WEEK FOUR - ELECTROPHYSIOLOGY OF NEURONS, SPINAL CORD AND NERVES, SOMATIC REFLEXES

Question 1

Explain why a cell has an electrical charge difference (potential) across its membrane

Answer 1

Unequal concentrations of ions across a membrane = electrical charge
due to disparities in concentration/permeability

Question 2

Explain how stimulation of a neuron causes a local potential

Answer 2

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’

Question 3

Describe events involved in the generation of an action potential

Answer 3

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

Question 4

Describe what is meant by a refractory period

Answer 4

Period of resistance to stimulation

Absolute refractory period
= No stimulus will trigger AP

Relative refractory period
= Only strong stimulus will trigger new AP

Question 5

Outline 4 reasons why local potentials differ from action potentials [MDRE]

Answer 5

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

Question 6

Explain nerve signal conduction in unmyelinated fibres

Answer 6

1. Threshold voltage in trigger cone begins impulse
2. Nerve signal [impulse] - chain reaction of
   sequential opening of voltage-gated NA= channels down entire length of axon
3. Nerve signal [nondecremental] travels at 2m/sec

Question 7

Explain nerve signal conduction in myelinated fibres

Answer 7

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.

Question 8

Outline the 5 steps involved in synaptic transmission at an excitatory cholinergic synapse

Answer 8

1. Nerve signal opens voltage-gated calcium channels in synaptic knob
2. Triggers release of ACh which crosses synapse
3. ACh receptors trigger opening of Na+ channels producing local potential [postsynaptic potential]
   l
4. When reaches -55mV, triggers AP in postsynaptic neuron

Question 9

Explain how synaptic transmission can result in inhibition

Answer 9

1. Nerve signal arriving at synaptic knob triggers release of other neurotransmitters eg GABA (g-aminobutyric acid)
2. GABA neurotransmitter released in the same way as ACh binds to GABA receptors = opening of Cl- channels
3. Chloride flows into postsynaptic neuron down its concentration gradient = inside of cell more negative = hyperpolarization
4. Postsynaptic neuron now less likely to reach threshold

Question 10

List and describe 3 methods of stopping the stimulation of the postsynaptic neuron

Answer 10

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

Question 11

State the 3 principal functions of the spinal cord

Answer 11

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

Question 12

Describe the gross structure of the spinal cord

Answer 12

Cylinder of nervous tissue within vertebral canal [thick as finger]

Extends through vertebral canal STARTING from foramen magnum to L1

1. cervical/brachial plexus
2. thoracic region
3. lumbar/sacral plexus
4. medullary cone
5. cauda equinae

Question 13

List and describe the 3 meningeal layers of the spinal cord

Answer 13

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

Question 14

Describe the cross-sectional anatomy of the spinal cord

Answer 14

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

Question 15

Describe and illustrate the pathway of the spinothalamic tract

Answer 15

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]

Question 16

Describe and illustrate the pathway/s of the lateral corticospinal tract

Answer 16

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

Question 17

Describe the anatomy of nerves and ganglia

Answer 17

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

Question 18

State the branches of a spinal nerve

Answer 18

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

Question 19

Name the 5 nerve plexi and their major branches

Answer 19

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

Question 20

Define a reflex and explain how reflexes differ from voluntary movement

Answer 20

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

Question 21

List and describe the general components of a typical reflex arc

Answer 21

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

Question 22

Describe the structure and explain the function of muscle spindles

Answer 22

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]

Question 23

Explain and illustrate how the stretch reflex functions by using the patellar tendon reflex as an example

Answer 23

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]

Question 24

Explain how the Golgi tendon reflex functions

Answer 24

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

Question 25

Explain how the flexor withdrawal reflex functions

Answer 25

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

Question 26

Explain how the crossed extensor reflex functions

Answer 26

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

Question 27

Exemplify the combined functioning of the flexor withdrawal and crossed extensor reflex by using the example of stepping on glass with the right foot

Answer 27

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