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

Nucleus

Stimulus

Axonhillock

Cellbody

Axon

Signaldirection

Presynapticcell

Synapse

Neurotransmitter

Synaptic terminals

Postsynaptic cell

Synapticterminals

• 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

OUTSIDEOF CELL

INSIDEOF CELL

Key

Na

K

Sodium-potassiumpump

Potassiumchannel

Sodiumchannel

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

1

Figure 37.11

Key

Na

K

Actionpotential

Threshold

Resting potential

Time−100

−50

0

50

Mem

bra

ne

po

ten

tial

(mV

)

Rising phase of the action potential

Depolarization

Falling phase of the action potential

Resting state

Undershoot

Sodiumchannel

Potassiumchannel

Inactivation loop

OUTSIDE OF CELL

INSIDE OF CELL

1

5

43

2

15

42

3

Figure 37.12-3

Axon

Plasmamembrane

Cytosol

Actionpotential

Actionpotential

Actionpotential

K

K

K

K

Na

Na

Na

1

2

3

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

Axon Myelinsheath

Schwanncell

Nodes ofRanvier Nucleus of

Schwann cell

Schwanncell

Node of Ranvier

Layers of myelin

Axon

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

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

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

Figure 37.15

Presynaptic cell Postsynaptic cell

Axon Synaptic vesiclecontaining neurotransmitter

Synapticcleft

Postsynapticmembrane

Ca2

K

Na

Ligand-gatedion channels

Voltage-gatedCa2 channel

Presynapticmembrane

1

2

3 4

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

(a) Hydra (cnidarian)

Spinalcord(dorsalnervecord)

Brain

(b) Planarian (flatworm)

(c) Insect (arthropod) (d) Salamander (vertebrate)

Sensoryganglia

Brain

Nervecords

Eyespot

Transversenerve

Segmentalganglia

Brain

Nerve net

Ventralnerve cord

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

Spinal cord

Central nervoussystem (CNS)

Peripheral nervoussystem (PNS)

Cranialnerves

Spinalnerves

GangliaoutsideCNS

Brain

Figure 38.5

Afferent neurons

Sensoryreceptors

Internaland external

stimuli

Autonomicnervous system

Motorsystem

Control ofskeletal muscle

Sympatheticdivision

Entericdivision

Control of smooth muscles,cardiac muscles, glands

Parasympatheticdivision

Efferent neurons

Peripheral Nervous System

Central Nervous System

(information processing)

Figure 38.6b

Medullaoblongata

Embryonic brain regions Brain structures in child and adult

Forebrain

Hindbrain

Midbrain

Telencephalon

Myelencephalon

Metencephalon

Forebrain

Hindbrain

Midbrain

Diencephalon

Cerebrum (includes cerebral cortex,white matter, basal nuclei)

Medulla oblongata (part of brainstem)

Pons (part of brainstem), cerebellum

Midbrain (part of brainstem)

Diencephalon (thalamus,hypothalamus, epithalamus)

Telencephalon

Myelencephalon

Metencephalon

Diencephalon

Mesencephalon

Mesencephalon

Embryo at 1 month Embryo at 5 weeks

Spinalcord

Child

Diencephalon

Midbrain

CerebellumSpinal cord

Pons

Cerebrum

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

ThalamusPineal glandHypothalamus

Pituitary gland

Spinal cord

Brainstem

Midbrain

Medullaoblongata

Pons

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

Amygdala

Olfactorybulb

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

Frontal lobe

Temporal lobeOccipital lobe

Parietal lobe

Cerebellum

Motor cortex (controlof skeletal muscles)

Somatosensory cortex (sense of touch)

Wernicke’s area(comprehending language)

Auditory cortex(hearing)

Broca’s area(forming speech)

Prefrontal cortex(decisionmaking,planning)

Sensory association cortex (integrationof sensoryinformation)

Visual association cortex (combiningimages and objectrecognition)

Visual cortex(processing visualstimuli and patternrecognition)

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