Lecture 3: Neural Circuits and Their Function Flashcards
What is a complete neural circuit?
from sensor (afferent) to effector (efferent) – can involve as few as two neurons
What are action potentials (spikes or nerve impulses) ?
electrical mechanism that nervous systems have evolved that allow for conduction of information across distances
What can extracellular recordings record?
AP – because of their large amplitude
What can intracellular recordings record?
PSP
What are post-synaptic potentials?
signals induced when a neurotransmitter activates receptors at a synapse
What is neuromodulation?
term for synapses whose effects are not simple PSPs
What are three classes of effects that can occur when a synapse is activated?
- excitation
- inhibition
- modulation
What is excitation?
activating the synapse causes EPSP
What is inhibition?
activating the synapse causes IPSP
What is modulation?
activating the synapse causes effects other than simple PSPs
these effects can often be changes to the strength of other neighbouring synapses on the postsynaptic cell – synapse ‘modulates’ its target
When is a synapse considered ‘activated’?
when AP occurs in presynaptic neuron’s axon (and axon terminal), which causes release of neurotransmitter from axon terminal – neurotransmitter will be detected by postsynaptic cell
What is grey matter?
CNS areas with somata and neuropil (dendrites, axon terminals and synapses)
What is white matter?
consists of (myelinated) axons linking the neurons in two or more regions of grey matter
Sensory vs. Motor Neurons
Polarity
SENSORY: often ‘pseudo-unipolar’ – dendrite joins axon, not soma
MOTOR: multipolar – one axon, many dendrites
Sensory vs. Motor Neurons
Location of Somata
SENSORY: outside CNS in dorsal root ganglion
MOTOR: (and dendrites) In ventral grey matter
Sensory vs. Motor Neurons
Location of Axon
SENSORY: enters spinal cord/hindbrain on dorsal side, into dorsal grey matter
MOTOR: Leaves spinal cord via ventral root and travels directly to skeletal muscle cells
Within a circuit, what is a neuron’s activity is generated from?
sum of its inputs across time and space – most neurons (including those involved in simple reflexes), receive synapses from many different neurons, and each synapse is activated every time its presynaptic axon fires an AP
What is spatial activation?
summation in different points of space (different synapses)
What is temporal activation?
summation in same space, but at different times (repeated activation)
What determines the postsynaptic neuron’s activity/output?
integrated total of all recent inputs, such as spatial and temporal summation (measured by APs generated in its axon hillock)
What is convergence?
property of postsynaptic neuron in which multiple inputs converge onto it – receiving information
Where does divergence occur?
in axons
Where does convergence occur?
in dendrites
Where does convergence occur?
in dendrites
What is divergence?
how many separate cells are being communicated with
What does dorsal grey matter contain?
interneurons processing sensory info
What does ventral grey matter contain?
somatic motor neuron somata
What effect does changing the position of motor neurons in the ventral horn have?
scrambling the position of motor neurons within the ventral horn scrambles motor behaviour
How are neurons mapped in the zones of grey matter?
between (and within) ten zones (I-X), different functional neuron subtypes are topographically mapped
How are neurons mapped in the dorsal horn (I-IV)?
sensory neurons that carry different information make synapses with distinct interneuron types concentrated in different zones (function-o-topy)
How are neurons mapped in the ventral horn (IX)?
motor neurons innervating muscle cells from neighbouring muscles are located next to each other (somatotopy), at any particular spinal cord level
- upper limb muscle motor neurons: somata in cervical (neck) region of spinal cord
- lower limb motor neurons: found lower in spinal cord
What do efferent neurons of ANS control?
non-motor organ systems – neurons innervate visceral organs, muscles, skin and limbs
How is the overall organization of ANS similar to SNS? (2)
- circuit organization of afferents, interneurons, and efferents
- somatotopic mapping – neuron cell body position within nervous system roughly corresponds to position of target organ in the body
What is the visceral nervous system?
part of PNS that consists of all the nerves that relay information between CNS and visceral organs
What is the autonomic nervous system?
part of PNS that regulates involuntary physiologic processes – including heart rate, blood pressure, respiration, digestion, and sexual arousal
How many neurons does the ANS pathway have?
two neurons (extra, compared to SNS) in their effector pathways (to reach their targets) – preganglionic and postganglionic
- preganglionic efferent neuron’s soma is located in lateral horn grey matter
- synapse between two neurons (and postganglionic neuron’s soma) are located in autonomic ganglia, outside CNS
How many neurons does the SNS pathway have?
single efferent neuron sends axon from ventral horn grey matter in spinal cord, to target organ (skeletal muscle)
motor neurons only have excitatory effects on their target muscles, and each skeletal muscle cell will only receive one synapse from one motor neuron
What is the antagonistic effect of the parasympathetic and sympathetic divisions of the ANS caused by?
caused by postganglionic neurons from the two divisions signalling onto the same target cells using different neurotransmitters
What is the enteric nervous system?
division of VNS that coordinates sensory and motor functions of digestive organs, and can operate independently of CNS
What are the two functions of visceral sensory neurons?
- send information to CNS
- directly synapse on interneurons and effector neurons of ENS
What do parasympathetic and sympathetic neurons synapse on?
parasympathetic and sympathetic divisions represent the CNS response to visceral sensory information, but mostly synapse onto ENS neurons, not effector cells
