chapter 1 Introduction to Neurophysiology Flashcards
Purpose of chapter
provide general introduction to neurophysiology with more detailed information on several selected topics and high-level overview of the CNS’s function.
Anyone working in a field associated with neuro-engineering must understand neurophysiology in great detail
example careers:
Basic and applied research
Device design and development
Neurology and neurosurgical clinical subspecialties
neuron (nerve cell):
a cell population considered as the functional unit within the human nervous system
neurons typically have
dendrites, axons, and axon terminals
resting membrane potential:
Neurons under resting conditions have an electrical potential across their plasma membranes
the intracellular space is negative, and the extracellular space is positive
resting membrane potential in healthy neurons is
[-40∶-70] mV. By convention, the extracellular fluid is assigned a voltage of 0.
the resting membrane potential is generally steady unless
altered by changes in local electrical currents.
resting membrane potentials exist due to
an excess of negative ions inside the neuron and an excess of positive ions outside the neuron.
a change in resting membrane potential is made possible by a distribution of
3 ions that are mobile across the neuron’s plasma membrane
state the ion, [intracellular], [extracellular]
Na^+ 15 mM 145 mM
Cl^- 7 mM 100 mM
K^+ 150 mM 5 mM
the excess of charged ions collect near the plasma membrane and their movement when the cell is excited underlies the development of an
action potential
If the concentration gradient for any ion is known, then the ___ across the plasma membrane for that ion can be calculated using the ____
relative equilibrium potential, Nernst equation.
i.e. one can estimate the electrical potential necessary to balance a given ionic concentration gradient across a membrane with the net flux being 0.
Nernst equation:
E_ion=61/Z log(C_out/C_in )
Where:
E_ion is the equilibrium potential for a given ion in mV
C_in is the intracellular concentration ion
C_out is the extracellular concentration of the ion
Z is the valence f the ion
61 is a combined constant which accounts for the Universal Gas Constant, temperature (37 C), and Faraday’s electrical constant
The nerve cell’s negative resting membrane potential is determined mainly by ___
Reason:
K+
If each of the 3 ions, Na+, K+, or Cl- become totally permeable across a given membrane, then:
E_Na=60 mV
E_K=-90 mV
E_Cl=-80 mV
Since nerve cells have negative resting membrane potentials, these potentials must be determined by either chloride or potassium ion distributions.
By measurements of ion movement, chloride ions are typically passively distributed across a given neuron’s surface membrane. Thus, chloride currents have negligible roles under resting conditions.
neurons typically have ___ within their surface membranes. Different neuron types have unique compositions.
ion-selective channels
gating:
the triggered openings of ion-specific channels
voltage-gated ion channels:
respond to changes in local membrane potentials of a given cell
ligand-gated ion channels:
that respond to specific biochemical factors (receptor activated by agonist
spontaneously active ion channels:
elicit random frequencies of opening and closing
leak channels:
somewhat continuously open though only allowing typically low ion flows
channels are classified by:
L4 control mechanisms L5 Examples: L6 voltage-gated L6 ligand-gated L6 spontaneous L6 leak L4 ion selectivity L5 Examples: L6 Na+ L6 K+ L6 Ca2+ L6 Cation non-specific L4 The direction in which the ion is going (e.g. inward and outward)
action potentials occur in nerve cells due mainly to ___
transient changes in cellular permeabilities of Na+ and K+ ions.
An initial local electrical depolarization causes the transient openings of voltage-dependent Na+ channels
when this occurs, the surface membrane potential reaches a threshold voltage of 10-30 mV above the given resting potential.
There is a 1-2 ms increase in sodium permeability which is interpreted as an increase in conductance.
This further depolarizes the cell and drives the membrane potential toward the sodium equilibrium potential.
Within 1 ms, these channels are actively inactivated.
The depolarization activates voltage-gated K channels.
This allows efflux from the cell which thus drives the membrane potential back towards the potassium equilibrium potential (which is more negative).
This excitation can be also be considered self-propagating. Adjacent cell membrane areas can be excited
Example: propagation down the nerve axon.
neurons form connections between themselves via ___
synapses, chemical or electrical
This is the primary mechanism by which information is transferred in the CNS.
neurons form connections between themselves via synapses, chemical or electrical
glia cells / neuroglia:
a category of nerve cells which are not neurons and therefore not a part of the primary mechanism by which information is transferred in the CNS but is essential for supporting the process.
