PERIPHERAL NERVOUS SYSTEM Flashcards
Neurulation
Neurulation begins in the third week of development and continues into the fourth week. The principal result of neurulation is the formation of the neural tube and neural crest cells.
- Folding process
- Neural tube is the embryonic precursor to the CNS
- Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.
Chemoattraction and chemorepulsion in the developing CNS
Cell detects a chemical gradient and moves in that direction, neurons extend many axons within the body to connect it with the target organs
8 weeks of embryonic development
The spinal cord occupies the whole length of the vertebral canal
- Vertebral column grows faster, the caudal end of the spinal cord (conus medullaris) will shift gradually upwards
- In the lumbar region, the dorsal root ganglia have very long dorsal and ventral roots joining them to the spinal cord
Sampling of CSF
Done by lumbar puncture at L3/4
-Spinal cord terminates at L1/L2 so the needle cannot damage it
Somatic motor reflex
Involves sensory receptors called proprioceptors that monitor the position of limbs, body movement and strain on the musculoskeletal system
-Sensory input with the efferent nerve goes to the spinal cord
Autonomic motor reflex
Unconscious motor reflexes
Sensory (afferent) neurons
Contains two sets of dendrite-like processes, one in the periphery and one in the spinal cord
- Cell boy is on a t-junction off the axon in the dorsal root ganglion
- AKA sensory nerve fibres
Sensory receptors
Found in all connective tissues
-Hair follicle receptors are a hybrid form between free and (wrapped around the hair and transmits pain and temperature) encapsulated (non-neural) endings and they respond to hair displacement
Free nerve endings
Forms very fine nerve plexus in the dermis and many other tissues
- Responds to chemical stimuli
- Chemoreceptors
Function of free nerve endings
- Receptors for noxious stimuli are formed from axons with free nerve endings, which branch profusely in the tissue and forms a fine matrix
- If there is an injury in the skin cells, the contents will spill out into the extracellular fluid, due to the different contents of liquid within the cell and the extracellular fluid, the nerve endings will pick up and respond to the change
- Especially changes in PH
Glabrous (non-hairy) skin
Contains:
1. Free nerve endings (transmits pain)
2. Nerve endings surrounded by specialized connective tissues called Capsules which determines the kind of stimulus the nerve ending will be sensitive to (eg. pressure/frequency)
Exampless: Meissner’s, ruffini ending and pacinian corpuscles
Meissner’s corpuscle
Touch
Ruffini’s ending
Stretch
Pacinian corpuscle
Vibration
Merkel’s discs
Touch
Capsules
Made of connective tissue
- When the nerve fibre first grows into the tissue, it is unencapsulated
- Cytokines released from the bare end of the nerve fiber causes local connective tissue cells to form a capsule
- Capsules form a mechanical filter
Receptive field
The area of skin innervated by a single nerve fibre
- Receptive fields are used for tactile discrimination
- The smaller the RF, the more the discriminate
- The palmar surface of the fingers or outer parts of the lips have a smaller size RF, improving ability to localize stimuli
- RF fields are larger on the proximal limbs
- RF overlaps and the brain localizes a stimulus by processing the information from many fibres simultaneously. This ensures that damage to single fibre does not leave any region of the skin anesthetic
- Progressive loss of nerve fibres (diabetic neuropathy) leads to progressive worsening in ability to localize stimuli as there is less and less overlap
Two point discrimination
Two point discrimination is the ability to discern that two nearby objects touching the skin are truly two distinct points, not one. It is often tested with two sharp points during a neurological examination and is assumed to reflect how finely innervated an area of skin.
Mechanism of the two point discrimination
Convergence creates large receptive fields:
- Convergence of primary neurons alloss simultaneous subthreshold stimuli to sum at the secondary sensory neuron and initiate an action potential
- Two stimuli that fall within the same secondary receptive field are perceived as a single point because only one signal point goes to the brain hence is NO two point discrimination
Small receptive fields are found in more sensitive area:
- When fewer neurons converge, secondary receptive fields are much smaller
- The two stimuli activate separate pathways to the brain. The two points are perceived as distinct stimuli ajd hence there is two point discrimination
Homunculus
How the brain processes the body, representing the motor/sensory distribution along the cerebral cortex
- the brain maps each sensory receptor onto the cortex rather than considering the area of the body where the sensory is located
- More receptors=larger representation
Muscle spindle and golgi tendon organ
Muscle and tendons contains these two types of receptors
- Muscle spindles are stretch receptors that detect changes in the length of the muscle
- Golgi tendon organ is a proprioceptive receptor organ that senses changes in muscle tension
Responding to stimulus
- Receptor potential strength and duration vary with the stimulus
- Receptor potential is integrated at the trigger zone
- Frequency and duration of action potentials is proportional to stimulus intensity and duration
- Neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal
Myelin sheath
Individual sensory and motor neuron axons have a sheath of fatty insulation called myelin wrapped around them
- This produced by connective tissue cells, the Schwann cells
- Has low electrical capacitance, charge cannot be stored on it, this forces current to flow only at the nodes
- Multiple layers of lipid membrane provides an electrically insulating layer so current flow can only occur at the nodes of ranvier, which is also necessary for good conduction of action potential
- Demyelinating diseases of peripheral nerves damage the myelin sheath and block conduction of action potentials eg. Multiple sclerosis
Epineurium
A whole peripheral nerve consists of several fascicle bundled together with blood vessels surrounded by epineurium
Fascicles
Groups of functionally related nerve fibres are collected together into nerve fascicles
- Each fascicle is surrounded by perineurium
- Individual sensory or motor nerve fibres are surrounded by a thin protective membrane called the endoneurium
Node of ranvier
Where two sheaths meet there is a small gap
- contains Na+/K+ ATPases, Na+/ Ca2+ exchangers and voltage gated Na+ channels
- The depolarized region generates action potentials
- Neurons are electrically active at the node of ranvier
- Action potentials jumps from one node to another
How are encapsulated sensory axon endings activated?
