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Harness as difference in electrical charge between the inside and outside of their cells. They are electrically active.
Charge difference of excitable cells
They are more negatively charged on the inside than the outside. This electrical potential difference is located immediately adjacent to the cell membrane.
Electrical activity requires two conditions
A selectively permeable membrane and a differential distribution of charged ions across the membrane
[Na+] inside and ouside
- Inside: 15mM
- Outside: 150 mM
[K+] inside and outside
- Inside: 150 mM
- Outside: 5 mM
Pumps 3 Na+ out of the cell for every 2 K+ in. Uses ATP and is not critical between every action potential, but needed in the long run to establish concentration gradient.
Na+ equilibrium potential
K+ equilibrium potential
Resting membrane potential and what influences it
- - It is -70 MV
- - Large diffusion of K+ out
- - Small diffusion of Na+ in
- - No diffusion of anionic proteins
Relative Ionic permeability
The ease with which an ion can travel across a membrane. The greater an ion's permeability, the greater the ion's influence on the membrane voltage.
Relative ion permeability of K+:Na+
50:1. So K+ is 50 times more permeable than Na+, because there are a greater number of open K+ channels at rest.
More positive inside the cell
More negative inside the cell
How can we depolarize a cell
- - Increase the permeability of the membrane to Na+
- - Decrease the relative permeability of K+
How can we hyperpolarize a membrane
- -Increase the permeability of the membrane to K+
- - Decrease the relative permeability of Na+
A graded potential whose amplitude and duration can vary depending on the strength and rate of the application or removal of the stimulus. The stronger the stimulus the greater the permeability change and the larger the generator potential. They have no refractory period so summation is possible. They must be converted into APs to travel long distances.
A depolarization increases the probability that the activation gate is ___
At rest most voltage-gated channels are in an available state, what does this mean?
- -The activation gate is closed and inactivation gate is open
- - the channel is non-conducting but is ready to be activated by a depolarization
Action potential threshold initiates a ____ loop
- Positive Feedback. The initial depolarization opens some avaiable Na+ channels. The Na+ influx results in further membrane depolarization, the more depolarized the
- better chance that the activation gate of available Na+ channels will be opened.
Channel Gating during Resting State
The activation gate of both channels is closed. The Na+ channel inactivation gate is likely open. (K+ has no inactivation gate)
Channel Gating during Rising Phase
Both the inactivation and activation gate of Na+ channel are open. The activation gate of K+ is closed.
Channel Gating during Falling Phase
The inactivation gate of Na+ channels has closed and the activation gate of K+ channels is open.
Channel Gating during After Hyperpolarization
The activation gate of Na+ channels is closed, and the inactivation gate remains closed. The activation gate of K+ remains open.
Going from apterhyperpolarization to resting state...
The Na+ channel inactivation gate opens and the channels is now int he available state. The closing of K+ activation gates results in the membrane returning to resting membrane potential
Absolute Refractory Period
The portion of the membrane that has just undergone an AP cannot be restimulated. This period corresponds to the time during which the Na+ gates are not in their resting conformation.
Relative Refractory Period
The membrane can be restimulated but requires a stronger stimulus than is usually necessary. This period corresponds to the time during which the K+ gates that were opened during the AP have not yet closed. (Na+ gates must be in resting conformation)
What happens during prolonged depolarization, eg. lethal injection?
- - Voltage gate Na+ channels will remain inactivated
- - Increased conductance of K+
- - AP generation would be impossible
- - lethal injection: high does of KCl elevates extracellular K+ levels = prolonged depolarization.
The passive movement of charge. Current enters the axon through ion channels, depolarizing that region of the membrane. the positive charge is attracted to adjacent negatively charged regions of the membrane, this spreads the depolarization.
Current leak during electrotonus
As current travels along the axon, charge leaks outwards across the membrane, aka electrotonic decay. Depolarization decreases as you move along the membrane.
The distance that an electrical even can propagate along a neuron is governed by:
The ratio of the Axial Resistance and the Membrane resistance. Increasing the diameter of the axon decreases axial resistance. Decreased axial resistance and/or a increase in membrane resistance increases the propagation.
Action potentials activate voltage-gated channels along axonal membranes to regenerate depolarization. "Boosting" with voltage-gated channels regenerates inward current and counteracts outward current leak in an unmyelinated axon.
What determines the rate of action potential propagation?