Examples: supporting functions Maintenance of cerebral homeostasis Formation of myelin Protection and support of neurons
The purpose of the myelin layer of a neuron (myelin sheath) is to
increase the speed at which generated electrical impulses propagate
major subpopulations of glia and some of their characteristics:
L4 astrocytes / astroglia:
L5 star-shaped
L5 provides physical and nutritional support for neurons
L5 clean up brain-debris
L5 regulate the contents of the extracellular space
L5 forms a structural scaffolding to hold neurons in place
L4 microglia:
L5 remove waste / cellular debris
L4 oligodendroglia
L5 provide the insulation (myelin) for neurons in the CNS
L4 Schwann cells
L5 provide insulation (myelin) for neurons in the PNS
The cell populations within the brain have a high degree of ____
They are ___ structures which continually turn over their ____ to create neural connections.
plasticity,
dynamic,
structural components and or alter their shapes
In the avg. human brain, there is one glia per ___
There are 2 neurons per ___
neuron,
3 glia in the cerebral cortex (gray matter)
Glia are important for ____
synaptic plasticity and synaptogenesis,
regulation of repair of neurons and neural pathways after injury or ischemic event
astrocytes can communicate with ___
neurons and even modify the signals they send and receive
L4 It is implied that this is done at the site of the synapse
about half of an individual’s gene pool contributes to ____
developing the brain and its respective cell populations.
A healthy adult contains about ___neurons.
10^10
The synapses between these neurons utilize on the order of 100 different ____
neurotransmitters, second messengers, and growth factors.
There are about___ different receptor molecules in the brain
These receptors can be configured into different ___
Examples: ___
300,
receptor complexes
dimers, trimers, complex heteromers
neurons are divided into 3 functional classes:
afferent, efferent, and interneuron
Afferent neuron:
convey sensory information from tissues and organs into the CNS
efferent neuron:
convey information away from the CNS to effector cells to induce a response
For each afferent neuron, there are about ____
10 efferent neurons, and 200k interneurons
simplistic interpretation of the relationship between afferent, efferent, and interneuron neurons:
Neural circuits exist to amplify weak signals, attenuate overly intense activity, emphasize contrasts, maintain rhythms (e.g. for respiration, keep groups of neurons in their optimal functional range (e.g. feedback adjustment of their gains
transported information throughout the nervous system can be considered to ___
If the information ___, it goes to multiple brain regions to have a global effect.
If the information ___, it goes to a specific cell or group of cells (e.g. nuclei or ganglion) to activate or inhibit a given neural function.
___ within a neural network in the CNS can be considered to occur at more global levels relative to overall CNS function.
converge or diverge.,
diverges,
converges,
Interposed divergence and convergence
Alt explanation: information from multiple brain regions (e.g. sites, groups of neurons, nuclei) may have simultaneous / instantaneous function impacts as convergent or divergent on given neural activities associated with specific brain functions.
Example: the ability to wake from sleep results from the large divergence of neural excitation arising from the reticular activating system (RAS) located in the brainstem.,
Divergence (of neural information):
Example:
Example:
can occur via axon collaterals which serve to make information accessible simultaneously to different parts of the CNS.
A given set of sensory information can be used for reflexive response and also mediating a sensory experience.
a single motoneuron innervates numerous muscle fibers which function as a contractile unit.
Convergence (of neural information):
thousands of axon collaterals can converge on the cell body of a single neuron.
It then depends on the sum and direction of the synaptic processes acting at each moment in time to determine whether an action potential will be induced to send information (signal) onwards.
Convergence allows a neuron to process or integrate incoming excitatory and inhibitory signals occurring at its membrane within milliseconds.
1.1
Temporal and spatial facilitation
___ in the CNS can have different design properties that enable various signaling functions.
two examples:
neural discharge patterns and network structures
temporal and spatial facilitation
Temporal facilitation:
L4 repetitive stimulation of an axon may result in an action potential.
L5 This is called Excitatory Post-synaptic potentials (EPSPs)
L4 EPSPs, when triggered in succession are additive and eventually become suprathreshold.
L5 This is possible because the durations of EPSPs are longer than the duration of axonal refractory periods.
L6 summation of EPSPs resulting in action potentials are not affected by the axonal refractory periods.
spatial facilitation:
The activation of a single axon produces subthreshold EPSPs but several axons innervating the same neuron triggered simultaneously give rise to a suprathreshold potential and thus an action potential.
CNS neurons receive inputs from ____
example:
inputs can be ____
inhibitory information is called ____
dozens to thousands of axons.
an avg. of 6k collateral branches terminate (form synapses) at a single motoneuron.
excitatory or inhibitory information
Inhibitory Post-synaptic potentials (IPSPs)
It is the ____which determines if an action potential is triggered.
L4 ____, then an action potential will occur
L4 ____, an action potential will not occur.
