NEURONS, SYNAPSES
AND SIGNALING
The neuron – structure and function
• Conducts long distance electrical signals and short
distance chemical signals.
• Cell body – includes nucleus and other organelles
• Dendrites – highly branched extensions that receive
signals from other neurons
• Axon – extension that transmits signals to other cells,
can be long, branched at end
FIGURE 37.2
Dendrites
Stimulus
Axon
hillock
Nucleus
Cell
body
Presynaptic
cell
Axon
Signal
direction
Synapse
Synaptic terminals
Synaptic
terminals
Neurotransmitter
Postsynaptic cell
• Synapse – junction between axon/dendrite
• Neurotransmitters – chemical messengers, pass
information from transmitting neuron to receiving cell
• Glia cells – support cells in nervous system
• Outnumber neurons
• In brain
• Nourish neurons, insulate axons, regulate the extracellular fluid
around surrounding neurons
Information processing
• Sensory neuron – interneuron – motor neuron
• Central nervous system – brain and spinal cord
• Peripheral nervous system – nerves
• Autonomic nervous system – involuntary actions
• Sympathetic – fight or flight
• Parasympathetic – maintenance
Ion pumps and channels
resting potential
• Inside of a cell is negatively charged relative to outside
• Membrane potential –charge difference between outside
and inside of cell
• attraction of opposite charges across the plasma membrane as
source of potential energy
• Resting potential – membrane potential for resting neuron
Resting potential
• Potassium – K+ - greater inside of cell
• Sodium – Na+ - greater outside of cell
• Gradients are maintained by sodium potassium pump
• S/P Pump – 3 K+ out for 2 Na+ in
• Existence of a voltage difference in resting neuron
• Ion channel – allows ions to move back and forth across
the membrane and generates a membrane potential
• Net flow of each ion across the membrane since neither
K+ or Na+ is at equilibrium
Figure 37.6
Key
Na
OUTSIDE
OF CELL
K
Sodiumpotassium
pump
Potassium
channel
Sodium
channel
INSIDE
OF CELL
Action Potentials - axons
• Neuron responds to stimulus – gated ion channels react
• Hyperpolarization – inside of membrane more negative
due to opening K+ channel, which diffuse out, shifting
membrane potential
• Depolarization – reduction in the magnitude of the
membrane potential, usually involves gated Na+ channels
opening and diffusing into the cell
• Action potential – massive change in membrane voltage
Figure 37.11
Key
Na
K
3
Rising phase of the action potential
4
Falling phase of the action potential
Membrane potential
(mV)
50
Action
potential
−50
2
INSIDE OF CELL
Inactivation loop
1
Resting state
2
4
Threshold
1
1
5
Resting potential
Depolarization
OUTSIDE OF CELL
3
0
−100
Sodium
channel
Time
Potassium
channel
5
Undershoot
Figure 37.12-3
Axon
Plasma
membrane
Action
potential
1
Na
K
Cytosol
Action
potential
2
Na
K
K
Action
potential
3
Na
K
Evolutionary adaptations of axon
• Wider axon – allows for less resistance to the flow of
currents
• Invertebrates differ from vertebrates
• Vertebrate axons have narrow diameters but do conduct action
potentials at high speeds
• Due to insulation – myelin sheath
• Myelin sheaths
in CNS – oligodendroglia
in PNS – schwann cells
Figure 37.13
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin
sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 m
Saltatory conduction
• Myelinated axons have gaps – nodes of Ranvier
• where voltge-gated Na+ channels are located
• Action potentials occur at nodes and pass over
myelinated sections – making conduction much faster
Figure 37.14
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
The synapse - communication
• Electrical and chemical synapses
• Most synapses are chemical synapses in the vertebrate
brain
• Release of neurotransmitters, held in vesicles, by the
pre-synaptic neuron
• Neurotransmitter diffuses across the synaptic cleft, to the
post synaptic membrane, which activates a specific
receptor
Presynaptic cell
Postsynaptic cell
Figure 37.15
Axon
Synaptic vesicle
containing neurotransmitter
1
Synaptic
cleft
Postsynaptic
membrane
Presynaptic
membrane
3
K
4
Ca2 2
Voltage-gated
Ca2 channel
Ligand-gated
ion channels
Na
Neurotransmitters
• Acetylcholine – nervous system functions, muscle
•
•
•
•
•
stimulation, memory formation, learning
Glutamate – AA – in invertebrates, at neuromuscular
junction rather than acetylcholine
GABA – (gamma-aminobutyric acid) – inhibitory
synapses, increase permeability to Cl-, Valium reduces
anxiety through binding to a site on a GABA receptor
Norepinephrine - excitatory
Dopamine and serotonin – affect sleep, mood, attention
and learning, Parkinsons, depression
Endorphins – decreasing pain perception
Evolution of the nervous system in the
Animal Kingdom
• Cnidaria – nerve net, contraction and expansion of
gastrovascular cavity
