WEEK FOUR - ELECTROPHYSIOLOGY OF NEURONS, SPINAL CORD AND NERVES, SOMATIC REFLEXES Flashcards
Explain why a cell has an electrical charge difference (potential) across its membrane
Unequal concentrations of ions across a membrane = electrical charge
due to disparities in concentration/permeability
Explain how stimulation of a neuron causes a local potential
stimulated by chemicals, light heat or mechanical disturbance
Opening of ligand-gated Na+. channels but not enough to stimulate the voltage gated Na+ channels
= depolarisation across cell membrane decreased
Na+ diffuses for short distance inside membrane = change in voltage called a ‘local potential’
Describe events involved in the generation of an action potential
More dramatic change in membrane - high density of voltage-gated channels occur [Trigger zone up to 5000 channels /μm^2 - normal is 75]
If threshold potential [-55mV] is reached
= voltage-gated Na+ channels open
[Na+ enters causing depolarisation]
past 0 mV [millivolts] Na+ channels close
K+ gates fully open - K+ exits cell
= repolarisation
Describe what is meant by a refractory period
Period of resistance to stimulation
Absolute refractory period
= No stimulus will trigger AP
Relative refractory period
= Only strong stimulus will trigger new AP
Outline 4 reasons why local potentials differ from action potentials [MDRE]
LP = variable in Magnitude
AP = either open or not
LP = Decremental [get weaker with distance]
AP = non-decremental
LP= Reversible as K+ diffuses out of cell - returns membrane voltage to resting potential
LP= can either be Excitatory or inhibitory
AP = always excitatory
Explain nerve signal conduction in unmyelinated fibres
- Threshold voltage in trigger cone begins impulse
- Nerve signal [impulse] - chain reaction of
sequential opening of voltage-gated NA= channels down entire length of axon - Nerve signal [nondecremental] travels at 2m/sec
Explain nerve signal conduction in myelinated fibres
Conduction of a nerve impulse= myelin is an excellent insulator with a high resistance to current flow.
Because myelin does not cover the nodes of Ranvier, current flows from one node of Ranvier to the next.
Outline the 5 steps involved in synaptic transmission at an excitatory cholinergic synapse
- Nerve signal opens voltage-gated calcium channels in synaptic knob
- Triggers release of ACh which crosses synapse
- ACh receptors trigger opening of Na+ channels producing local potential [postsynaptic potential]
l - When reaches -55mV, triggers AP in postsynaptic neuron
Explain how synaptic transmission can result in inhibition
- Nerve signal arriving at synaptic knob triggers release of other neurotransmitters eg GABA (g-aminobutyric acid)
- GABA neurotransmitter released in the same way as ACh binds to GABA receptors = opening of Cl- channels
- Chloride flows into postsynaptic neuron down its concentration gradient = inside of cell more negative = hyperpolarization
- Postsynaptic neuron now less likely to reach threshold
List and describe 3 methods of stopping the stimulation of the postsynaptic neuron
Diffusion
Some of released neurotransmitter molecules diffuse away from synaptic cleft
Enzymatic degradation
Enzymes break down neurotransmitters.
Reuptake of neurotransmitter
Many neurotransmitters are actively transported back into presynaptic neuron by endocytosis
State the 3 principal functions of the spinal cord
Conduction
Bundles of fibres passing efferent and afferent information spinal cord
Locomotion
Repetitive, coordinated actions of several muscle groups
Reflexes
Involuntary, stereotype responses to stimuli
Describe the gross structure of the spinal cord
Cylinder of nervous tissue within vertebral canal [thick as finger]
Extends through vertebral canal STARTING from foramen magnum to L1
- cervical/brachial plexus
- thoracic region
- lumbar/sacral plexus
- medullary cone
- cauda equinae
List and describe the 3 meningeal layers of the spinal cord
DAP acronym
Dura mater [outer]
- tough collagenous - filled w/ fat/blood vessels
Arachnoid mater [middle]
- simple squamous lining - filled w/ CSF
Pia mater [inner]
- attached to brain/spinal cord
Describe the cross-sectional anatomy of the spinal cord
Central area of grey matter = butterfly shape, surrounded by white matter in three columns
Grey matter = neuron cell bodies w/ LITTLE myelin
White matter = myelinated axons - carries signals to/from brainstem
Dorsal horns/root = sensory fibres/neurons
- cell bodies of INTERNEURONS [90% of neurons in brain]
Ventral horns/root = motor fibres/neurons
three columns
1. dorsal
2. lateral
3. anterior
Describe and illustrate the pathway of the spinothalamic tract
pain, pressure, temperature, light, touch, tickle
starts at sensory receptors [first order neurons]
decussates at spinal cord and travels up white matter on contralateral side to second order neurons
passes through medulla, midbrain, into cerebrum
and goes ^ ending at thalamus [contralateral side]
Describe and illustrate the pathway/s of the lateral corticospinal tract
TWO neuron pathway [carries movement related info from motor cortex –> spinal cord]
1. upper motor neuron
2. lower motor neuron
STARTS at motor cortex, goes DOWN through midbrain, pons, medulla
