Neuroscience 8 - PNS Flashcards
Describe the structure of a sensory neuron.
- Pseudounipolar
- One long dendrite, cell body in the middle, then a long axon
- May be type A, B or C
List the types of sensory neuron.
- Free nerve endings (pain receptor neurons)
- Encapsulated nerve endings in the pacinian corpuscle
Describe the structure of the somatic nervous system
- Consists of efferent nerves responsible for stimulating muscle contraction
- The basic route of nerve signals begins in the primary motor cortex
- Voluntary or automatic
- Nerves terminate at the NMJ
Describe the pathway through the ventral and dorsal horns
- Sensory input enters the dorsal horn of the spinal chord through the dorsal root, which passes through the dorsal root ganglion
- Motor nerves enter the ventral root to muscles.
- Motor and sensory neurons mix in the mixed spinal nerve
- The mixed spinal nerve splits into the anterior and posterior ramus
How many pairs of PNS nerves are there?
12 cranial nerves
31 spinal nerves
Briefly describe the structure of the autonomic nervous system
- Efferent nerves and ganglia stimulate effectors outside our control
- Parasympathetic/sympathetic
- 1st neuron is from hypothalamic nuclei to brain stem/spinal cord
- 2nd neuron (preganglionic) to the autonomic ganglia
- 3rd neuron postganglionic to the visceral effectors
What are the types of nerve fiber?
- A fibers
- B fibres
- C fibres
Describe the structure and location of A nerve fibers.
- Largest diameter
- Thick myelin sheath
- Somatic (sensory and motor neurones to skeletal muscle)
Describe the structure and location of B nerve fibers.
- Medium
- Myelinated
- Visceral (sensory and autonomic preganglionic)
Describe the structure and location of C nerve fibres.
- Unmyelinated, smallest diameter
- Sensory and autonomic motor neurones.
Which factors affect conduction speed?
- Myelination
- Larger fibres conduct faster due to the increased size and reduced resistance.
- Temperature
What is a nerve plexus?
- A network of nerve fibres originating from different levels associated with an organ
- Formed from merging of anterior rami of the spinal cord and blood vessels
- Mainly innervate the limbs
- Thoracic ventral rami don’t form nerve plexuses
What is a dermatome?
- An area of skin supplied by a single sensory spinal nerve root.
- Stacked along the thorax and the abdomen, longitudinally across the limbs
- Used to assess spinal damage with a skin prick test
What are visceral afferents?
- They pass through the spanchnic nerves and plexuses
- This is to get to the spinal dorsal root - they are not a part of the ANS
What is a myotome?
- Muscles supplied by a single motor spinal nerve root
What is the clinical significance of myotomes?
They allow testing for root level muscle weakness.
What is a peripheral neuropathy?
Damage or disease affecting the PNS nerves
What symptoms will you get if you have damage to the sensory nerves?
Tingling, pain and numbness due to loss of sensation
What symptoms will you get if there is damage to the motor nerves?
Weakness to the hands and feet
What symptoms will you have if there is damage to the autonomic nerves?
Changes in heart rate/blood pressure
What are the two types of neuropathy?
- Mononeuropathy is where a single nerve is affected
- Polyneuropathy is where several nerves are affected
What are the causes of neuropathies?
- Metabolic
- Toxic
- Inflammatory
- Traumatic
- Genetic
- Idiopathic
Describe the structure of a PNS nerve.
- Endoneurium is around each neuron/axon
- Perineurium is around a fascicle (a bundle of neurones/axons)
- Epineurium is around the entire nerve
What is the perineuriums function?
It gives the nerve its main tensile strength
What us the composition of the epineurium?
- Dense collagenous
- Good blood supply
- Some fatty tissue
What is neuropraxis?
- A reversible block in conduction due to selective demyelination of the axon sheath
- The endoneureum and axon is still intact
- Often due to nerve compression
What is axontmesis?
- Demyelination and axon loss
- Epineurium and perineurium remain intact
- Still some continuity within the nerve
- Degeneration occurs below and slightly proximal to the site of injury
What is neurotmesis?
- The most severe form of nerve injury
- Associated with complete nerve division and disruption
- Commonly seen after toxic or ischemic injuries
- There is damage to the epineurium (around the entire nerve), so no nerve growth
What is wallerian degeneration?
- A process that occurs when a nerve fiber is cut or crushed and the part of the axon distal to the injury degenerates.
- This is also known as anterograde or orthograde
What is chromatolysis?
Dissolution of the nisl bodies in the cell body of a neuron
What are the uses of EMG in diagnosis?
It is used to distinguish between myopathies and muscle weakness due to nerve pathology
What is a nerve conduction study?
- A diagnostic technique where the speed of impulses is evaluated
- Assesses the peripheral nerves only
What is somatosensory evoked potential (SEP)?
- The site of a lesion is evaluated by assessment of amplitude and latency of responses
- Peripheral and central pathways
- Used in MS diagnosis
What spinal level innervates the upper limbs?
- The brachial plexus innervates the upper limbs (C5 to C8 and T1)
- Cervical nerves
Which section of the spinal cord innervates the lower limbs?
The lumbar nerves
True or false: A peripheral nerve may contain myelinated or unmyelinated fibres but not both
TRUE
What is the name for the supporting cells of the dorsal root ganglia?
Satellite cells
Describe how the mixed spinal nerve divides
- Dorsal ramus
- Ventral ramus
- Grey and white ramus communicans
What is the ventral ramus?
Contains nerves that serve ventral parts of the trunk and upper/lower limbs (somatic and visceral sensory and motor information)
What is the dorsal ramus?
Contains nerves that supply dorsal portions of the trunk (visceral/somatic motor and somatic sensory information to and from skin/muscles of back)
What are the white and grey ramus communicans?
- Convey autonomic signals of the sympathetic nervous system, communicating the spinal nerve and sympathetic trunk
- Gray ramus carry postganglionic nerve fibres from ganglia to their destination (and pregnaglionic that don’t synapse)
- White ramus carries preganglionic nerve fibres from the spinal cord to paravertebral ganglia
- Different colours due to differences in myelination
What is the difference between a ramus and a root?
- Rootlets combine to form roots
- Roots combine to form the spinal nerve, then split again to form rami
