Physio Topic 6: Peripheral Nervous System: Efferent Division

peripheral nervous system

• all neural structures outside the brain and spinal cord

• provides links to and from the external & internal environment

• includes:

  • sensory receptors

  • peripheral nerves

  • associated ganglia

  • motor endings

efferent division

efferent pathways of the PNS transmit impulses from the CNS to effectors/targets in the body

somatic motor neurons

• targets are mostly skeletal muscles (voluntary)

autonomic neurons

• targets are mostly smooth and cardiac muscles & glands (involuntary)

autonomic division

• autonomic = self-governing

• mostly involuntary and unconscious control

• 2 branches: sympathetic & parasympathetic

  • work together

sympathetic branch

• speeds up heart rate etc.

• promotes intense activity

• mass activation (acts as a unit)

  • activates everything connected, simultaneously

• “fight or flight responses”

• fast & hard

parasympathetic branch

• slows heart rate etc.

• regular “at rest” functioning

• organs individually activated

  • not mass activation

• “rest & digest” functions

  • responsible for most of digestive system

autonomic control

• control centers in hypothalamus, pons & medulla

• some spinal reflexes (urination & defecation → when young)

• intrinsic part of our homeostatic mechanisms

• antagonistic control is common (via dual innervation)

  • ex. heart rate

autonomic pathways

• always 2 neurons in series

  • preganglionic neuron from CNS → autonomic ganglion

  • post ganglionic neuron from autonomic ganglion → target

autonomic pathways: sympathetic branch

• thoracic & lumbar origins

• short preganglionic and long postganglionic neurons

• ganglia in 2 ganglion chains alongside spinal cord + some collateral ganglia (not in chain)

autonomic pathways: parasympathetic branch

• brain stem and sacral origins

• long preganglionic and short postganglionic neurons

• ganglia near or on targets

• vagus nerve contains about 75% of parasympathetic fibers

  • at top (brain stem)

autonomic neurotransmission

• sympathetic pathway: always Ach in preganglionic & NE in postganglionic

• parasympathetic pathway: always Ach in preganglionic & Ach in postganglionic

autonomic synapses

• neuroeffector junctions - not typical synapses (between neuron + target)

• varicosities instead of axon terminals

  • still function like synapse + have branches = but more spread out, whole are over one junction → can effect many cells at once

• allows postganglionic neuron to affect a large target area

• multiple types of receptors allow neurotrans in the ANS to have diverse effects

autonomic synapse process

• neurotrans released into interstitial fluid but other processes are as much as usual

  • same, but broader area

• 1. action pot arrives at the varicosity

• 2. depol open voltage-gated Ca2+ channels

• 3. Ca2+ entry triggers exocytosis of synaptic vesicles

• 4. NE binds to adrenergic recep on target

• 5. recep activation ceases when NE diffuses away from the synapse

• 6. NE is removed from the synapse

• 7. NE can be taken back into synaptic vesicles for re-release

• 8. NE is metabolized by MAO

autonomic receptors (sympathetic branch)

• 2 main types: alpha +beta

• do dif things: inhib / exitatory

• dif organs have dif types

• dif types of receptors = produce dif signals despite sympathetic branch only releasing NE

autonomic receptors (parasympathetic branch)

• produces AcH at target end

• 2 types of recep

  • nicotonic (excit) = opens channel (respond fast)

    • activated = nicotine

  • muscarinic (either) = little slower

    • muscarine = activated

adrenal medulla

• central part of the adrenal glands linked to sympathetic nervous system

• secretes epinephrine (adrenaline) directly into the blood enabling mass activation of multiple organs

  • specific to sympathetic

somatic motor division

• “of the body”

• controls skeletal muscles

• one neuron not two, always excitatory (long) + myelinated

  • message needs to go fast → quick movements

• connects to muscle fibers at neuromuscular junctions covered by Schwann cells

  • nervous sys meets muscular sys

somatic v autonomic layout

• somatic motor reflex = single motor straight to target

• autonomic motor reflex = multiple neurons involved

somatic motor neurons and the neuromuscular junction

• form from several cells merging tg

  • muscle cell = muscle fiber

• neuromuscular junction = axon terminals, motor end plates on the muscle membrane, Schwann cell sheaths

  • jelly-like blob = prevents incoming action pot from effecting anything but motor end plate

• somatic motor neuron branches as its distal end

• also includes skeletal muscle fiber and motor end plate

neuromuscular junction

• motor end plate

• schwann cell sheath

• axon terminal

• mitochondrion

motor end plate

• at the neuromuscular junction

• high surface area = filled w/ nicotinic receptors

  • up + down memb

• basically a synapse → sets off muscle contraction

• parts: synaptic vesicle (ACh), presynaptoc memb, synaptic cleft, nicotinic ACh receps, postsynaptic memb is modified into motor end plate

neuromuscular junction

• an action potential arrives at the axon terminal, causing voltage-gated Ca2+ channels to open

• calcium entry causes synaptic vesicles to fuse w/ the presynaptic memb (exocytosis) and release ACh into the synaptic cleft

depol of muscle fiber at the neuromuscular junction

• the nicotinic challenge recep binds 2 ACh molecs, opening a nonspec monovalent cation channel

• the open channel allows Na+ and K+ to pass

• net Na+ influx depol the muscle fiber

• 2 binding sites (ACh molec not enough)

• binding = conf change = K Na out