Sem 2 Module 3 - The Autonomic Nevous System & CNS Neurotransmitters Flashcards
What does the somatic nervous system do?
Innervates skeletal muscles
What are impulses which lead to skeletal muscle contractions are along?
Pathways which descend from the brain:
- From the motor cortex for voluntary movements
- From other parts of the brain for involuntary movements e.g. from the respiratory centre in the brainstem.
What are efferent arms of reflex arcs in somatic nervous system?
- In spinal nerves e.g. for deep tendon reflexes
- In some cranial nerves e.g. for head-turning reflexes
What is the Autonomic nervous system?
- The ANS innervates smooth muscles, cardiac muscles and glands
- Every organ includes at least of these tissues (even skeletal muscle)
- In a skeletal muscle, striated muscle cells are innervated by somatic motor nerves,,
- But the smooth muscle cells of arterioles are innervated by autonomic motor nerves
Where are impulses which modify the function of smooth, cardiac muscle and glands carried on?
Pathways which descend from the hypothalamus
e.g. pathways from the hypothalamus (thermostat) to sweat glands
What are the efferent arms of reflex arcs in the autonomic nervous system?
- In spinal nerves e.g. leading to increase in heart rate when you feel sudden pain
- In some cranial nerves e.g. leading to constriction of the iris in response to bright light
What does the hypothalamus control?
Both the endocrine system and autonomic system
What is the effect of autonomic nerves?
- The functions of organ can be profoundly affected by changes in the activities of tissues such as smooth muscle
- The autonomic nervous system does not turn such functions “on” or “off”, but rather modulates them —- turns them “up” or “down”
What are the two divisions of the autonomic nervous system?
- Sympathetic
- Parasympathetic
What is the sympathetic division of the ANS?
- In general, the sympathetic nervous system is the “up” control for organs which are more active when you are frightened (“fight or flight”, acute stress), excited, or exercising
What is the parasympathetic division of the ANS?
- Is the “up” control for organs that are more active when you are relaxed, and promote build-up of body reserves e.g. digestive organs
What are autonomic effects on specific organs?
Major activities regulated by autonomic nervous system include:
- Heart rate —> Strength of contraction
- Local blood flow and blood pressure
- Digestive activity
- Respiratory airways resistance
What are the autonomic pathways?
In the somatic nervous system, a chain of neurons and their axons conveys signal from the motor cortex of the cerebrum to skeletal muscle.
Upper motor neuron:
- Motor cortex —> corticospinal tract —> anterior horn of grey matter
Lower motor neuron:
- Anterior horn of grey matter —> Anterior root of spinal nerve —> skeletal muscle
Thus, the pathway from the CNS to skeletal muscles consist of single neuron, the lower motor neuron and its axon
What are the two neurons that are in the pathway from the CNS to effector tissues?
- THE FIRST NEURON - Has its cell body in the CNS, with the axon reaching out of the CNS to an autonomic ganglion.
There it synapses with the cell body of —>
THE SECOND NEURON - From its cell body in an autonomic ganglion, the axon reaches out to its effector tissue- The first neuron is therefore termed presynaptic - The second neurone is postsynaptic
Where are the cell bodies of sympathetic presynaptic neuron located?
In the T1 and L2 segments of the spinal cord
In this region they make up what horns of grey matter?
Lateral
Lateral horns are only found between T1 and L2
What happens in the Sympathetic pathways?
Some postganglionic fibres run up from the topmost sympathetic ganglia into the head:
- this explains how the eye receives sympathetic innervation — no sympathetic fibres in cranial nerves
In the sympathetic nevous system, a single preganglionic neuron may synapse with many postganglionic neruons —-> single impulse in a preganglinoic neuron transmitted to a large number of effector tissues
- Thus, sympathetic impusles may be “broadcast” and produce generalized effects.
What is the neurotransmitter between sympathetic ganglionic fibres and their effector tissues? and the transmission?
Neurotransmitter —> Noradrenalin
Transmission —-> Adrenergic
What is the exception – when a neurotransmitter between sympathetic postganglionic fibres and sweat glands?
It is Acetylcholine
What is the adrenal medulla controlled by?
Hypothalamus
What does the adrenal medulla do?
