Flashcards in Lesson 4: Peripheral Nervous System (PNS); Autonomic Nervous System (ANS) Deck (125):
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
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?
What is the term for diminished reflex?
What are the three ways sensory info travels?
1. Input to simple reflexes (monosynaptic reflex) - one sensory neuron that synapses on one motor neuron
2. Input to complex reflexes - involve one or more interneurons, may stimulate complex motor responses usually in the spinal cord level
3. 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?
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?
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).
What is this type of injury: Severe as to cause complete severance of the axon AND disruption and misalignment of the Schwann cell sheath
What is this type of injury: - With heavier compression, the axon may be severed while the Schwann cell sheath and fibrous coverings remain intact
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?
1.Death of the lost (distal) piece: Wallerian degeneration from the point of injury distally and
2.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.
Hos is Myasthenia Gravia a LMNL that is not a peripheral nerve injury?
Receptors on the postsynaptic membrane (on muscle) are destroyed by the patient’s immune system and there are a limited number of receptors to receive the signal.
Each impulse, there are fewer and fewer receptors available, eventually leading to “fatigue” of the synapse. Repetitive stimulation causes failure at the neuromuscular junction not along the nerve itself
What types of injuries are the nerves in the limbs vulnerable to?
-Compression from disc extrusion and excessive stretch
What types of injuries are sensory neurons susceptible to?
-Damage from disc protrusion at the root level
-Nutritional damage such as diabetic neuropathy at the root level.
What infectious organisms are alpha motor neurons susceptible to?
What symptoms doe the have?
-Nutritional damage such as diabetic neuropathy and
-Autoimmune diseases such as Myasthenia Gravis
Areflexia, rapid muscle atrophy, flaccid paralysis, and absence of muscle tone.
What two systems is the ANS broken into?
1. The sympathetic nervous system that responds to emergency an
2. The parasympathetic nervous system that maintains general body functions
What do neurotransmitters do?
Molecules released from neuron’s presynaptic membrane at the axon terminus into the synaptic cleft. They bind to the post-synaptic membrane receptors that in turn alter the resting potential of the postsynaptic membrane of the receiving neuron
What does acetylcholine involved with?
Neuromuscular junction, parasympathetic nerve endings.
Regulates voluntary or reflexive motor movements. Also regulates autonomic functions. Is primary NT of the ANS
What is nor-epinephrine involved with?
Sympathetic nerve endings, to spinal cord - influence is inhibitory
What is dopamine involved with?
Subbstantia nigra cells release it into basal nuclei (Parkinson’s Disease)
What does serotonin do?
Involved in very many functions including pain - send projections to lower brainstem and spinal cord
Where is GABA found?
In basal nuclei (reduced in Huntington’s Chorea)
What do enkephalins and endorphins do?
Pain modulation (reducing pain signals)
What does substance P do?
Important in pain
What are the two types of degenerative changes that follow axonal sectioning?
Axonal (retrograde), reaction and Wallerian (anterograde) degeneration.
Where do retrograde degenerative changes occur? Where does the injury exist?
Retrograde degenerative changes occur in the cell body in response to sectioning the axon. Axonal injury extends from the site of injury to the cell body
Where do Wallerian degenerative changes occur?
Wallerian degeneration, the degenerative changes occur in the axon region detached from the cell body
What is chromatolysis?
Swelling of the cell body and shifting of the nucleus from its central position to the periphery; starts with axon hillock and cell nucleus, followed by degeneration of Nissl bodies and the displacement of the cell nucleus from the center to the periphery of the soma
How do neuroglial cells react to cellular injuries and brain tissue necrosis? (2)
By multiplying in number (hyperplasia) and by increasing their size (hypertrophy)
How are neutrophils involved in lesions of the brain?
Infection fighting - scavenger white blood cells arrive at the lesion site within a few days of an injury
What do microglia do for a lesion in the brain?
Microglia are primary scavengers or nervous system. Their function is to phagocytose the cellular debris
Which structures are involved in regeneration of axons?
Schwann cells and fibroblasts contribute significantly to axonal regeneration in the PNS.
What is the process of axonal regeneration of the PNS?
Schwann cells fill the interval between opposing ends of the nerve fibre. Sheath of Schwann and endoneurial connective tissue, forms a tube from the proximal fibre end leading to the distal end - guides the growth of the peripheral axon.
What is the process of axonal regeneration of the CNS?
Axons severed in the CNS also undergo regrowth and sprouting similar to those in the PNS; but unknown factors prevent damaged neurons from reconnecting to the distal axonal segments and reinnervating their target structures.
In the hierarchy of motor function, what are the 4 anatomic levels?
In the hierarchy of motor functions, which structure is the first level in the regulation of sensorimotor functions?
