Chapter 11: Peripheral Nervous System Flashcards
what is peripheral nervous system
- includes all neural tissue outside the CNS
- provides communication between the CNS and the rest of the body
- has two divisions: efferent (motor) and afferent (sensory)
what are the parts of the efferent division?
1) autonomic nervous system
2) somatic nervous system
what is the autonomic nervous system?
- acts largely unconsciously to regulate involuntary processes including heart rate, blood pressure, respiration, digestion
- innervates most effector organs (i.e cardiac muscle, smooth muscle, glands)
- works with endocrine and behavioural state system to maintain homeostasis
two divisions of the autonomic nervous system
1) sympathetic nervous system = fight or flight
2) parasympathetic nervous system = rest and digest
what is the somatic nervous system?
- associated with the voluntary control of skeletal muscles and skin
what is the goal of the autonomic division?
- to ensure homeostasis is maintained
- one system always predominates depending on the state the body is in
at rest = the parasympathetic nervous system dominates.
body is excited or stressed = sympathetic nervous system dominates
describe the concept of dual innervation of the autonomic nervous system
- some effector organs are innervated by both divisions of the autonomic nervous system
- results in opposing antagonistic effects between the two autonomic divisions
- primary function is to help regulate organs and MAINTAIN HOMEOSTASIS
where does dual innervation not occur?
- sweat glands
- blood vessels
- adrenal medulla
what is the role of antagonistic control?
- necessary for homeostasis because it allows a physiological variable to either increase or decreases
- depends on dual innervation of a organ
- is used in autonomic branch of nervous system (one is excitatory and one is inhibitory)
- controls most internal organs
what does “fight or flight” mean?
- the sympathetic nervous system is most active during periods of excitation or physical activity
- the body prepares to cope with threatening situations
- heart rate increases, blood pressure increases, dilate pupils, energy stores are mobilized etc
what does “rest and digest” mean?
- the parasympathetic nervous system is most active during resting conditions
- both stimulates the digestive organs (digestion & absorption of nutrients) and inhibits the cardiovascular system (decreasing heart rate)
how does the autonomic nervous system maintain homeostasis?
- balances the activity levels of the parasympathetic and sympathetic nervous system
–> at low energy demands, heart doesn’t need to work hard because the body doesn’t demand blood = parasympathetic dominates
–> when body is active, energy demand increases and heart works harder = sympathetic dominates
how does the central nervous system regulate the autonomic nervous system?
- sensory information about the state of an organ or tissue is continuously relayed to homeostatic control centres in brain stem (hypothalamus, pons, medulla)
- the hypothalamus, pons and medulla act as an integrating centre. they take info and illicit a response
- the control centres tightly regulate bodily functions and regulate autonomic output (via sympathetic and parasympathetic systems) to maintain systemic homeostasis
- this means that heart rate, blood pressure, temperature etc are controlled and adjusted depending on the state of the body
what areas of the brain (CNS) regulate autonomic function?
- mainly the hypothalamus (known as “centre for homeostasis”), the pons and medulla
- these are called the integrating centre
anatomy of the autonomic nervous system
- consists of two efferent pathways containing 2 types of neurons (pre-ganglionic and post-ganglionic) that communicate between CNS and effector organs
- the neurons communicate via synapses in peripheral structures called autonomic ganglia
what is a visceral reflex?
- automatic changes in the functions of organs in response to changing conditions in the body
- the motor response of the body can be modified, facilitated, or inhibited by the CNS (hypothalamus, medulla, pons)
what is the concept of autonomic regulation (tone)?
- autonomic tone refers to the ongoing activity of the ANS when the body is at rest
- is the balance between sympathetic and parasympathetic activity
- tone can increase or decrease depending on the activity of the effector organ
what is the function of pre-ganglionic and post-ganglionic neurons?
- pre-ganglionic neurons travel from the CNS to the ganglia
- post-ganglionic neurons travel from the ganglia to the effector organs
–> usually a single pre-ganglionic neuron synapses with 8-9 post-ganglionic neurons
what is the autonomic ganglia?
- a cluster of synapses between pre-ganglionic and post-ganglionic neurons
- in each ganglion, there are axon terminals of pre-ganglionic neurons
- in each ganglion, there are cell bodies and dendrites of post-ganglionic neurons
how do sympathetic and parasympathetic neuron pathways differ?
1) the pathways point of origin within the CNS
2) location of the autonomic ganglia
in sympathetic division:
- most pathways originate in thoracic and lumbar regions of spinal cord
- ganglia is proximal to spinal cord
- short pre-ganglionic neurons that synapse with ganglia
- long post-ganglionic neurons that go to effector tissue
- stimulates tissue metabolism and alertness
in parasympathetic division:
- pathways originate in brain stem and leave via cranial nerves
- ganglia is proximal to effector/target organs
- long pre-ganglionic neurons to synapse with ganglia
- short post-ganglionic neurons to effector organs
- stimulates digestion and lowers HR
what is the role of the vagus nerve in the parasympathetic nervous system?