- - The diameter of the fibre
- - The amount of membrane capacitance
Diameter of the fibre
Increasing the diameter results in decreased axial resistance. This increases the flow of current which results in more charge per until time reaching neighbouring membrane segments. Neighbouring membrane segments reach AP threshold more rapidly.
- A capacitor is two conductive plates separated by an insulator. So capacitance refers to the distance between extracellular and intracellular fluid across the membrane. it is inversely proportional to the distance between the two membranes.
- -Less capacitance = less time required to charge the membrane to threshold, greater rate of AP firing
- Membrane capacitance can be reduced by increasing the thickness of the membrane with myelin.
Formed from schwann cells in the PNS and oligodendrocytes in the CNS. The myelin coating acts as an insulator to prevent current leakage. Myelinated fibres APs travel up to 50 times faster. Myelin has no channels so less energy is requires, less ATPase activity.
Between nodes of ranvier...(myelin)
- -The AP travels elecrotoncally - passively
- - The capacitance of the membrane is low
- - The charge time of the membrane is short
- - The membrane is resistance is increased
- - Depolarization propagates rapidly
At the node of ranvier
- - Active propagation of the action potential
- - Membrane capacitance is greater
- - the charge time of the membrane is longer
- - There is a concentration of Na+ channels
- - The action potential slows down
Where do synapses occur?
On the cell body and dendrite of the postsynaptic neuron
Steps in synaptic transmission
- 1. AP propagation in presynaptic neuron
- 2. Calcium enters into the synaptic knob due to depolarization at end of axon
- 3. Neurotransmitter is released into synaptic cleft by exocytosis
- 4. Neurotransmitter binds to postsynaptic receptors
- 5. specific ion channels open in the subsynaptic membrane.
What is the result of the binding of a neurotransmitter toa receptor
Induces a conformational change in the receptor, opening a channel pore. The resultant ion movement through the pore generates synaptic current. This generates a postsynaptic potential
Postsynaptic potential (PSP)
Is either excitatory or inhibitory. Determines whether or not the postynaptic neuron will fire. You can have temporal and spatial summation of PSP's. Involve the passive movement of charge so have no refractory period. They are graded.
Temporal summation of PSP's
You can add PSP's up until you have a strong enough depolarization to cause an AP.
Spatial Summation of PSP's
Taking information from the different branches (different synaptic clefts on dendrites that all go into the same cell body) and adding all the information up.
Excitatory synaptic transmission and EPSP
Usually requires the activation of more than one excitatory synapse to initiate an AP. EPSP is formed by the binding of excitatory neurotransmitters to bring Vm closer to AP threshold.
Bind to receptors that generate depolarizing PSP's to create an EPSP by bringing Vm to AP threshold. The most common one is glutamate. AMPA type glutamate receptor allws both Na+ and K+ ions through the open pore, this generates at EPSP with an equilibrium potential of 0.
Inhibitory synaptic transmission and IPSP
When inhibitory neurones bind to receptors that generate PSP's that keep Vm from reaching AP threshold. An IPSP will either move Vm away from AP threshold or hold it at ECl preventing any subsequent depolarization. It is centered around the soma.
Bind to receptors that generate PSP's and keep Vm from reaching AP threshold. A common one is GABA. GABA receptor gated channels allow the entry of Cl- ions generating and IPSP with a potential equal to ECl.
A specialized synapse between an alpha motor neuron and skeletal muscle. It innervates at least one muscle fibre - motor unit. It is always excitatory, and an end plate potential is always large enough to cause muscle contraction.
Neuromuscular transmission steps
- 1. AP in alpha motor neuron propagates to nerve terminal
- 2. It opens voltage gated calcium channels in the axon terminal
- 3. calcuim entry triggers the release of acetylcholine via exocytosis
- 4. Ach activates nicotinic receptors in muscle membrane. The activation of these receptors causes a larger suprathreshold end plate potential in the muscle fibre.
- 5. The EPP triggers and AP in the muscle cell memebrane (sarcolemma).
a bundle of axons situated outside the CNS
a bundle of axons travelling within the CNs
The forebrain subdivides into
telencephalon and diencephalon
The midbrain becomes
the hindbrain becomes
the metencephalon (pons) and the medulla. The cerebellum is an outgrowth of the hindbrain.