In cortical control, which reflexes are activated first? Which system is operational at higher hierarchical levels?
How is general motor function in the spinal cord and brainstem organized?
Organized ipsilateral to its output and reflex input. Spinal alpha motor neurons and their axons innervate the muscles ipsilaterally. (applies to LMN injuries)
Dorsal horn - spinal cord: what kind of info does it carry, where to?
Contain secondary sensory nerve cells that receive bodily sensory info through dorsal root ganglia fibers
Ventral horn - spinal cord: what kind of info does it carry, where to?
Contain motor (lower) nerve cells, which project through the anterior roots to activate muscles, glands and joints
Where do the fibres of the dorsal and anterior roots travel before merging to form a spinal nerve?
Pia, subarachnoid space, arachnoid membrane, dura, intervertebral foramina
When the spinal nerve exits the intervertebral foramina, what does it divide into, and what are the functions of the (2) structures?
Dorsal ramus fibers: of each spinal nerve are concerned with the muscles and skin in posterior part of the body. Ventral ramus fibers: of spinal nerve supply anterior body parts – upper and lower limbs
Lower motor neuron cell body - why is it important?
LMN cell body provides the output pathway via its axon, which travels through the ventral root and peripheral nerves to innervate a skeletal muscle, where it forms multiple axon branches to activate many muscle fibres. LMN is final common pathway
(Alpha motor neuron)
What three bundles or longitudinal axons does white matter have?
Which direction and what type of fibres?
Dorsal - ascending sensory fibres
Lateral and Anterior - descending motor and ascending sensory fibre bundles
Lateral Corticospinal Tract: what parts of the body does it manipulate?
Where do the fibres cross the midline?
Where do they synapse?
Intervenes in control of skeletal muscles during delicate, skilled manipulation of distal parts of limbs, (fingers, toes)
Cross midline at lower medulla
Synapse on ventral horn alpha motor neurons
Anterior Corticospinal Tract: What parts of the body does it manipulate?
Where does it cross the midline?
Where do they synapse?
Regulates precision in movements of axial and girdle muscles
Fibers continue descending ipsilaterally.
Uncrossed fibers eventually cross midline before synapsing on ventral horn alpha motor neuron
Which extrapyramidal tract is this: regulates neck and body twisting movements with extensor support for startle reflexes in response to visual and auditory stimuli?
Which extrpyramidal tract is this: projects vestibular impulses to spinal LMNs. Regulates extensor muscle tone, fibers control the reflexive adjustment of the body and limbs to keep the head stable?
Which extrapyramidal tract is this: transmits impulses from red nucleus to spinal LMN to regulate muscle tone for limb extension and posture in support of the body against gravity?
Ascending tracts - autonomic pathways - what impulses do they serve?
Impulses are pain, thermal sensation, touch proprioception, kinesthesia
What two types of nerve cells does gray matter have?
Motor nerve cells, either anterior motor neurons or interneurons
What is the primary function of the gamma motor neurons?
Regulate the length of the spindle fibers and modulate the excitability of the annulospiral primary endings. This regulates the stretch reflex muscle tone and allows the CNS to regulate its own state of excitability
Where do gamma motor neurons leave through and where do they send projections?
What does this cause?
Leave though the ventral nerve root muscle fibers, stretching central parts of muscle spindles – sending projections to alpha motor neurons
Causes contraction of extrafusal fibres of the muscle
How are motor functions of the spinal cord influenced?
Influenced indirectly by descending impulses from motor cortex and brainstem motor centres.
What is the first function of the spinal cord?
Reflexive motor response
What are extrafusal/intrafusal fibres controlled by?
Alpha motor neurons Gamma motor neurons
What type of response do nuclear bad fibres mediate? Nuclear chain fibres?
Nuclear bag fibres mediate dynamic sensory responses, Nuclear chain fibres mediate static responses.
What type of receptors are golgi tendon organs? What do they innervate?
Sensory muscle receptors, innervate the tough tissues that attach muscles to bones
Stretch Reflex: what is it?
What it the process?
Is a reflexive muscle contraction in response to a stretching/lengthening of the muscle spindles.
Sensory inputs from stretched muscle spindles monosynaptically activate the alpha motor neurons, efferent fibre to muscle complete reflex arc and cause contraction of the extrafusal fibres. Reflexive contraction of the muscle restores it to a resting position, decreasing the sensory spindle impulses.
What to factors indicate pathology in the nervous system?
Reduced (hypoactive) or increased (hyperactive) quality of muscle reflexes
What is a clonus reflex?
Repeated contraction followed by relaxation of the muscle during a stretch reflex.
Which spinal cord disorder is this: a lesion at a specific spinal level; below that level, sensory and motor functions are impaired?
