Nervous systems

Dendrites

recieves input from other neurons

Cell body

intergration of synaptic potentials

Axon

conduction component of the neuron, propagates action potentials to the axon terminals 

action potential 

transmits information and triggers synaptic output 

Synaptic terminals

output of the neuron, secretion of neurotransmitter, alters activities of other cells

resting membrane potential concentrations

Na+ is more concentrated outside the cells and K+ more concentrated inside the cell

Resting membrane potential first part 

Mainly K+ channels open, NA+/K+ pump maintains high intracellular K+ conc. and creates chemical conc. gradient, K+ ions diffuse in both directions across the membrane

Resting membrane potential second part 

Higher probability of K+ ions hitting channels from inside neuron creating net outward current of K+, an excess of negative ions inside creates an electrical gradient which opposes K+ outflow. 

Equilibrium with gradients

Electrical gradient counteracts chemical concentration gradient

Before an action potential

Most voltage-gated sodium and potassium channels are closed

Depolarisation

Voltage gated Na+ channels open first and Na+ flows into the cell

Rising phase of action potential 

When reaching the threshold, membrane potential becomes more positive, more Na+ channels open and so on, this is a self sustaining process 

falling phase of action potential

Voltage gated Na+ channels become inactivated, Voltage gated K+ channels open and K+ flows out of the cell

Undershoot/hyperpolarisation

Voltage gated K+ channels are still open (in addition to permanently opened ones), more K+ flows out than during rest

Electrical synapse

Cytoplasmic channels (gap junctions) directly connect adjacent neurons, allow direct electrical current flow for rapid transmission, transmission is bidirectional

Chemical synapse

Neurotransmitter cross cleft and bind postsynaptic receptors, slower than electrical synapses but more flexible, transmission is unidirectional, excitatory or inhibitory 

Chemical synapse processes 1 and 2

1) synthsis and packaging neurotransmitter in synaptic vesicles 2) Action potential causes Ca2+ influx which triggers fusion of the vesicle with the presynaptic, results in neurotransmitter release 

Chemical synapse process 3 

3) Neurotransmitter diffuses across the synaptic cleft and binds to and activates receptors in postsynaptic cell 

Natural toxins often block synaptic transmission

Botox blocks neurotransmitter release, bungarotoxin preventing binding of neurotransmitter at receptor 

two mechanisms of terminating synaptic transmission

enzymatic breakdown of neurotransmitter in the synaptic cleft, reuptake of neurotransmitter by presynaptic neuron 

Chemical synapse can be …

excitatory or inhibitory

Synaptic integration

Neuron combines synaptic inputs to determine its output, summation of postsynaptic potentials

IPSP

Inhibitory postsynaptic potential, decrease probability of action potential generation

EPSP

Excitatory postsynaptic potential, increase probability of action potential generation 

Stretch reflex inhibitory interneuron

Sensory neuron also activates inhibitory interneurons, these inhibits antagonistic motor neurons, prevents antagonistic muscle contraction, sensory neuron of stretch receptor synapses on motor neurons to the same muscle

Stretch reflex 

Motor neuron excited, extensor muscle contracts, leg extensor muscle stretched, muscle spindle generates action potentials 

Reflexes 

a model for how circuits generate behaviour, automatic responses, often mediated by spinal cord or brainstem, simple and reproducible, clear input-output pathways

Reflexes are useful for

studying information processing, synaptic plasticity, and basic learning, less suited for complex cognition but form essential building blocks for larger networks

Sensory neurons, afferent neurons

detect stimuli and convert them to electrical signals

Interneurons 

integrate and modulate information 

Efferent neurons, motor neurons

send signals to muscles or glands causing action