The telencephalon is composed of
the cerebrum and the basal ganglia
The site of the highest level of neuronal processing. Responsible for motor activity, sensory perception, conciousness, etc... cerebral cortex is outer layer of grey matter.
involved in motor control
Diencephalon is made up of
Thalmus and Hypothalmus
Important relay station for processing information going to the cerebrum.
Fundamental role in regulation autonomic bodily functions
The brainstem consists of...
Midbrain, pons and medulla. All levels of the brainstem contain tracts of white matter (Carrying info from cortex). The brainstem also contains the nuclei responsible for the function of the cranial nerves.
A group of cell bodies in called a ____ in the CNS and a ___ in the PNS
- - Nucleus in CNS
- - Ganglia in PNS
spinal cord---> brain
brain ---> spinal cord
- Receives and integrates all incoming sensory synaptic input. It forms the central core of the brainstem and has 3 functions:
- -Ascending: responsible for the generation and maintenance of arousal and consciousness.
- -Descending: Responsible for the generation and maintenance of muscle tone.
- - Respiratory and cardiovascular centers are located in the medulla.
Spinal Cord Segments
- -8 cervical
- -12 thoracic
- -5 lumbar
- -1 coccygeal
Are filled with CSF. CSF leaves the ventricular system via apertures between the cerebellum and the medulla.
Produces CSF constinuously
3 membranes on the outer surface of the CNS
- -Pia Matter
- -Arachnoid membrane
- -Dura matter
Highly vascularized, adheres to the surface of the brain and spinal cord. It faithfully follows the indentations of the surface. Innermost layer.
Fits loosely over the surface. Creates a space between itself and the pia matter called the subarachnoid space, beneath which is situated the CSF. It has a rich blood supply, and the CSF is resorbed back into the blood at this point.
Very thick and though, has a protective function. Outermost layer.
Functions of the CSF
- - Buoyancy effect : protects against the tendency of various forces to distort the brain.
- - Excretion of wastes
- - Transport of hormones
Carry information towards the CNS
Carry information away from the CNS
Peripheral Nervous System: Nerves
- Nerves are composed of bundles of axons
- -sensory (afferent)
- -motor (efferent)
- -innervated striated muscle fibres
- -have their cell body in the CNS (except autonomic)
- - release acetylcholine - which acts via nicotinic receptors at neuromuscular junctions
-have their cell body outside the CNS in sensory ganglia
- In the PNS are either sympathetic or parasympathetic.
- -There are 2 neurones in the pathway from the CNS to the peripheral organ.
- 1. Preganglionic
- 2. Postganglionic
Preganglionic autonomic neurone
- has its cell body in the CNS
Postganglionic autonomic neurone and the relation of its ganglion
- It is an autonomic ganglion in the periphery.
- -in the sympathetic system the ganglion is close to the CNS and releases norepinephrine.
- -in the parasympathetic system the ganglion is close to or actually within the target organ and releases acetylcholine.
Length of neurones in parasympathetic system
- Preganglionic is long
- Post ganglionic is short
Length of neurones in sympathetic system
- Preganglionic is short
- Post ganglionic is long
Functions of the ANS
- Controls vegetative systems (coordinates with endocrine system)
- Blood Pressure (HR, SV, resistance)
- GI motility
- Salt/Water balance
- Pupillary reflexes
- Sexual function
Control of ANS
- Reflexes at the spinal cord level
- medulla - within the brainstem
- prefrontal cortex
Catabolic effects of sympathetic nervous system
- increased heart rate, SV, and BP
- increased blood flow to skeletal muscle
- decreased blood flow to skin
- fight or flight response -release of epinephrine from adrenal medulla stimulates skeletal muscle glycogenolysis
Anabolic effects of parasympathetic nervous system
- decreased HR, SV, and BP
- increased GI motility
- Relaxation of sphincters in esophagus, stomach and bladder
- done by acetylcholine
Both sympathetic and parasympathetic systems are activated during intense conflict situations
originate inside the cranium and can carry afferent, efferent and ANS fibers
OLFACTORY - sense of small (sensory)
OPTIC - Vision (Sensory)
OCCULOMOTOR - moves eyeball medially (motor, voluntary) and constricts pupil (motor autonomic-parasympathetic)
TROCHLEAR - Moves eyeball (motor voluntary)
TRIGEMINAL - Mastiation (motor, voluntary) and touch, temperature and pain from face and head (sensory).
ABDUCENS - moves eyeball laterally (motor voluntary). III and VI work together by information carried in the medial longitudinal fasiculus.