Its acts in some ways like a sympathetic ganglion. It receives sympathetic preganglionic fibres, which synapse with medullary cells. These cells in turn, like sympathetic postganglionic neurons, release noradrenalin
What is noradrenalin secreted with?
Adrenalin
What is noradrenalin and adrenalin secreted into?
Adrenalin and noradrenalin are secreted not onto specific effector cells but into the bloodstream
So therefore what else does the adrenal medulla act as?
An endocrine gland
Where are the cell bodies of parasympathetic preganglionic neurons in the CNS located in?
- Grey matter nuclei in the brainstem
- Grey matter of the spinal cord in sacral segments (S2-S4)
What can neurons of the first type do?
- Receive “instructions” from the hypothalamus in the form of descending impulses
- Receive afferent imulses which enter the brain through cranial nerves, or which ascend through the spinal cord
- Such impulses may serves as the afferent arms of cranial parasympathetic reflexes, with the parasympathetic preganglionic neurons serving ast he efferent arms.
What can neurons of the sencon type, like sympathetic preganglionic neurons do?
- Receive “iinstructions” from the hypothalamus in the form of descending impulses
- Receive afferent impulses which enter the spinal cord
- Such impulses may serve as the afferent arms of spinal parasympathetic reflexes
What happens in parasympathetic pathways?
- The axon (fibre) of a parasympathetic preganglionic neuron leaves the skin in a cranial nerve (or leaves the spinal cord in a sacral spinal nerve)
- Axon of a parasympathetic preganglionic neruons runs all the way to it effector organ.
- Synapses with the postganglionic neuron in a ganglion - close to or within the organ itself
- The neurotransmitter at these synapses is acetylcholine
- Transmission is cholinergic
The axon of the postganglionic neruon then runs a short distance to its effector tissue within that organ
- Thus, the parasympathetic preganglionic fibres are long, and postganglionic fibres are short
The neurotransmitter between parasympathetic postganglioinic fibres and their effector tissues is acetylocholiine
- and the tranmitter is cholinergic
Which cranial nerves carry parasympathetic fibres?
III (Oculomotor)
VII (Facial)
IX (Glossopharyngeal)
X (Vagus)
What are the sympathetic effects?
- Exercising, excited or frightened —> more efferent impulses in sympathetic pathways (fewer in parasympathetic)
- Sympathetic system serves as “up” control enhanced e.g. heart rate
- “Down” control diminished e.g. blood flow through the skin
- The overall effect is to mobilize the body for action
What happens when sympathetic pathways are activated?
The effects are widespread
- Single preganglionic neruon may synapse with many postganglionic neurons —> large number of effector tissue “broadcast”
- The release of adrenalin and noradrenalin from the adrenal medulla into the blood —> “Sympathetic” stimulation even if no direct postganglionic neurons
- Effects of direct sympathetic stimulation intensified and prolonged
What are the parasympathetic effects?
When you are happy and relaxed, the hypothalamus sends more efferent impuses down parasympathetic pathways (and fewer down sympathetic pathways)
- Functions for which the parasympathetic system serves as an “up” control will be enhanged e.g. digestive activities
- Functions for which it is a “down” contorl will be diminished e.g. heart rate
- The overall effect is to build up your resources such as nutrient storage
What is one specific parasympathetic pathways?
Via vagus nerve (X)
- Heart —> Reduced rate of contraction
- Lungs —> Bronchoconstriction
- Digestive tract —> Increased digestive functions
What is another specific parasympathetic pathway?
Via oculomotor nerve (III)
- Eye —> Pupil constriction, lens thickening for near vision
What is another specific parasympathetic pathway?
Via facial nerve (VII) & Glossopharyngeal (IX)
- Salivary glands —-> increased secretion of saliva
What is another specific parasympathetic pathway?
Via nerves from sacral segments of spinal cord
- Bladder —-> Urination
- Rectum —-> defecation
- Genitals —-> Erection of clitoris, penis (increased blood flow in erectile tissue)
What are the autonomic transmitters and receptors?
- There are multiple types of receptors in the autonomic nervous system
- Pharmacologists produce drugs - selective for only one type of receptor
- Selectively affect one organ while leaving others alone
- Drugs that stimulate receptors - Agonist
- Drugs that inhibit recptors - Antagonists
What about the neurotransmitter at all autonomic ganglia is acetylocholine?
