Peripheral Nervous System Flashcards
Can neurons divide?
No, neurons cannot divide. As you age, neurons are lost. They have plasticity so that others can take over the job if needed.
Neuron body structure
- Neurons can be very long and extend throughout the body
- Longest living cell
- Largest volume of a cell
- Most similar to skeletal muscle cells due to ability to be very long.
Main regions of the neuron
- Dendritic zone- receives input
- Axon- conducts excitation
- Telodendritic zone- transmits output
Where is the decision to send the signal on made?
Cell body at the axon hillock
Cell body or perikaryon
Located within dendritic zone; associated with axon
Come in variety of different shapes and sizes
- Larger processes result in larger cell bodies
- Faster transmission requires larger axon and larger cell body
Dendrites
Highly branched processes
Telodendrites
- Highly branched region at the end of axon
- Have terminal expansions (bulbs) that store and release neurotransmitters
Why does a neuron have a very developed nucleolus?
Needs a lot of proteins/neurotransmitters for passing signal onward
Neuronal communication
Neurons are very interconnected. Will be connected telodrites to cell body, telodrites directly onto the telodrites of another neuron. (axodendritic synapse, axosomatic synapse, axoaxonic synapse)
Can be rerouted to another neuron if one dies/is lost
Types of neurons
- Multipolar
- Bipolar- found in sensory
- Unipolar- found in sensory
Supporting cells of the PNS
Neurolemmocytes
- Schwann cells
- Satellite cells
Schwann cells
- Ensheath or myelinate axons
- Less than 1mm in length so need many schwann cells to enclose entire length of axon
- In both non-myelinated and myelinated nerve fibers
Non-myelinated nerve fiber and schwann cells
Each schwann cell cell ensheaths a number of axons but there is no myelin sheath formed around each axon
Myelinated nerve fiber and schwann cells
- Each schwann cell ensheathes only one axon (through a overlapping of inner layers) in a myelin sheath
- Very small in diameter
Satellite cells
Encapsulate neuron bodies in ganglia
Satellite cells and non-myelinated ganglia
Encapsulate the neuron bodies in the ganglia
Satellite cells and sensory (cranio-spinal) ganglia
Form tight capsule around the ganglia
Satellite cells and autonomic ganglia
Form loose capsule around ganglia
Myelin formation in PNS
- Myelin sheath is interrupted at nodes. The nodes are covered by cytoplasmic processes of adjacent Schwann cells which have a continuous basal lamina
- Have internodes and paranodes
Myelin formation in CNS
- Nodes are not covered.
- There are thicker nodes and thinner internodes in CNS than PNS
- One oligodendrocyte can provide myelin sheath to as many as 50 axons
Components of Peripheral Nervous System
Includes both cranial and spinal nerves
- Have both roots (proximal part of nerve) and distal branches
- And ganglia
Nerve fiber
Made up of 1 axon, its associated myelin sheath, and neurolemmocytes
Afferent vs. efferent nerve fibers
Afferent
- Axons of unipolar neurons located in sensory (cranio-spinal) ganglia
Efferent
- Axons of multipolar neurons located in CNS or in autonomic ganglia
Somatic vs. visceral nerve fibers innervation
Somatic
- Innervate skin, skeletal muscle and joints
Visceral
- Innervate cardiac & smooth muscles, and glands
Neuron types in visceral efferent pathways
- Preganglionic neurons: cell bodies in CNS
- Postganglionic neurons: cell bodies in ganglia, non-myelinated axons having varicosities; NT release and diffuse from multiple sites, and postsynaptic specializations are not evident
Components of a nerve
A nerve is made up of thousands of axons either non-myelinated (ensheathed) or myelinated by neurolemmocytes
Parts:
1. Endoneurium- fibrocyte and collagen fibers surrounding neurolemmocytes
2. Perineurium- surrounds nerve fascicles. Has two layers: a inner cell layer & a fibrous layer
3. Epineurium- connective tissue surrounding multiple fascicles.
Layers of perineurium
- Inner cellular layer (epitheloid tube)
- Perineural epitheloid cells (squamous) arrange in concentric layers that are joined by tight junctions providing a cell barrier. Each layer is covered by basal lamina on both sides. The cells proliferate arounf some receptors
- Outer fibrous layer- collagenous connective tissue merges with epineurium
Ganglia
Localized enlargement of a nerve containing neuron cell bodies
Sensory ganglia
- Contain primary afferent neurons (unipolar neurons)
- Cell bodies are superficial and nerve fibers towards center
- Centrally placed nucleus with dispersed Nissl’s substances
- Satellite cells form tight capsule around neuron cell bodies
Ex. dorsal root ganglia (of spinal cord) and ganglia in cranial nerves
Autonomic ganglia
- Multipolar neurons in autonomic nerves
- Eccentric nucleus and marginally distributed Nissl’s substance
- Loose encapsulation by satellite cells
- Cholinergic presynaptic neurons synapse on postganglionic neurons
- Postsynaptic neurons can be cholinergic and adrenergic
- Synaptic vesicles have electron-dense core
CNS components
Includes the tube (brain and spinal cord)
Central processing unit that makes decisions on what to do
PNS components
- Brings sensory input to the CNS so the CNS can make the decision and then makes the decision happen
- Involves both the cranial and spinal nerves
Parts of the brain and spinal cord
Spinal cord
1. Cervical
2. Thoracic
3. Lumbar
4. Sacral
Brain
1. Forebrain
2. Midbrain
3. Hindbrain
Breakdown of the Peripheral Nervous System
- Somatic (skeletal muscles and skin)- conscious decisions
- Afferent (sensory)
- Efferent (motor) - Visceral (cardiac muscles, smooth muscles, glands)
- Afferent (sensory)- General system- pain
receptors in visceral
organs - Special system- senses
- Efferent (motor)- autonomic, unconscious control
- General system- pain
What makes up a spinal nerve?
- Dorsal root (sensory): somatic and visceral afferents
- Ventral root (motor): somatic and visceral efferents
Autonomic nervous system
separated into Parasympathetic (craniosacral) feed and breed & Sympathetic (thoracolumbar) fight or flight divisions
Sympathetic nervous system
- Fight or flight
- More generalized response
- Thoracic-lumbar T1-L5
- Post-synaptic neurons release norepinephrine
- Adrenergic: epinephrine, dopamine, isoproterenol
Parasympathetic nervous system
- Rest and digest or feed and breed
- More specific and discrete response
- Post-synaptic neurons release acetylcholine
- Cholinergic agents: nicotine, muscarine, caffeine, pilocarpine
- Anticholinergic: atropine
Organs that only have sympathetic innervation
- Sweat glands
- Pilo-erector muscles
- Uterus
- Kidney