FACIAL - Muscles of facial expression (motor, voluntary), lacremal and salivary glands (motor, autonomic -para) and tastebuds of anterior 2/3 of tongue (sensory).
AUDITORY - Hearing from cochlea and gravity, motion and position of the head from vestibular apparatus (sensory).
GLOSSOPHARYNGEAL - Pharynx swallowing (motor, voluntary), salivary glands (motor, autonomic-para), tastebuds in anterior 1/3 of tongue, monitors CO2 and O2 in blood by carotid chemoreceptors, and monitors BP by carotid sinus baroreceptors (sensory)
VAGUS - Pharynx swallowing, larynx phonation (motor, voluntary), slows heart rate and controls abdominal secretions (motor autonomic-para), sensory info from baroreceptors and chemoreceptors and from GI tract.
ACCESSORY - Swallowing and shoulder shrugging (motor, voluntary)
HYPOGLOSSAL - Tongue (motor, voluntary)
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
The sympathetic innervation of the head comes from which spinal nerves?
The upper thoracic spinal nerves
Preganglionics from the sympathetic system arise from...
All thoracic and the first and second lumbar segments
Parasympathetics are carried in which nerves?
Cranial nerves III, VII, IX, and X, and sacral 2,3 and 4.
Preganglionics of the parasympathetic system arise from...
sacral segments 2, 3,and 4.
The special senses are modalities carried by cranial nerves, they are:
- olfaction - I
- vision - II
- taste - VII and IX
- hearing and balance - VIII
The general (somatic) senses are detected from all parts of the body and transmitted to the CNS via:
IV (trigeminal) and all spinal nerves except C.1
Are transducers that convert one form of energy into another form. They detect stimuli and turn them into action potentials.
Types of sensory receptors
- photoreceptors - rods and cones of retina
- thermoreceptors - changes in temperature, central (hypothalmus) and peripheral (skin)
- nociceptors - pain
- mechanoceptors - mechanical stimuli
a depolarization of the peripheral, receptive portion of the sensory axon. It is cause by a sensory stimulus (except in rods and cones, the GP is a hyperpolarization).
The GP is similar to the EPSP in that:
- It can be graded in amplitude (the bigger the stimulus, the bigger the GP)
- doesn't cause the membrane to be refractory
- is not actively propagated
GP are different than AP's which
- are all or none
- cause the membrane to be refractory
- are actively propagated by regenerating themselves along the axonal membrane
The mechanism of a GP depends on the type of receptor, but in general it is due to:
the opening or closing of ion channels resulting in a depolarization
The GP of somatosensory mechanoreceptors:
- is a direct effect of mechanical stimuli on stretch sensitive channels
- is non selective and allows both Na and K to pass
- the net result is a depolarization
The GP's of nociceptors, photoreceptors and chemoreceptors
is a G-coupled mechanism that influences the channel indirectly
How is stimulus intensity coded?
- frequency coding
- population coding
the greater the intensity, the greater the frequency of action potentials in individual axons
with increased intensity, more individual receptors are recruited
Slowly adapting receptors
Important in areas where it is valuable to maintain information about a stimulus. They monitor static, unchanging stimuli such as maintained muscle length and maintained pressure (ruffini endings)
Rapidly adapting receptors
detect the onset of the stimulus or respond with a slight depolarization when the stimulus is removed. Detect change in time (vibration - pacinian corpuscle) or change in space (meissner's corpuscle)
- a rapidly adapting skin receptor
- contains concentric layers of CT wrapped around the peripheral terminal of an afferent neuron
- as the stimulus continues, the pressure energy is dissipated because the layers slip, the receptor no longer responds and you get adaptation.
Meissner's corpuscles, pacinian corpuscles and endings surrounding hair follicles are:
- rapidly adapting
- involved in discriminative touch
- meissner's are abundant in fingertips
merkel's endings and ruffini endings
- slowly adapting
- signal non-changing features of tactile stimuli - maintained pressure
how is pain and temperature detected?
- by receptors which are free nerve endings
- there is no associated capsule or specialization
what are the two ways peripheral nerve fibres are classified?
- 1. based on conduction velocity, uses A,B,C, designation
- -A: fastest conduction velocity, large diameter, myelinated
- -B: smaller diameter, ssi
- 2. based on the diameter and uses I,II,II,IV
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