Which spinal cord lesion is this: involvement of axonal bundles may impair both sensory and motor systems?
What is Lower Motor Neuron Syndrome?
What are the clinical signs?
Lesions, peripheral nerve destruction can result in complete muscle weakness and total flaccidity. Muscle fibres are disconnected from motor efferents and thus cannot receive descending cortical impulses and reflexive sensory input. Affected muscle fibres gradually degenerate
Clinical signs = flaccid paralysis, absent reflexes, muscular fibrillation, atrophy of muscle. Occur unilateral to the lesion
Brown-Sequard Syndrome: Which side of the body does the lesion affect?
What is the Babinski sign response?
What are clinical signs?
Lesion in corticospinal tract on the ipsilateral half of the body: there is spastic paralysis in the right arm and leg (UMN symptoms). Babinski sign presence is abnormal, marked by extension of big toe by scraping sole of the foot
Ipsilateral discriminative sensory loss: loss of vibratory and discriminative sensation in the ipsilateral half of the body
Contralateral pain and temperature sensation loss: affects pain and temperature from the left (contralateral) side of the body below the level of the lesion
What are the symptoms that suggest a complete spinal cord lesion?
Paralysis and sensory loss bilaterally below the level of the lesion with spared functions above
What are the symptoms that suggest a spinal hemisection?
Ipsilateral loss of position and vibratory sensation below the level of the lesion, ipsilateral body paralysis, and contralateral loss of pain and temperature
What are the symptoms that suggest a peripheral or central lesion?
Paralysis and sensory loss affecting the same single limb suggest a lesion in peripheral nerve or in the cortex
Spinal Dura Mater: how many layers is it?
Does it have the cranial periosteal layer?
What is it attached to?
Single-layer meningeal membrane;
lacks cranial periosteal layer. Is rostrally attached to foramen magnum – opening in occipital bone through which spinal cord passes.
Spinal Arachnoid Membrane: Where does it start and extend?
What is it filled with?
-Begins at foramen magnum and extends to cauda equina.
Is filled with CSF;
Spinal Pia Mater:
What does it adhere to?
Which fibres pierce it?
How is it attached to the dura?
-Innermost layer surrounds and adheres to spinal cord. Fibers of dorsal and ventral spinal roots pierce the pia.
Is attached to surrounding dura mater by series of denticulate ligaments on both sides
What does the ANS regulate?
Involuntarily regulates visceral body functions by controlling the cardiac muscles, smooth muscles, and glands
How do efferents of the ANS travel?
Do they directly innervate the visceral organs?
ANS efferents travel to the target structures via the secondary ganglia, which are located in the PNS.
Do not directly innervate visceral organs - project neural impulses to the visceral organs through sympathetic and parasympathetic channels of ANS
In the ANS, what are the preganglionic/postganglionic neurons?
Preganglionic fibres project to a peripheral ganglion - brainstem and spinal cord. Postganglionic fibres regulate the activity of the specific visceral organ and glands.
Where do efferent fibres of the ANS travel and terminate in the sympathetic system?
Efferent fibres of preganglionic cells leave through the ventral spinal roots, travel through the white communicating ramus of each spinal nerve. Fibres terminate on the postganglionic neurons in the sympathetic chain or on other postganglionic sympathetic neurons in the abdomen or around aorta.
Which system is working when the following symptoms arise : heart rate accelerates, arterial pressure rises, blood sugar level increases, and blood flow diverts from visceral structure to skeletal muscles
Which system is working when the following symptoms arise: ctivities include decreasing heart rate, lowering blood pressure, constricting pupils, and increasing digestion
What does the parasympathetic nervous system do?
Restores spent metabolic energy.
Where do the efferents of the parasympathetic nervous system innervate?
Visceral structures in the head, neck, thorax, abdominal cavity, and pelvic organs
Which nerve is the largest source of preganglionic parasympathetic fibres?
In the visceral afferent system, where do -Sensory fibres from thoracic, abdominal, pelvic viscera travel and enter the spinal cord?
Travel through sympathetic nerves, reaching sypmathetic chain. Enter the T1-L2 region of the cord - ascend into dorsal lemniscus and anterolateral systems
Where do fibres from the bladder, rectum and accessory genital organs travel in the visceral afferent system?
Travel through splanchnic nerves and enter spinal cord through S2-S4 nerves
Which brain structure controls the ANS?
What does activation of the anterior/posterolateral hypothalamus stimulate?
Where do hypothalamic impulses travel in the central autonomic pathway?
Transmitted to the midbrain via a descending component of the medial forebrain bundle,
relayed caudally through synaptic relays in brainstem RF, convey impulses to visceral motor nuclei of brainstem and spinal cord