Peripheral Nervous System Tissue Flashcards
Neuron definition
Cells that respond to stimuli by altering the ionic gradient across their plasma membranes. Functional units of both the CNS and PNS.
Three main parts of neurons
Cell body (soma), dendrites and axon
Cell body function
contains nucleus and most of the cell’s organelles and serves as as the synthetic or trophic centre for the entire neuron
Cell body components
Numerous free polyribosomes and highly developed RER, indicating active production of cytoskeletal proteins for transport and secretion. Basophilic Nissl polysomes appear under a light microscope. Golgi apparatus, located in cell body, mitochondria found throughout the cell.
Dendrite function
principle signal reception and processing sites on the neurons. conduct electrical impulses to cell body.
Dendrite structure
Short branched projections leading to the cell body. Thinner dendritic processes as they bench, with cytoskeletal elements predominating in the distal regions.
Dendritic spine function
Most synapses on dendrites in the CNS occur at the dendritic spines, which are dynamic membrane protrusions on the dendritic branches.
What sort of structures are dendritic spines?
Plastic, as they change morphology depending on actin filaments (neural plasticity)
What is neural plasticity involved in?
Underlies adaptation, learning and memory development
What are axons?
Long slender projections that arise from the cell body and conduct electrical impulses away from the soma to other cells
Axon structure
Very long process, that can be up to a metre in length. Plasma membrane of the axon is called the axolemma and its contents are the axoplasm.
Where does the axon originate from?
The Axon hillock on the soma
What is terminal aborisation?
Axons have small branches called collaterals, which each end with an axon terminal (terminal bouton). This contacts another neutron at a synapse to initiate an impulse in that cell.
What do axons lack and how is this overcome?
Axons lack a protein synthesis machinery, so all the organelles and proteins required by the axon (neurotransmitters) must be transported from the cell body after synthesis.
Anterograde transport and the microtubule involved
Movement away from the cell body, kinesin
Retrograde transport and the microtubule involved
Movement along dynein towards the soma.
What is transported via anterograde transport?
organelles and macromolecules synthesised by the cell body
What is transported via retrograde transport?
Materials taken up by endocytosis, including viruses and toxins
Two types of polarity exhibited by neurons
Structural and functional
Explain structural polarity
One end of the neurone specialised to receive impulses (dendrites contain neurotransmitter receptors at synapses) the other end is specialised to send signals (axon terminals contain vesicles with acetylcholine)
Explain functional polarity
Unidirectional impulse propagation
How is neuronal form maintained?
By the cytoskeleton
Cytoskeletal components
microtubules, neurofilaments and microfilaments
Microfilaments function
Actin molecules allow changes in shape at dendritic spines, neural plasticity. Act as a scaffold for signal transduction systems
Neurofilament functions
Maintain axonal structure
What are Schwann cells + function?
A special form of glial supporting cell, provide axon insulation and maintain a constant ionic microenvironment.
Structure of myelin sheath
In myelinated neurones in the PNS, Schwann Cells wrap around the axon to form multiple layers of Schwann cell membrane called the myelin sheath, which is mainly lipid. The Schwann cell membranes fuse together to form the mesaxon.
What are the spaces between myelin sheath called?
Nodes of Ranvier
Function of nodes of Ranvier
Increase the speed of transmittance, as saltatory conduction can take place
Satellite cells structure
Form a thin glial layer around each large neuronal cell body in the ganglia of the PNS
Satellite cell functions
Trophic or supportive effect on the neurones, insulating, nourishing and regulating their micro environments
Difference in structure in unmyelinated axons
Still enveloped in Schwann cells, however they do not wrap many times to form a myelin sheath. Have a uniform distribution of voltage gated ion channels and no nodes of Ranvier so conduction is much slower
How are nerve fibres organised in nerve?
- individual cell wrapped in endoneurium
- group of cells with endoneurium wrapped in perineurium to form fasicles
- fascicles are grouped together and covered by one final epineurium
What are the coatings all examples of?
Connective tissue
Structure of each connective tissue
endoneurium- reticular fibres, scattered fibroblasts and capillaries
perineurium- flat fibrocytes, edges sealed together by tight junctions, form 2-6 layers
epineurium- dense irregular fibrous coat
Function of perineurium
Regulates diffusion, thus acts as the blood b nerve barrier
Structure of dorsal root
- white matter surrounding brown matter
- afferent root is the dorsal root, containing a dorsal root ganglia just before hand
- sympathetic soma present in ganglia
Sensory ganglia structure
contains large neuronal soma
associated with sheetlike extensions of small glial satellite cells.
cells supported by a distinct connective tissue capsule.
Autonomic ganglia structure
intramural ganglia- found present in certain organs, associated with parasympathetic neurones
paravertebral- found adjacent to vertebral column
Enteric ganglia
2 types -myenteric and submucosal plexuses
Sensory neurone terminal structure
axon terminals called terminal boutons are distal terminations of the telondendria of an axon
somatic motor neurone terminal structure
Neuromuscular junction that releases ACh onto a motor end plate. End plate is studded with nAChRs
sympathetic autonomic motor neurone terminal structure
axon contains many varicosities that release neurotransmitter
6 different types of sensory nerve endings
Hairs, Meissner corpuscle, Ruffini endings, Merkel discs, Pacinian Corpuscle and free nerve endings
Explain root hair plexus
A web of sensory fibres surrounding the base of hair follicles in the reticular dermis that detects the movement of hairs
Merkel cell structure and function
Each associated with expanded nerve endings which function as tonic receptors for light touch and sensing an object’s texture
Meissner’s corpuscle structure and function
elliptical structure consisting of a sensory axon among flattened Schwann cells arranged perpendicular to the epidermis in the dermal papillae.
Initiate impulses when light touch disturb the shape, enabling the mechoreceptor to transduce an impulse
Ruffini cell structure and function
collagenous fusiform capsules, anchored firmly to the surrounding connective tissue with sensory axons stimulated by stretch and torque
Pacinian Corpuscle structure and function
large, oval shaped structures found deep in the reticular dermis and hypodermic with an outer capsule formed of thin concentric lamellae of flattened Schwann cells and collagen. Surround a highly branched, unmyelinated axon. Stretch mediated sodium channels
sense coarse touch, rough textures, pressure and vibrations.
Also located in walls of bladder, where detects if bladder is full
Free nerving ending structures and functions
Found in the papillary dermis and extend into the lower epidermal layers which respond primarily to high and low temperatures, pain and itching
