C2.2 Neural Signaling

C2.2.1 Neurons as cells within the nervous system that carry electrical impulses

• Dendrites: Short branched nerve fibers

  • Receive signals from other neurons and transmit them to cell body

• Axons: Elongated nerve fibers

  • Transmit electrical impulses from the cell body to other neurons/muscles/glands

C2.2.2 Generation of resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions

• Membrane potential: Difference in voltage inside/outside a cell membrane

• Resting potential: Membrane potential when an impulse is not being transmitted (-70mV)

  • Sodium-potassium pumps transfer 3 sodium ions out and 2 potassium ions in using active transport

  • Pumped ions leak back across the membrane by facilitated diffusion (K+ leaks faster)

  • Negatively charged proteins/organic anions inside the nerve fiber

• Sodium-potassium pump action

  • 3 Na+ bind to pump inside the cell

  • ATP is hydrolyzed, Pi binds to pump and causes conformational change

  • Pump opens, Na+ released from cell

  • 2 K+ ions bind to pump outside the cell

  • Pi is released, pump returns to original shape

  • K+ moved into cell

C2.2.3 Nerve impulses as action potentials that are propagated along nerve fibers

• Nerve impulse: A signal transmitted along a nerve fiber

• Action potential: A rapid change in the cell’s membrane potential

• Depolarization: Membrane potential changes from negative to positive (reaches about +35mV)

• Repolarization: a change back from positive to negative

C2.2.4 Variation in the speed of nerve impulses

• Positive correlation between axon diameter and speed of nerve impulse conduction (reduces resistance)

• Myelination: Nerve fibers covered in Schwann cells with gaps called nodes of Ranvier

  • Nerve impulses jump from nodes of Ranvier to the next

  • faster than unmyelinated nerve fibers

• Negative correlation between speed of conduction of nerve impulses and animal size

C2.2.5 Synapses as junctions between neurons and between neurons and effector cells

• Synapse: Junction between cells in the nervous system

  • between sensory receptor cells and neurons

  • between neurons

  • between neurons and muscle fibers/gland cells

• Effectors: Cells that carry out response to a stimulus (e.g. muscle fibers/gland cells)

• Synaptic gap: Space between axon and dendrite of neurons

• A signal can only pass in one direction across a typical synpase

• Neurotransmitters carry signals across the synaptic gap

C2.2.6 Release of neurotransmitter from a presynaptic membrane

• At presynaptic cell:

  • Nerve impulse propagated along the neuron until it reaches the end and the presynaptic membrane

  • Depolarization of presynaptic membrane causes calcium ion channels to open, ions diffuse into the neuron

  • Vesicles containing neurotransmitter to move to and fuse with presynaptic membrane

  • Neurotransmitter released into synaptic gap by exocytosis

• Calcium functions as a chemical signal triggering exocytosis of neurotransmitters in presynaptic neuron

C2.2.7 Generation of an excitatory postsynaptic potential

• At post synaptic cell:

  • Neurotransmitter binds to receptors in postsynaptic membrane → ion channels open

  • Ions diffuse into the postsynaptic neuron → membrane potential rises

  • Triggers action potential → propagates away from the synapse

  • Neurotransmitter in synaptic gap is broken down and reabsorbed by presynaptic neuron

• Acetylcholine is one of the most common neurotransmitters

• Acetylcholinesterase: Enzyme present in synaptic gap that breaks down acetylcholine into choline and acetate

  • choline is reabsorbed by presynaptic neuron and converted back to acetylcholine by recombining with acetyl group

C2.2.8 Depolarization and Repolarization during action potentials

• Action potential is only initiated if the threshold potential is reached

• Voltage causes Sodium ion channels to open (threshold potential)

  • sodium ions diffuse out → depolarization

• Potassium channels open

  • potassium ions diffuse out of the axon → repolarization (back to -70mV)

C2.2.9 Propagation of an action potential along a nerve fiber/axon as a result of local currents

• Local current: Depolarization causes different sodium ion concentrations to neighboring part of axon

  • Inside axon: sodium ions diffuse along inside the axon to neighboring part that is still polarized

  • Outside axon: sodium ions diffuse from polarized part back to the part that just depolarized

• Local currents reduce the concentration in the part of the neuron that is not depolarized yet → membrane potential rises to about -50mV

  • sodium channels in axon membrane open → depolarization

  • propagated along the axon

C2.2.10 Oscilloscope traces showing resting potentials and action potentials

• Measured by placing electrodes on each side of the membrane

• Action potential → spike

C2.2.11 Saltatory Conduction in myelinated fibers to achieve faster impulses

• Myelination of nerve fibers

  • Schwann cells layers

  • Prevents ion movements → action potentials only occur at nodes of Ranvier

• Na+/K+ pumps are clustered at the Nodes of Ranvier

C2.2.12 Effects of exogenous chemicals on synaptic transmission

• Exogenous chemical: A chemical that enters the body of an organism from an external source.

• Neonicotinoids: Synthetic compounds that bind to acetylcholine receptors of synapses in central nervous system of insects

  • Acetylcholinesterase cannot break down neonicotinoids → irreversible binding to receptors

  • Receptors blocked → no transmission

  • Paralysis and death

• Cocaine: Binds to dopamine reuptake transporters

  • Causes dopamine to build up in the synaptic gap → continuously excites postsynaptic neuron

  • Stimulates transmission of dopamine

C2.2.13 Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials

• GABA is an inhibitory neurotransmitter

  • binds to receptor → chloride channel opens

  • chloride ions enter → hyperpolarization of the postsynaptic neuron

• Hyperpolarization → less likely for action potential to be propagated along the neuron → reduces neurotransmission

C2.2.14 Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron

• Excitatory neurotransmitters → towards threshold potential

• Inhibitory neurotransmitters → threshold potential not reached

• Summation: Multiple releases of excitatory neurotransmitter combine to cause an action potential

C2.2.15 Perception of pain by neurons with free nerve endings in the skin

• Pain receptors (at nerve endings of sensory neurons) in the skin detect stimuli and convey impulses to central nervous system

  • have channels for positive ions → threshold potential reached → nerve impulses pass through sensory neuron to spinal column

  • Interneurons in spinal cord relay impulse to cerebral cortex

  • Impulse reaches sensory areas of cerebral cortex → pain sensation

• Stimuli: chemical substances, excessive heat, puncture of skin

C2.2.16 Consciousness as a property that emerges from the interaction of individual neurons in the brain

• Emergent property: Caused by interactions between elements of a system

• New properties emerge at each level of biological organization

• Consciousness is an emergent property

  • Emerges from interactions of individual neurons in the brain