They are activated by physical distortion of their terminal membrane
- They have mechanically sensitive sodium channels in their membranes
- When the axon is bent, it allows sodium ions to enter the cell which then becomes depolarized
- The depolarization is not an action potential, it is called a receptor potential which triggers an action potential
Frequency of action potentials
Weak pressure on the skin (in the case of slowly adapting receptors), bends the axon slightly and produces a small receptor potential which triggers action potentials at a low rate
Stronger pressure increases the frequency of action potentials; it takes less time to start each one.
The maximum frequency is limited by refractory period the axon
Intensity of a cutaneous stimulus is coded by the frequency of action potentials in itss sensory axon
Refractory period
The refractory period is a period of time during which a cell is incapable of repeating an action potential. In terms of action potentials, it refers to the amount of time it takes for an excitable membrane to be ready to respond to a second stimulus once it returns to a resting state.
Rapidly adapting receptors
Only respond at the beginning of a stimulus
- They fatigue after a second or so to a sustained steady stimulus
- Eg. Pacinian corpuscles and Meissner’s corpuscles
Slowly adapting stimulus
Will continue firing at a sustained stimulus but at a gradually reducing rate
- Ruffini’s endings
- Merkel’s disks
Speed of conduction
Linked to diameter of the fibers
-Velocity is 6 times the fiber diameter
Ganglia
Structures containing cell bodies of neurons and glial cells
- Posterior root ganglion contains all cell bodies from the sensory neurons
- Anterior root ganglion contains the motor pathway
- If the dorsal roots are severed between the dorsal root ganglion and the spinal cord, the sensory axons cannot regenerate into the spinal cord SPINAL PARALYSIS
Spinal nerves can be damaged as the point where they leave through the intervertebral foramina
Stenosis: Slipped disc, growth of bones in the vertebra, enlarged ligament
Pinched nerve: Compression of a nerve
Foraminal stenosis: Narrowing/tightening of the foramina
Referred pain
Pain perceived at a location other than the site of the painful stimulus
- Nociceptors from several locations converge on a single ascending tract in the spinal cord, pain signals from the skin are more common than pain from internal organs and the brain associates activation of pathway with pain in the skin
- Pain of the internal organs is therefore often sensed on the surface of the body, indicating distress
Neuromuscular junction
Chemical synapse between a motor neuron and a muscle fiber, allows motor neuron to transmit a signal to the muscle fiber, causing contraction
-Where nerve input is translated into contraction of the muscle
Activation of motor neuron
- Stretching stimulates sensory receptors (muscle spindle)
- Sensory neuron becomes excited
- Within the spinal cord, sensory neuron activates the motor neuron
- Motor neuron becomes excited
- The effector is the muscle that contracts and relieves the stretching of the muscle spindle
Peripheral nerve injury: Loss of ability of the brain to communicate with muscles and organs
When a peripheral nerve is cut:
- The distal part is disconnected from its cell body and it degenerates
- Distal Schwann cells unwrap themselves from the dead fragments and divide to form a continuous line of cells lining the distal endoneurial sheaths (embryonic state)
- The proximaal cut ends of the nerve fibers form growth cones and start to grow back down inside the sheaths, guided by chemical factors (cell adhesion molecules) on the surface of schwann cells
- Symptoms: loss of reflexes, hyposensitivity, muscle weakness, atrophy, areflexia
Peripheral nerve regeneration
Microtubules in the regrowing axon transport growth-related materials down to the growth cone
1. At the edge of the cone there are actin filaments which extend out in filopodia
- The tips of the filopodia attach to the tissue and then the actin contracts to pull the cone along towards the denervated tissue
- Behind the growth cone, schwann cells proliferate and start to wrap myelin around the nerve fibre. (initially thin and slow conducting, enlarges with time but never to original diameter)
Nerves may take months or years to regenerate and some may never, the further distal the injury, the more likely there is to be successful regeneration
