Neural Signalling

neurons - specialized cells carrying electrical impulses

• sensory neurons - transmit nerve impulses from sense receptors → central nervous system (CNS)

• interneurons - transmit nerve impulses btwn neurons 

  • located in CNS

• motor neurons - transmit nerve impulses from CNS → effector

  

Structures in motor neurons

• dendrites - short nerve fibers

  • receive info from neurons via neurotransmitters

• cell body (soma) - cell nucleus + cytoplasm

  • take info from dendrites + determines at axon hillock if action potential fires

• axon

  • action potential moves along axon → axon terminals

  • covered in Schwann cell extensions (myelin)

• Schwann Cells

  • produce myelin for motor neurons 

  • maintain strength of action potential

• axon terminals

  • where neurotransmitters are released

    • released as a result of action potential

• node of ranvier

  • allow saltatory conduction

    • electrical signal jumps from one node to next 

Resting potential - electrical potential difference (voltage) of cytoplasm of a neuron relative to surroundings at rest

• for neurons = -70 mV

  • negative bc of distribution of ions inside/outside plasma membrane

    • inside of neuron = high concentration of organic negative neurons

Sodium potassium pump uses ATP to maintain resting potential of a neuron (active transport)

• pumps OUT 3 Na (sodium)

• pumps IN 2 K (potassium)

• against concentration gradient

• example of exchange transporter

  • sodium and potassium transported in opposite directions

Action of sodium potassium pump

• 3 NA attach to sodium ion binding sites → ATP attaches to pump and hydrolyzes → phosphate remains on protein pump → ADP released → phosphate makes pump change shape → sodium moves across axon membrane → sodium released OUT of cell

• 2 K attach to potassium ion binding sites → phosphate released from pump → pump returns to original shape → potassium moved INTO cell

Nerve impulses - action potentials propagated along nerve fibers

• action potential involves flow of sodium + potassium ions

• electrical

  • involves flow of ions along nerve fibers (ex axons)

Action potential - change of electrical potential difference (voltage) as nerve impulse moves across nerve fiber

• involve depolarizaiton/repolarization of nerve fiber along axon

• Depolarization - increase of potential difference across membrane of a cell

  • neuron at -70 mV gets stimulus → reaches threshold potential (-55mV) → voltage gated sodium channels open → sodium diffuses into nerve fiber → potential difference (voltage) increases to +30 mV

• Repolarization - restoration of potential difference to resting potential across membrane of a cell

  • potential difference reaches +30 mV → voltage gated sodium channels close → voltage gated potassium channels open → potassium moves out of cell → potential difference decreases below resting potential (-70 mV) (hyperpolarization)

• sodium potassium pump restores resting potential by pumping sodium out of cell + potassium into cell

Threshold potential - minimum potential difference required across a membrane to initiate an action potential

• voltage gated sodium channels open at -55 mV → sodium ions enter nerve fiber

Depolarization → influx of sodium ions into nerve fiber

• sodium enters nerve fiber → spread out via diffusion → create local currents

  • movement of sodium ions in local channels in axon → decreases potential difference to threshold potential → action potential propagated along nerve fiber

Oscilloscopes - used to measure changes in potential difference of axons during action potential

animal size increases → speed of nerve impulses decreases

• negative correlation example

Large animals have adaptations to increase speed of nerve impulses

• wide axon diameter

  • diameter increases → speed of nerve impulse increases

    • positive correlation

  • squid have diameter of 1 mm → speed of nerve impulses 35 ms^-1

• myelination of axons

  • speed of small, unmyelinated axons as low as 0.5 ms^-1

myelin - multilayer of phospholipids + proteins

• surrounds axons

  • wrapped around axons via Schwann cells → form myelin sheath

    • spaces btwn myelin sheath = nodes of Ranvier

• insulating layer

• increases speed of nerve impulses

  • can be as high as 150 ms^-1 compared to unmyelinated 0.5-10 ms^-1

• myelinated neurons can do saltatory conduction

Coefficient of determination R² - determines strength of relationship btwn 2 variables

• high = strong relationship

• low = weak relationship

Saltatory conduction- the way an action potential jumps btwn nodes of Ranvier as it moves down an axon

