2.1 Flashcards
What are the 2 neurons that carry somatic motor signals from the brain to the effector?
Upper motor neuron (UMN): originate in brain and brainstem; form connections with lower motor neurons in the ventral horn of the spinal cord; stays in CNS
Lower motor neuron (LMN): located in ventral horn of SC; starts in CNS, axon carries signal to muscles and glands
Function of the ANS
Achieve homeostasis
Communication between body and environment
The 3 parts of the ANS
Sympathetic: fight, flight, or freeze
Parasympathetic: rest and digest
Enteric: digestion
Compare autonomic and somatic neurons
Autonomic: preganglionic and postganglionic neuron
- Ganglion: group of cell bodies in ANS
Somatic: upper and lower motor neuron
Sympathetic vs parasympathetic motor neurons
Sympathetic
- 90% of the preganglionic neurons are located in the lateral cell column, the rest are in the central autonomic nucleus and spinal interacalatus nucleus
- postanglionic neurons are located in the sympathetic chain ganglia
- norepinephrine, epinephrine, acetylcholine
Parasympathetic:
- preganglionic cell bodies are located in parasympathetic nuclei of 4 cranial nerves and lateral horn
- postganglionic cell bodies are located in various ganglia in the head and throughout the body
- acetylcholine is the principal neurotransmiter
Location of ganglia
Length of each neuron
Neurotransmitters
Sensory input to the ANS
There are no sensory autonomic neurons
Primary visceral sensory (feeling full; blood pressure) neurons carry signals to the ANS
These fibers travel with other structures to synapse with sensory ganglia of cranial and spinal nerves.
ANS connections in the brain and brainstem
The sensory signals travel to various nuclei of the brainstem
Nuclei in the brainstem relay messages to different areas of the thalamus and cerebral cortex, so that ANS motor signals are controlled on multiple levels
Functions of the SNS
Release sweat
Contraction of arrector pilli
Dilation
Vasoconstriction
Accelerates heartbeat
Increase force of contraction
Reduce peristalsis in alimentary canal
Dilate pupils
Contract smooth muscles to open eyelids
Increase peristalsis carrying spermatozoa
Relax smooth muscle for easier breathing
Release epinephrine
Neurons in the SNS
Short preganglionic neurons originate in the lateral horn of the spinal cord and exit in the thoracolumbar region to the sympathetic ganglia.
- the neurons traveling to the adrenal medulla travel directly to their target
The long postganglionic neurons originating in the ganglion chain then travel outward and terminate on the effector tissues.
SNS preganglionic fibers synapse on…
Sympathetic chain (trunk) at level of exit
Sympathetic chain (trunk) at levels higher or lower
A prevertebral ganglion
- Paravertebral: sympathetic trunk
Where do SNS postganglionic fibers travel?
Postganglionic fibers travel with other nerves and structures
- Re-enter spinal nerves to travel to muscles on skin structures
- Travel with cranial nerves
- Travel with arteries
- Travel to specific organs
- Travel within a plexus
SNS special fibers
All sympathetic nerves to the head and neck have cell bodies in the superior cervical ganglia
- Internal carotid nerves: sympathetic functions in the head
- External carotid nerves: sympathetic functions in the face
Regions of SNS ganglia
- Cervical
- Thoracic
- Splanchnic: visceral, GI, genitourinary (GU)
Ganglia give rise to nerves and plexuses which serve the organs of each region
PSNS functions
Constrict pupils
Contract to relax suspensory ligaments
Tear and serous secretion flow
Decrease heartbeat
Bronchioconstriction
Respiratory, pancreas, and alimentary canal gland secretion
Alimentary canal peristalsis
Relax sphinceter muscles
Facilitates bile release
Stimulates erection
Neurons of PSNS
Long preganglionic neurons
- Cell bodies in the: brainstem or spinal cord
- Axons exit: cranial or sacral
Short postganglionic neurons
- Cell bodies in ganglia
Synapses
- On the ganglia in or near the target organ
Motor components of the ENS
Motor control is largely though two plexuses:
- Meissner’s submucosal plexus
- Auerbach’s myenteric plexus
Neurotransmitters of the ANS
PSNS is considered primarily cholinergic, and SNS is considered primarily adrenergic.
All presynaptic neurons release Ach
Types of cholinergic receptors
Nicotinic receptors are on postsynaptic neurons of both PSNS and SNS
Muscarinic receptors are on target organs of PSNS
Types of andrenergic receptors
Alpha receptors respond to epinephrine > norepinephrine
- Vascular muscle and pupils of eyes
- Constrict or dilate
Beta receptors respond to norepinephrine > epinephrine
- Also respond to isoproterenol: common drug for asthma treatment
- Cardiac and bronchial cells
ANS control of urination
SNS: Relaxes bladder wall, contracts the urethral sphincters
PSNS: Contracts bladder wall, relaxes the urethral sphincters
Somatic: Contracts external sphincter
Neurogenic bladder
A common MS occurrence is a spinal cord white matter lesions above T12.
This results in a spastic bladder and loss of voluntary control of the external sphincter.
A spastic bladder initiates an emptying reflex at lower bladder volumes and is analogous to muscle spasticity and hyper-reflexivity seen in upper motor neuron disease.
This combination leads to urgency (depending on the degree of sensation that is still intact) and incontinence.
Horner Syndrome
Sympathetic trunk compression→
- Loss of sympathetic pupil dilation
- Loss of SNS tone to superior tarsal m.