• Planarian – cephalization – eye spot, nerves, nerve cords,
simple CNS
• Insects – ganglia –clusters of neurons, brain, ventral
nerve cord
• Vertebrates – CNS – brain and spinal cord
• PNS - nerves
FIGURE 38.2
Eyespot
Brain
Nerve
cords
Nerve net
Transverse
nerve
(a) Hydra (cnidarian)
(b) Planarian (flatworm)
Brain
Brain
Ventral
nerve cord
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglia
Segmental
ganglia
(c) Insect (arthropod)
(d) Salamander (vertebrate)
Glia cells
• Nourish, support and regulate the functioning of neurons
• Astrocytes – hold blood vessels close, aid in nourishment
• Oligodendroglia – make myelin sheath in CNS
• Microglia – phagocytic,
CNS - PNS
• Gray matter – consists mainly of cell bodies and dendrites
• White matter – consists of myelinated axon bundles
• Brain – consists of 100 billion neurons
Figure 38.4
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Figure 38.5
Central Nervous
System
(information processing)
Peripheral Nervous
System
Afferent neurons
Efferent neurons
Sensory
receptors
Autonomic
nervous system
Motor
system
Control of
skeletal muscle
Internal
and external
stimuli
Sympathetic Parasympathetic
division
division
Enteric
division
Control of smooth muscles,
cardiac muscles, glands
Figure 38.6b
Brain structures in child and adult
Embryonic brain regions
Telencephalon
Cerebrum (includes cerebral cortex,
white matter, basal nuclei)
Diencephalon
Diencephalon (thalamus,
hypothalamus, epithalamus)
Mesencephalon
Midbrain (part of brainstem)
Metencephalon
Pons (part of brainstem), cerebellum
Myelencephalon
Medulla oblongata (part of brainstem)
Forebrain
Midbrain
Hindbrain
Midbrain
Hindbrain
Mesencephalon
Metencephalon
Diencephalon
Cerebrum
Diencephalon
Myelencephalon
Midbrain
Pons
Forebrain
Embryo at 1 month
Telencephalon
Medulla
oblongata
Cerebellum
Spinal cord
Spinal
cord
Embryo at 5 weeks
Child
Brain region functions
• Cerebrum – skeletal muscle contraction, center for
learning, emotion, memory and perception
• Cerebellum – coordinates movement and balance,
learning and remembering motor skills.
• Diencephalon –
• thalmus – input center for sensory information
• Hypothalmus- thermostat, biological clock
• Regulates pituitary gland therefor regulates hunger and thirst, fight or
flight, role in sexual and mating behaviors.
The brain stem
• Midbrain – receives sensory information, coordinates
visual reflexes
• Pons and Medulla – 2 way conduction from spinal cord to
brain
• Helps to coordinate large scale body movements, control several
automatic, homoestatic functions: breathing, heart and blood
vessel activity, swallowing, vomiting and digestion.
Figure 38.6d
Diencephalon
Thalamus
Pineal gland
Hypothalamus
Pituitary gland
Brainstem
Midbrain
Pons
Medulla
oblongata
Spinal cord
Emotions – Limbic system
• Biological clock regulation –
• Typically regulated by cycles of light and dark
• Coordinated by a group of neurons n the hypothalmus in
conjunction with sensory information from the eyes.
• Brain reward system and drug addition
• Drugs alter the transmission of signals in the synaptic pathway
formed by neurons.
• Mouse party
Use imaging of the brain to understand the brain
Positron emission tomagraphy (PET)
Magnetic resonance imaging (MRI)
Figure 38.8
Thalamus
Hypothalamus
Olfactory
bulb
Amygdala
Hippocampus
Cerebral Cortex
• Controls
• Language and speech – Broca’s area and Wernicke’s area
• Both in left side of brain…
Left side of brain is also more adept at math and logical operations
Right side – recognition of faces and patterns, spatial relations and
nonverbal thinking.
Frontal lobe – decision making
Figure 38.11
Motor cortex (control
of skeletal muscles)
Frontal lobe
Somatosensory cortex
(sense of touch)
Parietal lobe
Prefrontal cortex
(decision
making,
planning)
Broca’s area
(forming speech)
Temporal lobe
Auditory cortex
(hearing)
Cerebellum
Wernicke’s area
(comprehending language)
Sensory association
cortex (integration
of sensory
information)
Visual association
cortex (combining
images and object
recognition)
Occipital lobe
Visual cortex
(processing visual
stimuli and pattern
recognition)
Evolution of cognition in Vertebrates
• Perception and reasoning that constitute knowledge
• Human evolution…larger cranial capacity
• Hypothesis – evolution of a highly convoluted cerebral
cortex
• Primates, and cetaceans (whales and dolphins)
• Birds – lack convoluted cortex but have organization of clustered
neurons in top layer of brain, the pallium
Senses
• Sensory receptor – sensory transduction - transmission –
perception
• Types of sensory receptors
• Mechanoreceptors – pressure, touch, stretch, motion and sound
• Electromagnetic - light, electricity and magnetism
• Thermoreceptors – heat and cold
• Pain – extreme pressure or temp
• Chemoreceptors – solute concentration, smell, taste