Neurophysiology (Part 2) Flashcards
Describe the optic pathway from the retina to the CNS
Axons from the retinal ganglion cells form the optic nerves and optic tracts, these synapse in the lateral geniculate body of the thalamus where they then ascend to the visual cortex in the geniculocalcarine tract
Nerve fibers from the ____ field cross at the optic chiasm and ascend contralaterally whereas nerve fibers from the ____ field remain uncrossed and ascend ipsilaterally.
temporal
nasal
What is hemianopia?
loss of vision in half of the visual field of one or both eyes
What are the 4 planes in which lesions can occur in the optic pathway?
1) optic nerve
2) optic chiasm
3) optic tract
4) geniculocalcarin tract
Lesion in the optic nerve causes loss of vision where?
Complete blindness on the side in which the optic nerve was cut
Lesion in the optic chiasm causes loss of vision where?
Loss of vision in the temporal fields in both eyes
Lesion in the optic tract causes loss of vision where?
Loss of temporal field vision in the eye opposite the lesion and nasal vision in the eye on the same side of the lesion (left tract lesion = right temporal field loss and left nasal loss)
Lesion in the geniculocealcarine tract causes loss of vision where?
Loss of temporal field vision in the eye opposite the lesion and nasal vision in the eye on the same side of the lesion (left tract lesion = right temporal field loss and left nasal loss) with macular sparing
The human ear is sensitive to tones with frequencies between __ and ____ Hz
20 - 20,000 Hz
*most sensitive only to 5000 Hz
Sound intensity greater than __ dB cab damage the auditory apparatus and those greater than __ dB can cause pain
100
120
What is the fluid in the scala vestibuli and scala tympani called? The composition of this fluid is similar to what?
perilymph
extracellular fluid (high sodium)
What is the fluid in the scala media called? The composition of this fluid is similar to what?
endolymph
intracellular (high potassium)
What are the 2 types of receptor cells in the organ of Corti?
inner and outer hair cells
*there are more outer hair cells
What are the 4 steps in auditory transduction?
1) sound waves are transmitted to the inner ear and cause vibration of the organ of Corti
2) this vibration causes bending of cilia on the hair cells by a shearing force as the cilia push against the tectorial membrane
3) the bending of the hair cells produces a change in K+ conductance of the hair cell membrane (depolarization can occur)
4) Depolarization opens voltage-gated Ca2+ channels which causes a release of excitatory NT which activates the afferent cochlear nerves and information is transmitted to the CNS
Describe the structure of the membranous labyrinth
It contains 3 perpendicular semicircular canals and 2 otolith organs, both in which are filled with endolymph and are surrounded by perilymph
Rotation of the head to the left stimulates the ____ semicircular canals, and rotation of the head to the right stimulates the ____ semicircular canals.
left
right
What are 2 tests that can test the integrity of the vestibular pathway?
- Barany test
> person is rotated in a special chair and nystagmus is observed - Caloric test
> head is tilted back 60 degrees and then the ear is rinsed with warm or cold water to induce nystagmus
What does audition involve?
The transduction of sound waves into electrical energy, which can then be transmitted in the nervous system
How are sound waves produce?
With increases (compression) and decreases (decompression) in pressure
What is the usual range of frequencies in human speech?
300 to 3500 Hz
What does the external ear consist of?
the pinna and the external auditory meatus (canal)
What does the middle ear consist of?
the tympanic membrane and a chain of auditory ossicles called the malleus, incus, and stapes
What does the inner ear consist of?
a bony labyrinth that consists of 3 semicircular canals and a membranous labyrinth that consists of a series of ducts called the scala vestibuli, scala tympani, and scala media.
What is formed from the bony and membranous labyrinths of the inner ear?
the cochlea, which is a spiral-shaped structure comprised of 3 tubular canals or ducts that contains the organ of Corti
What is auditory transduction?
The transformation of sound pressure into electrical energy
Describe the process of sound makes as it travels to the inner ear
The external ear directs sound waves into the auditory canal which transmits the sound waves onto the tympanic membrane. When sound waves move the tympanic membrane, the chain of ossicles also moves, pushing the footplate of the stapes into the oval window and displacing the fluid in the inner ear.
What is considered the sensory transduction apparatus?
the organ of Corti
What structures in the inner ear are the sites of auditory transduction?
auditory hair cells
What cranial nerve serves the organ of Corti?
CN VIII the vestibulocochlear nerve
The frequency that activates a particular hair cell depends on what?
the position of that hair cell along the basilar membrane
Hair cells located nearest the stapes (base) respond best to ____ frequencies.Hair cells located at the apex of the basilar membrane respond best to ____ frequencies
high
low
What is the vestibular system used to maintain?
equilibrium or balance by detecting angular and linear accelerations of the head
Where is the vestibular organ located?
with the temporal bone, adjacent to the auditory apparatus (the cochlea)
What does the vestibular organ consist of?
a membranous labyrinth within the bony labyrinth
The semicircular canals are used to detect what?
