Lesson 4: Peripheral Nervous System (PNS); Autonomic Nervous System (ANS) Flashcards
In the trunk and limbs, what are the peripheral nerves called? In the head and neck?
In the trunk and limbs: “somatic nerves”; in the head and neck, “cranial nerves”.
Where do sensory neurons travelling in peripheral nerves have their cell body? Where do their axons extend to, and synapse?
Dorsal root ganglion
Spinal cord
Motor neuron, interneuron, sensory neuron
Where are the cell bodies of motor neurons that run in the peripheral nerves?
Inside the spinal cord in the ventral horn
What typically is the mixed peripheral nerve composed of? Where are their cell bodies?
Bundles of sensory axons with their cell bodies in the dorsal root ganglion and (travelling in the opposite direction - goes up!) motor axons with their cell bodies in the ventral horn of the spinal cord - go down!
What is the term for an absence of reflex?
Areflexia
What is the term for diminished reflex?
Hyporeflexia
What are the three ways sensory info travels?
- Input to simple reflexes (monosynaptic reflex) - one sensory neuron that synapses on one motor neuron
- Input to complex reflexes - involve one or more interneurons, may stimulate complex motor responses usually in the spinal cord level
- Sensory signals enter the cord in neurons that form beginning of a sensory pathway to conduct sensory information to higher centers of the brain along “sensory pathways”
What is the term for the special neurons that produce basic motor patterns without voluntary activation?
Constantly active special neurons called central pattern generators
What would happen if you cut the (mixed) spinal nerve?
You would cut both motor and sensory axons. There would then be no sensory input and no motor output, therefore no reflex activity
Which segment is marked efferent fibres? Where do they go?
Ventral root (motor root) and contains only axons from motor neurons travelling away from the central nervous system
What is the mechanism that occurs when someone has Polio?
Motor nerve is damaged, the reflex is lost but there is still sensation travelling in the sensory nerve (because the damage is in the cell body and doesn’t affect the mixed part of the nerve).
Is a peripheral NS lesion
What is the function of the autonomic nervous system?
Carries info to and from the viscera (including blood vessels). There are central and peripheral components of the ANS
What is the peripheral nerve made of?
What does each axon do?
Bundles of axons from sensory and motor neurons. Each axon conducts its action potential to terminate at the end terminus and synapse on the next neuron or target organ.
What does a neuron depend on to fire?
The number and frequency of stimulation it receives from all sources ending on the dendrites, dendritic spines, and cell body
Whether those stimuli are excitatory or inhibitory
What are general senses, and special senses (+example)?
General: Common to many areas of the body and involve less elaborate receptors
Special: Have specialized receptors that are essentially little sensory organs - ex. vestibular function
What are examples of general senses?
The modalities of light (fine) and crude touch, pain, temperature, stretch (muscle, joint capsule, skin, and tendon), position sense from the limbs, two point discrimination, vibration, and stereognosis
What is the most common receptor? What does it do?
Bare nerve ending” that picks up signals from tissue damage or stretch and is registered as pain
What are the components of the sensory pathways GENERALLY?
Primary Neuron
Secondary Neuron
Tertiary Neuron
What is the start for the primary sensory neurons? Where does it end?
Begins at a sensory receptor - cell body is in the dorsal root ganglion.
The primary sensory neuron ends at the synapse to secondary neuron cell body inside the spinal cord
Where does the secondary neuron begin? Where does it end?
Where do those from the limbs/head + neck end in the thalamus?
Dorsal horn of the spinal cord.
They all end in the thalamus. Those from the limbs end in the VPL nucleus of the thalamus and those from the head and neck end in the VPM nucleus of the thalamus
Where does the tertiary neuron begin? Where does it end?
Begins in the thalamus and ends in the cerebral cortex (parietal lobe).
What type of receptor is the muscle spindle?
What roles does it have?
A mechano-receptor that plays an important role in posture, position sense, skilled activities, and muscle responsiveness
What does the muscle spindle monitor?
Muscle length (static) and rate of change in muscle length (dynamic)
What type of muscle fibre is the spindle? What is it attached in parallel with?
Is intrafusal muscle fibre and is attached in parallel with extrafusal (skeletal muscle) fibres
How are muscle contractions initiated and modified?
Through either the gamma-motor or the alpha-motor systems.
What happens when there is increased activity of one system for muscle contractions?
Increased activity of one system is reflexively accompanied by increased discharges in the other system, which causes the muscle to assume a new and appropriate length
What happens to extrafusal muscle fibres when alpha-motor neurons are simulated?
Causes the extrafusal muscle fibres to contract. Intrafusal fibres become slack and baggy, spindles lose their sensitivity to muscle length.