• more faster/energy efficient than non-saltatory condutction

  • myelin insulating axon → faster action potential propagation

• action potentials only occur at nodes of Ranvier

  • sodium potassium pumps + voltage gated channels clustered at nodes of Ranvier

  • action potentials jump node → node traveling down an axon

Synapse - junctions btwn 2 neurons OR neuron and an effector (muscles, glands, etc)

• nerve impulses ONLY pass from presynaptic → postsynaptic membrane 

Nerve impulses transmitted across synapses via neurotransmitters

• action potential arrives at end of presynaptic neuron → triggers opening of voltage gated calcium ion channels → calcium moves into synaptic knob via facilitated diffusion → triggers movement of vesicles w neurotransmitters to presynaptic membrane

• vesicles fuse w presynaptic membrane → contents released via exocytosis → neurotransmitters diffuse across synaptic cleft → neurotransmitters bind to specific receptors on postsynaptic membrane → sodium ion channels open

• sodium moves into postsynaptic neuron → depolarization → action potential (if excitation exceeds -55 mV) → neurotransmitter removed from synaptic cleft → stops stimulation of action potential → neurotransmitter reabsorbed or broken down 

  

Acetylcholine - neurotransmitter existing in many synapses (ex. neuromuscular junction)

• neuromuscular junction - synapses btwn axon terminals or motor neurons + muscle fibers

• motor neuron releases acetylcholine into synapse → acetylcholine binds to muscle fiber sarcolemma → ligand gated channels for acetylcholine open → sodium ions come into muscle fiber

• muscle fiber is positively charged inside → signal travels down T tubules alongside sarcoplasmic reticulum → voltage gated calcium channels on SR open → acetylcholinesterase breaks down acetylcholine → signal ends

arrival of action potential at axon terminal → release of acetylcholine into synaptic cleft

• acetylcholine diffuses across synaptic cleft → acetylcholine binds to transmembrane acetylcholine receptors on postsynaptic membrane → ligand gated channels open → threshold hit → voltage gated sodium channels open → sodium flows into neuron → excitatory postsynaptic potential

exogenous chemicals - come from sources outside of living things 

• may interfere w synaptic transmission

  • neonicotinoids - bind to cholinergic protein receptors + prevent binding of acetylcholine

    • often used in pesticides

      • can kill non-target insects like bees

    • acetylcholine binds to nicotonic cholinergic receptors on postsynaptic membrane 

      • neonicotinoids also bind to these receptors + have similar structure to nicotine/acetylcholine

        • binds irreversibly to acetylcholine receptors in insects → blocks synaptic transmission → paralysis/death in insects

      • acetylcholine receptors in humans are a different shape → neonicotinoids less toxic to humans

  • cocaine - prevents reuptake of dopamine

    • dopamine - neurotransmitter responsible for feelings of reward + motivation

    • dopamine transporters - remove dopamine from synaptic clefts btwn neurons in brain

      • cocaine binds to/blocks dopamine transporters → prevents reuptake of dopamine

        • lots of dopamine remains in synaptic cleft → continue binding to dopamine receptors on postsynaptic membrane → continued excitation of reward pathway

types of neurotransmitters

• excitatory 

  • increase likelihood of action potential being generated at postsynaptic neuron

  • open sodium channels → allow sodium ions to enter neuron

• inhibitory 

  • decrease likelihood of action potential being generated at postsynaptic neuron

  • bind to protein receptors allow negative ions to enter neuron → neuron is hyperpolarized → potential difference lower than resting potential

neurons have many synapses with other neurons

• inhibitory and excitatory neurotransmitters can be released at different synapses → send both signals out to cell body

• summation of effects = process that the combined influence of neurotransmitters determines if neuron generates action potential or not 

  • all or nothing consequence

    • either threshold is reached and action potential propagated… or not

nociceptors - group of unspecialized free nerve endings that serve as pain receptors

• can respond to range of stimuli

  • high temp

  • acid

  • chemicals (ex. capsaicin) 

• nociceptor is stimulated → sodium channels on nociceptor open → sodium flows into nociceptor → threshold potential reached → action potential generated → travels to brain → pain !

emergent properties come from interactions of component parts

• whole is greater than sum of parts

• ex. consciousness from neurons in the brain