Angular or rotational acceleration of the head. There are 3 canals, one in each plane
What is the enlargement at one end of each of the semicircular canals called?
an ampulla
Each ampulla contains vestibular hair cells, which are covered with a gelatinous mass called what?
a cupula
What is the significance of the cupula?
During angular acceleration of the head, the cupula is displaced, causing excitation or inhibition of the hair cells
The otolith organs are used to detect what?
linear acceleration (gravitational forces)
In which planes are the otolith organs activated?
In every one; because of the bilateral arrangement of the otolith organs, every possible orientation of the head can be encoded by excitation or inhibition of the vestibular hair cels.
What is nystagmus?
A vestibular reflex that occurs in response to angular or rotational acceleration of the head in which the eyes will initially move in the opposite direction of the rotation in an attempt to maintain a constant direction of gaze followed by a rapid eye movement in the same direction as the head’s rotation in an attempt to fix on a new position in space
Nystagmus is in the ____ direction of the head’s rotation.
same
What is postrotatory nystagmus?
If rotation is stopped abruptly, the eyes will move in the direction opposite that of the original rotation. During this period, the person tends to fall in the direction of the original rotation because they think they are spinning in the opposite direction.
What do posture and movement depend on?
A combination of involuntary reflexes coordinated by the spinal cord and voluntary actions controlled by higher brain centers
What is a motor unit?
A single motoneuron and the muscle fibers that it innervates
Movements requiring fine motor control, motoneurons innervate ___ muscle fibers
few
Muscles involved in large movements, motoneurons innervate ___ muscle fibers
thousands
What is the force of contraction of a muscle graded by?
The recruitment of motor units (size principle)
What fires first, small or large motor units? Explain…
Small motonuerons innervate a few muscle fibers, and because they have the lowest thresholds, they fire first.
Large motoneurons innervate thousands of muscle fibers, and because they have the highest thresholds they fire last.
Do large or small motoneurons generate the most force?
large
What does the size principle state?
As more motor units are recruited, progressively larger motoneurons are involved and greater tension will be generated
What are the 2 types of motoneurons?
- α Motoneurons
- γ Motoneurons
What do α motoneurons innervate?
extrafusal skeletal muscle fibers
What do γ motoneurons innervate?
specialized intrafusal muscle fibers, a component of the muscle spindles
What is the function of the muscle spindle?
to sense muscle length
What is the function of the γ motoneurons?
They adjust the sensitivity of the muscle spindles, so they respond appropriately as the extrafusal fibers contract and shorten
Why are the α and γ motoneurons coactivated?
So that muscle spindles remain sensitive to changes in muscle length even as the muscle contracts and shortens
What are the 2 types of muscle fibers?
- intrafusal
- extrafusal
What do extrafusal fibers constitute, what are they innervated by, and what is their main function?
the majority of skeletal muscle
α motoneurons
used to generate force
Can intrafusal fibers generate force?
No they are no large enough
How are muscle spindles arranged in reference to the extrafusal fibers?
parallel to them
What are the 2 types of intrafusal fibers present in muscle spindles?
- Nuclear bag fibers
- Nuclear chain fibers
Which of the 2 types of intrafusal fibers are more prevalent in the muscle spindle?
nuclear chain fibers
There are generally 5 or 6 nuclear chain fibers and only 2 nuclear bag fibers found in a muscle spindle
Describe the structure of nuclear bag fibers
They are larger and their nuclei are accumulated in a central “bag” region
Describe the structure of nuclear chain fibers
They are smaller and their nuclei are arranged in rows
Are muscle spindles innervated by sensory or motor neurons?
Both
What are the 2 types of sensory afferents that innervate muscle spindles?
Group Ia and Group II afferent nerves
What region of the muscle spindle does the single group Ia efferent nerve innervate?
The central region of both the nuclear bag fibers and the nuclear chain fibers
What region of the muscle spindle do the group II efferent nerves innervate?
The nuclear chain fibers primarily
Of the 2 types of sensory nerves that innervate muscle spindles, which has the faster conduction velocity?
Group Ia
What are the 2 types of γ motoneurons that innervate muscle spindles?
dynamic and static γ motoneurons
Dynamic γ motoneurons synapse on nuclear ___ fibers in “plate endings, whereas static γ motoneurons synapse on nuclear _____ fibers in “trail endings”
bag
chain
MOVE TO SLIDE 55
Which are faster α or γ motoneurons?