- about 75% of all parasympathetic neurons are running through the vagus nerve
- most parasympathetic innervation is to the head, neck and internal organs via the vagus nerve (originates in brain stem)
- sensory info comes from internal organs to brain and parasympathetic info goes brain to internal organs
what other nerves play a role in the parasympathetic nervous system?
- cranial nerve III (occulomotor)
- cranial nerve VII (facial)
- cranial nerve IX (glossopharyngeal)
- sacral spinal nerves
what are the 2 primary neurotransmitters in the peripheral nervous system?
- acetylcholine
- norepinephrine
what are neurons that releases neurotransmitters referred to as?
release acetylcholine = cholinergic
release norepinephrine = adrenergic
what specific neurons release acetylcholine and norepinephrine?
acetylcholine is released by:
- pre-ganglionic and post-ganglionic neurons of parasympathetic branches
- pre-ganglionic neurons of sympathetic branch
norepinephrine is released by:
- all post-ganglionic neurons in sympathetic branch
what are the two classes of cholinergic receptors? where are they located?
1) nicotinic receptors
- located on cell bodies and dendrites of sympathetic and parasympathetic post-ganglionic neurons
2) muscarinic receptors
- located on effector organs of parasympathetic nervous system (heart, smooth muscle of eye, digestive tract)
what are the two pathways of chemical signalling in the autonomic nervous system?
in SYMPATHETIC nervous system:
- ACh released from the pre-synaptic neuron binds to nicotinic cholinergic receptors on the postsynaptic membrane
- it is excitatory and depolarizes the post-synaptic neuron
- NE released from the post-ganglionic binds to the adrenergic receptors on the effector/target tissue
in PARASYMPATHETIC nervous system:
- ACh released from the pre-synaptic neuron binds nicotinic cholinergic receptors on the post-synaptic membrane
- it is excitatory and depolarizes the post-synaptic neuron
- then the post-ganglionic neuron releases ACh, which binds to muscarinic cholinergic receptors on the effector organ or target tissue.
- this action can either be excitatory or inhibitory
what is the difference between parasympathetic and sympathetic signalling pathways?
- parasympathetic pathways only use acetylcholine as the neurotransmitter
- sympathetic pathways use both acetylcholine and norepinephrine as the neurotransmitter
what if a neuron doesn’t secrete NE or ACh?
- it is called a non-adrenergic, non-cholinergic neiron
- the neuron secrete adenosine, ATP, NO or somatostatin instead
- the secretion depends on where their pre-ganglionic fibres leave the spinal cord
what are the receptors on effector tissues in the sympathetic nervous system?
- NE binds with adrenergic receptors
- all adrenergic receptors act via G-proteins
1) there is an alpha class of adrenergic
–> a1 = excitatory effect
–> a2 = inhibitory effect
2) there is a beta class of adrenergic
–> b1 = excitatory effect
–> b2 = inhibitory effect
–> b3 = stimulates fat cell lipolysis
what are the receptors on effector tissues located in the parasympathetic nervous system?
- ACh binds with cholinergic receptors
- all cholinergic receptors are G-protein coupled
–> muscarinic receptors are located on effector organs
–> the receptor is inhibitory or excitatory depending on the subtype
what is a neuroeffector junction?
- the synapse between an efferent neuron and its effector organ
- is the target region
how does a neuroeffector junction differ from a typical synapse?
- the post-ganglionic neuron do not have discrete axon terminals
- instead, neurotransmitters are released from several swellings along the axons called varicosities
- the varicosities contain vesicles with the neurotransmitter
how is a neurotransmitter released from a variscositie?
- when an action potential arrives at the varicosity, depolarization occurs and causes a voltage gated calcium channel to open
- this causes calcium to enter the cytosol and stimulate the release of the neurotransmitter via exocytosis
- the neurotransmitter is released over the surface of target cells and binds with its receptor on the target cell
- the neurotransmitter is terminated and removed from the synapse via diffusion away from the receptor, re-uptake or degradation by an enzyme
what enzymes can eliminate neurotransmitters?
1) acetylcholinesterase acts on acetylcholine to break down into acetate and choline
2) monoamine oxidase acts on catecholamines
what is the adrenal medulla?
- the inner part of the adrenal gland
- releases hormones (epinephrine and norepinephrine) that regulate your autonomic nervous system
- is the endocrine component of the sympathetic nervous system
how does the adrenal medulla secrete epinephrine into the blood?
- pre-ganglionic neurons innervate the adrenal medulla (located above the kidney) instead of synapsing on post-ganglionic neurons
- the medulla consists of modified sympathetic post-ganglionic cells (called chromaffin cells)
- when stimulated by the SNS, the adrenal medulla releases 80% epinephrine and 20% norepinephrine, and very small amount of dopamine
- these are secreted directly into the bloodstream to function as hormones and sent to target tissues