L4 ____, an action potential will not occur.
net amount of overall positive excitation (depolarization) at a given point in time
If there are more EPSPs than IPSPs at a given point in time
If there are more IPSPs than EPSPs at a given point in time
If an equal number of IPSPs and EPSPs occur on a neuron at the same time
The CNS is composed of the ____
brain and the spinal cord
The brain is protected by the ____and the spinal cord is protected by the ____
cranium, vertebral column
relative to other animals, the human brain has a highly developed ____.
frontal cortex and larger visual systems
The frontal cortex is associated with ____
Examples: ____
executive functions
self-control, planning, reasoning, abstract thought
The human cerebral cortex is fairly symmetrical with respect to the ____
Each hemisphere is arbitrarily divided into 4 lobes: ____
Their names derive from the ____
left and right hemispheres.
frontal, parietal, temporal, and occipital
bones which overlie them.
1 exception: The border between the frontal and parietal lobes is shifted backwards to the central sulcus
central sulcus:
a deep fold that marks the boundary between the primary motor cortex and the somatosensory cortex.
From a functionality POV, a given brain region may contribute to ____
Example:
Example:
to specific nervous system activity.
The motor cortical on the right side of the brain (anterior to the central sulcus) control motor functions on the left side of the body and vice versa
the brain regions responsible for 3D spatial resolution (as in knowledge of your body relative to the surrounding environment) are in the right temporal cortex
Current understandings of the anatomy and physiology of the nervous system derive from ____
Recent techniques such as ___ have been used to associate an individual’s functional abilities with specific brain regions or to monitor abnormal functions in a given region.
studies on human motor disorders and experiments on animals in which specific lesions of focal injuries were placed in a given neuronal tract, brain nuclei, or brain center.
functional MRI (FMRI)
antagonist inhibition:
The Ia afferents of a muscle spindle (length sensor) in a given muscle in which the spindle lies in parallel with the muscle fibers (extrafusal) make excitatory synapses with the motoneurons innervating that muscle (agonistic excitation).
Through interneurons, this results in inhibitory synapses with motoneurons innervating the antagonistic muscles.
This is also called reciprocal inhibition.
The antagonistic muscles in this case receive forward inhibition. (see figure 1.11, left panel)
Feedback inhibition:
This occurs when inhibitory interneurons act on the cells by which they themselves were activated. (see figure 1.11, center panel)
lateral inhibition:
This is a form of feedback inhibition in which the inhibitory interneurons are connected in a way such that they act on the excited cell and neighboring cells with the same function. (see figure 1.11, right panel)
positive feedback:
Interneurons send excitatory signals back to the cells from which they received the signal, and possibly to neighboring cells with similar functions.
synaptic potentiation:
Repeated use of a synapse can cause considerable of the synaptic potentials.
These changes can be associated with biochemical changes within that given cell (e.g. phosphorylation of various proteins)
synaptic depression:
Postsynaptic potentials during or after a tetanic stimulation are smaller than those initiated by a single stimulus
circuits that include ____ are typically considered reflexes.
sensory pathways (afferent) and an effector response (efferent)
alt exp.: reflexes may also denote a complete neuronal circuit extending from the peripheral receptor to the CNS to the peripheral effects. (see fig. 1.12)
Generally, all ____are considered to participate in reflexes of some kind.
receptors
monosynaptic reflex:
This is the most basic reflex found in the CNS
It is also noted as the stretch reflex which is elicited by a skeletal muscle stretch
In this case, there is only one synapse in the CNS between the afferent and efferent neurons.
Specifically, the Ia afferents from the muscle spindle send collaterals directly onto alpha motoneurons as an EPSP which in turn causes action potentials; no interneurons are involved. Yet, there is also a synapse on the muscle fiber involved, known as the neuromuscular junction. (see fig. 1.13)
reflex facilitation:
Facilitation of the stretch reflex within the leg can occur by increasing the neural gain in that given circuit such as by increasing one’s voluntary activity within the arms (i.e. the Jendrassik maneuver)
Thus, via excitatory synapses coming from neurons associated with the arms, the neurons to be activated in these reflexes are closer to their threshold potentials for extended periods, and fewer additional excitatory inputs (e.g. from afferent activations) are needed to elicit reflex responses.
most reflexes within the human CNS are composed of ____
numerous neural networks with many interneurons (polysynaptic) with both convergent and divergent projections.
Except for the ___ stretch reflex, all reflex arcs in humans are ____
monosynaptic, polysynaptic
polysynaptic reflexes:
contain multiple interneurons between the receptor and effector
examples of polysynaptic reflexes:
suckling reflex cough reflex clonus or tremor flexor reflex cross extensor reflex scratch reflex