γ motoneurons of the intrafusal fibers are smaller and slower than the α motoneurons that innervate the extrafusal fibers
Muscle spindles are ____ receptors whose function is what?
stretch
to correct changes in muscle length when extrafusal fibers are either shortened or lengthened
Describe the events that occur when a muscle is stretched (i.e. muscle spindle reflex)
1) muscle is stretched
2) extrafusal fibers are lengthened and because they run parallel, the intrafusal muscle fibers are lengthened as well
3) this increase in length is detected by the sensory nerves that innervate them:
- group Ia afferent fibers detect velocity of change
- group II afferent fibers detect the length of the muscle fiber
4) activation of group Ia afferent fibers stimulate α motoneurons in the spinal cord which causes the muscle to contract because they innervate the extrafusal fibers
5) γ motoneurons are coactivated with the α motoneurons which ensures that the muscle spindle will remain sensitive to changes in muscle length even during the contraction
Spinal cord reflexes are _______ motor responses that respond to specific kind of stimuli.
stereotypical
What is the neuronal circuit that directs spinal cord reflexes?
the reflex arc
What does the reflex arc include?
- sensory receptors
- sensory afferent nerves, which carry info to the spinal cord
- interneurons in the spinal cord
- motoneurons, which direct the muscle to contract
What are the 3 types of spinal cord reflexes?
- stretch reflex
- Golgi tendon reflex
- flexor-withdrawal reflex
What is the simplest of all spinal cord reflexes?
The stretch (myotatic) reflex
What is an example of the stretch reflex?
the knee-jerk reflex
How many synapses are involved in the stretch reflex?
one
What is the stimulus for the stretch reflex?
stretch (lengthening) of the muscle
What type of sensory afferents are involved in the stretch reflex?
Ia
What is the response of the stretch-reflex?
contraction of the muscle
Describe the steps involved in the stretch reflex
1) the muscle is stretched
2) group Ia afferent fibers in the muscle spindle are activated and their firing rate increases
3) group Ia afferents enter the spinal cord and synapse directly on and activate α motoneurons which innervate the homonymous muscle
4) Activation of the α motoneurons causes contraction of the muscle that was originally stretched
5) this contraction decreases the stretch on the muscle spindle and the firing rate of the group Ia afferents returns to normal
- Information is also sent from the spinal cord to cause contraction of synergistic muscles and relaxation of antagonistic muscles
What is an example of the Golgi tendon reflex?
the clasp-knife reflex (which is abnormal)
**in reference to the way a pocket knife closes
What is Golgi tendon reflex aka?
the inverse myotatic reflex (opposite of the stretch reflex)
How many synapses are involved in the Golgi tendon reflex?
2
What is the stimulus for the Golgi tendon reflex?
contraction (shortening) of the muscle
What type of sensory afferents are involved in the Golgi tendon reflex?
Ib
What is the response of the Golgi tendon reflex?
relaxation of the muscle
Describe the steps involved in the Golgi tendon reflex
1) the muscle contracts and extrafusal fibers shorten, activating the Golgi tendon organs attached to them
2) group Ib afferent fibers are activated and synapse on inhibitory interneurons in the spinal cord
3) the inhibitory interneurons synapse on α motoneurons inhibiting them, producing relaxation of the homonymous muscle
4) as the homonymous muscle relaxes, the reflex also causes synergistic muscles to relax and antagonistic muscles to contract
When is the clasp-knife reflex apparent?
In patients with hypertonicity or spasticity
What is an example of the flexor-withdrawal reflex?
touching a hot stove
How many synapses are involved in the flexor-withdrawal reflex?
many
What is the stimulus for the flexor-withdrawal reflex?
painful or noxious stimulus
What types of sensory afferents are involved in the flexor-withdrawal reflex?
type II, III, and IV afferents
Describe the steps involved in the flexor-withdrawal reflex
1) painful stimulus occurs
2) groups II, III, and IV afferents are activated
3) these flexor reflex afferents synapse on multiple interneurons in the spinal cord
4) on the same side of the pain stimulus flexor muscles contract and extensor muscle relax causing a withdrawal from the noxious stimulus
5) on the opposite side of the pain stimulus extensor muscles contract and flexor muscles relax producing extension which helps to maintain balance
6) the contracted muscles remain contracted for a period of time after the reflex is activated because of a persistent neural discharge called an afterdischarge
What is the reflex called that produces extension on the contralateral side of the painful stimulus?
crossed-extension reflex
In summary, the flexor-withdrawal reflex produces flexion on the ____ side and extension on the _____ side
ipsilateral
contralateral
Describe vestibular hair cells
There is a large kinoclium and a cluster of stereocilia
If the sterocilia are bent toward the kinocilium what happens?
The hair cell depolarizes and there is an increased firing rate in the afferent vestibular nerves
If the sterocilia are bent away from the kinocilium what happens?
The hair cell hyperpolarizes and there is an decreased firing rate in the afferent vestibular nerves
What direction of head movement excites the utricle? What directions inhibit it?
forward or lateral tilt
tilting the head back or medially inhibit it
What direction of head movement excites the saccule?
head movements in all directions
Where do afferent nerves from vestibular hair cells terminate?
in vestibular nuclei of the medulla
Where areas do the vestibular nuclei of the medulla project to?
- Extraocular muscles
- Spinal cord motoneurons
- Other part of the brain stem
- Cerebellum