How is the altered spindle sensitivity corrected when alpha-motor neurons are stimulated?
Rubrospinal, vestibulospinal, and reticulospinal tracts reflexively discharge gamma motor neurons - straighten the intrafusal fibers by contracting the end portions of the intrafusal fibers. The repositioned spindles regain their sensitivity to muscle length.
How is a reflexive muscle contraction initiated?
Induce tension to spindles by shortening the ends of the intrafusal fibers through the activation of gamma-motor neurons alters the resting state
Once gamma-mediated contraction occurs, what happens to the spindle? What do the annulsopinal endings do?
Stretches the spindles, also increases sensitivity of spindles and their afferent fibers.
The annulospinal endings send a volley of action potentials to the alpha-motor neurons on type Ia sensory fibers, shortening the extrafusal fibers
What occurs once the muscle has contracted enough to decrease the stress on the centre of the intrafusal fibres?
Rate of the type Ia firing decreases, and extrafusal fibers cease contracting. New desired muscle length permits maintenance of equilibrium in which the activity in type Ia fibers is below threshold.
How do dynamic muscle responses begin? What does this cause in the alpha motor neurons, and result for the muscle?
Stretch of the muscle mass and/or the muscle spindle; the simultaneous distortion of the primary sensory nerve endings causes a surge of sensory input to the alpha-motor neuron - leads to the contraction of the muscle mass, shortening the muscle
In the dynamic muscle response, when do the dynamic afferents from the type Ia nuclear bag fibres stop?
When the intrafusal fibres cease stretching
How much do static afferent fibres of the muscle spindle fire? How much do dynamic fibres fire?
Static afferent fibers: fire constantly
Dynamic fibres: fire at a rate directly related to the rate of change in length
Where does the gamma motor neuron receive input from? (To help keep muscles responsive to gravity and changes in body position)
Reticulospinal and vestibulospinal tracts.
What is reciprocal inhibition?
Neuronal arrangement in which the stimulation of one group of neurons causes inhibition of the motor neurons to the paired (antagonistic) muscle through the spinal interneurons.
Ex. arm flexion
What it the term for when there is simultaneous contraction of opposing muscles due to abnormal reflex activity, thwarting movement?
Co-contraction
What is a motor unit?
A single alpha motor neuron and all the muscle fibres it innervates
Each motor neuron innervated a number of myofibres, creating a unit
Why do motor units vary in size?
Depending on the precision needed in that muscle.
Ex. Many small units, each able to do something different, giving a lot of precision to the contraction of the muscle OR few large units with little precision
What muscles have many small motor units? Large motor units?
Muscles in vocal apparatus; muscles of the hip
What does the strength of a contraction depend on?
Number of motor units stimulated at the same time.
What is this clause: If a sensory receptor (for example, vibration) is stimulated, that sensory neuron is the “only hope” that the signal will get to the spinal cord. Once the signal arrives at the spinal cord, the primary neuron will communicate with many neurons, but before that, the single sensory neuron is the “only hope” to receive the signal from its receptor. If many nearby neurons also sense that modality, then there are many signals, but each one is the only hope for its receptor.
“Only hope clause” - Important difference between the PNS and CNS
What is a symptom/sign?
Symptom: something perceived and reported by the patient, and not observable by the examiner - pain
Sign: Something that can be observed and measured - motor loss is a sign
When a peripheral nerve is damaged, how is it classified a lower motor neuron lesion?
Test the motor component. Calling it a LMNL distinguishes between loss of motor function due to damage of the peripheral (lower motor neuron) compared with loss of motor function due to CNS damage involving the UMN
What is this type of injury: Light compression to axon’s contents - may be reversible (for example, hitting your funny bone).
Neuropraxia
What is this type of injury: Severe as to cause complete severance of the axon AND disruption and misalignment of the Schwann cell sheath
Neurotmesis
What is this type of injury: - With heavier compression, the axon may be severed while the Schwann cell sheath and fibrous coverings remain intact
Axonotmesis
Any time an axon is severed, where will it regrow from?
Will grow again from the point of the injury at a rate of 1mm/day
What are the two steps in recovery from peripheral nerve lesions?
- Death of the lost (distal) piece: Wallerian degeneration from the point of injury distally and
- Regeneration: regrowth from the point of injury Guided by the SHEATH
What is the result of the sheath being severed but remaining aligned?
Sheath not aligned?
Sheath is aligned?
Sheath is severed but remains aligned: recovery is fairly good.
Sheath is not aligned, the axon has lost its guidance, loses its way and fails to reinnervate target
Sheath is aligned the axon grows to the target, reinnervates the muscle and begins to send impulses, maturation of the nerve and myelination occurs again.