both pathways decussate in medullary pyramid
synapse with lower motor neuron in ventral horn of grey matter
ENDS at spinal cord
Describe the anatomy of nerves and ganglia
nerve = bundle of axons [long portion of neuron]
neuron = nerve cell w/ axon, cell body + dendrites
nerve covered in epineurium
ganglia = cluster of neuron cell bodies in PNS
dorsal root ganglion = sensory cell bodies [larger than ventral root
State the branches of a spinal nerve
proximal branches
dorsal root
ventral root
cauda equinae
distal branches
dorsal ramus
ventral ramus
meningeal ramus
rami communicantes - found at all levels of spinal cord
Name the 5 nerve plexi and their major branches
8 cervical [neck]
Hypoglossal nerve - controls tongue movement [allows more air input]
Phrenic nerve - innervates diaphragm [allows air input]
12 thoracic [armpit]
Median nerve - innervates four fingers except pinky
5 lumbar [lower back]
Femoral nerve - controls hip flexors, knee extensions
5 sacral [pelvis]1 coccygeal
Sciatic nerve - innervates gluteus, hamstrings, calf muscles
Define a reflex and explain how reflexes differ from voluntary movement
Predictable sequence of actions by glands or muscles in response to particular stimulus - do not occur in brain - but in spinal cord
Voluntary movement is under our control, can be slow/fast, uses higher/lower motor neurons and variable, not stereotyped
List and describe the general components of a typical reflex arc
Sensory receptor
Distal end of sensory neuron [dendrite]
Eg baroreceptor, thermoreceptor, nociceptor
Sensory neuron
Carries information from receptor to dorsal horn of spinal cord OR to brainstem
Integrating centre [interneuron]
Point of synaptic contact between neurons in gray matter of spinal cord/ brain stem
Motor neuron
Carries motor impulses from spinal cord –> skeletal muscles
Effector
Carry out reflexive response
[contraction/shortening]
In SOMATIC reflexes, effector = ALWAYS SKELETAL MUSCLE
Describe the structure and explain the function of muscle spindles
Stretch receptors embedded in skeletal muscles
Monitor length of muscle and how fast they change in length
composed of:
- sensory neurons [afferent info to brain on muscle length + speed of length change]
-gamma motor neurons [keep spindle fibres at good length for responding to stretch]
Explain and illustrate how the stretch reflex functions by using the patellar tendon reflex as an example
Patellar tendon reflex = monosynaptic reflex arc
impact on patellar ligament causes stretch in patella tendon
stretch detected by muscle spindle - found in quadriceps
muscle spindle stimulates sensory neurons that travel to spinal cord through dorsal root ganglion
sensory neurons synapse directly with motor neurons in ventral horn
excitatory motor neurons cause contraction of quadriceps [extensor]
reciprocal innervation = prevents muscles from working against each other
= inhibitory motor neurons cause relaxation of hamstrings [flexor]
Explain how the Golgi tendon reflex functions
Monitor tension in tendons produced by muscle contraction = prevents excessive muscle contractions/ uneven contraction
golgi tendon stimulated by increasing muscle tension
afferent sensory neurons sent to spinal cord through dorsal root ganglion
motor neurons supplying agonist [contracting] muscle are inhibited
motor neurons to antagonist [relaxing] muscle activated
Explain how the flexor withdrawal reflex functions
Protects from damage in response to painful stimulus
Nociceptor - pain - detects noxious stimuli
Ipsilateral as stimulus [eg stepping on glass] and response [muscle contraction] are on SAME side of body
Explain how the crossed extensor reflex functions
Flexor withdrawal reflexes only useful if body can still maintain balance which is maintained by extending other leg
Injured leg
Flexors [hamstrings] contract and extensors [quads] relax to lift leg from ground
Supporting leg
Flexors relax and extensors contract to stiffen leg so it can support weight of body
Exemplify the combined functioning of the flexor withdrawal and crossed extensor reflex by using the example of stepping on glass with the right foot
Ipsilatral Side [RF[
Stepping on glass w/ RF nociceptors of pain-sensitive neuron
Axon of neuron travels/ send afferent signals to spinal cord through dorsal root → dorsal horn and synapses on multiple INTERNEURONS and also decussates onto other side of spinal cord
EXCITATORY interneuron stimulates motor neuron - travels through ventral horn → ventral root to stimulate flexor muscle [hamstring]
= lifting leg away from painful stimulus
INHIBITORY interneuron inhibits alpha motor neuron to ipsilateral extensor muscles [quadriceps] - allowing knee joint to relax
Contralateral Side [LF]
EXCITATORY interneuron excites an alpha motor neuron - travels out of ventral horn → ventral root to ACTIVATE extensor muscle [QUADRICEP] of LHS = postural support
INHIBITORY interneurons synapse on alpha motor neuron, inhibiting activity in flexor muscle [HAMSTRING] of LHS leg
Withdrawal reflexes involve regulation of both ipsilateral and contralateral muscles
- reciprocal innervation to stimulate a flexor muscle and inhibit corresponding extensor
