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
