language, math, logical operations, serial processing,
bias for detail
Right – pattern and face recognition, spatial relations, emotional processing,
simultaneous processing of many kinds of information, bias for context
Language and speech
Left frontal lobe – Broca’s area – speaking
Posterior portion, temporal lobe –
Wernicke’s area – hearing
The cerebral cortex controls voluntary movement and cognitive functions
Emotions – results of interplay of many brain regions – the limbic system (ring
around brainstem)
Cerebral cortex – amygdala, hippocampus, olfactory bulb
Thalamus
Hypothalamus
Emotions manifested as laughing, crying – interaction with sensory areas
Feelings to functions controlled by brainstem – aggression, feeding, and sexuality
Amygdala – recognition of emotional
content of facial expressions
emotional memories
The cerebral cortex controls voluntary movement and cognitive functions
Memory and learning
Short-term memory
Activation of hippocampus =>
long-term memory
Influence of positive or negative
emotions (mediated by amygdala)
Mechanism in Aplysia
Interneuron => serotonin
=> closes K+ channels
=> prolonged depolarization
=> more Ca2+ diffuse into terminals
=> more neurotransmitter released
The cerebral cortex controls voluntary movement and cognitive functions
Long-term potentiation – increase in the strength of synaptic transmission
Positive feedback > enforcing synaptic connection
The cerebral cortex controls voluntary movement and
cognitive functions – summary
Each side of the cerebral cortex has four lobes – frontal, temporal, occipital,
and parietal – which contain primary sensory areas and association areas.
Information processing in the cerebral cortex
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. In the somatosensory
cortex and the motor cortex, neurons are distributed according to the part of
the body that generates sensory input or receives motor commands.
Lateralization of cortical function
The left hemisphere is normally specialized for high-speed serial
information processing essential to language and logic operations. The right
hemisphere is stronger at pattern recognition, nonverbal ideation, and
emotional processing.
The cerebral cortex controls voluntary movement and cognitive
functions – summary
Language and speech
Portions of the frontal and temporal lobes, including Broca’s area and Wernicke’s
area, are essential for generating and understanding language.
Emotions
The limbic system, a ring of cortical and noncortical centers around the brainstem,
mediates primary emotions and attaches emotional “feelings” to survival-related
functions. The association of primary emotions with different situations during human
development requires parts of the neocortex, especially the prefrontal cortex.
Memory and Learning
The frontal lobes are a site of short/term memory and can interact with the
hippocampus and amygdale in consolidating long-term memory. Experiments on
invertebrates and vertebrates have revealed the cellular basis of some simple forms
of learning, including sensitization and long-term potentiation.
Consciousness
Modern brain-imaging techniques suggest that consciousness may be an emergent
property of the brain based on activity in many areas of the cortex.