- All postganglionic neurons have nicotinic recptors for acetylocholine
- Such receptors are named after nicotine, a drug which act as an agonist at these recptors (activates them)
The parasympathetic system uses acetylocholine as its tranmitter at effector tissues
- Muscarinic receptors for acetycholine
- Named after muscarine, a drug which acts as an agonist at these receptors
What are examples of different receptors that account for different actions of transmitters and drugs?
Beta blockers –> inhibit receptors in both the heart and lung bronchioles
- Heart has beta 1 receptors: beta blockers lower blood pressure by slowing the heart
- Lung has beta 2 receptors: beta blockers constrict the bronchioles, making it harder to breathe
Propranolol —> blocks both types of beta receptor and hsould not be used to lower blood pressure in a person who is asthmatic
Metoprolol —> can be used to slow the heart without endangering asthmatic because it is specific for beta 1 and does not cause constriction of the bronchioles
What are neurotransmitters?
Neurotransmitters - Chemicals that allow one neve cell to talk to another nerve cell - can also be found in the brain and spinal cord —- the central nervous system
What are CNS neurotransmitters?
- Our whole brain consists of about 40 different types of CNS neurons (classified by the type of neurotransmitter)
What is the classification of neurotransmitters?
Amino Acids
- Excitatory: Glutamine (Glu), aspartate, cysteic acid, homocysteic acid
- Inhibitory: Glycine, Y-aminobutyric acid (GABA), alanine, taurine, serine
Monoamines
- Noradrenalin (NA), adrenalin (Adr), dopamine (DA), serotonin, histamine
More classification of neurotransmitters:
Acetylcholine (Ach)
Neuroactive Peptides
What are Amino Acid Peptides?
Inhibitory transmitter – e.g. GABA
- Important in interneurons of spinal cord, cerebellum & hippocampus
- Involved in motor control, spasticity & sleep/wakefulness
- We need inhibitory control to avoid excessive excitation (e.g. seizures & epilepsy)
What are Monoamines?
Monoamines are a class of related chemicals - Noradrenalin - Adrenalin - Dopamine - Serotonin Diverse neural circuits - Autonomic arousal - Cardiorespiratory control - "Reward" - Affect - Motor control - 5-HT involved in cognition, behaviour, sleep/wake cycles, mood, vomiting and pain (migraine) Implicated in mental illness e.g. Schizophrenia & depression
What is Acetylcholine (Ach)?
Ach is widespread in the PNS
In CNS
- Reticular formaiton
- Basal forebrain
- Basal Ganglia
- Anterior Spinal Roots
What are the roles of Ach in the PNS?
- Cognition
- Memory
- Consciousness
- Motor control
Low levels in Huntington’s Disease and Denentias such as Alzheimer’s disease
Muscarinic and Nicotonic receptors in the CNS
What are neuropeptides?
E.g. Vasopressin, oxytocin, encephalon’s and endorphins (opioids)
- Formed into he cell body
- May cause excitation or inhibition of target neurons
- May have potent behavioural effects when injected parenteral or intracerebrally
- Difficult to identify their functions
B Endorphins
Neuropeptide Y
Vasoactive intestinal peptide (VIP)
Substance P
What are the behavioural effects of Neuropeptides?
Huge range of behavioural effects:
- Hunger/satiety
- Sleep/wake cycle
- Temperature regulation
- Pain modulation
- Reward circuits
What are neurotransmitter imbalances in disease state?
In many CNS disorders appears to be imbalances the levels of neurotransmitters in particular areas of the brain
How do we know this?
From post-mortems - tracts and neurons have degenerated in particular areas
- Effects of monoamines “balance” Ach
- Motor control
- Mood (affect)
- Thought process
Simplistic - highly complex and not well understood
What are neurotransmitter imbalances in disease state examples?
Depression - deficiency of NA and 5-HT in areas of the brain related to mood and an excess of Ach
- Antidepressant drugs - SSRIs, TCAs & MAOIs increase the levels of monoamines
Parkinson’s disease - damage to the DA-containing neurons and a deficiency of DA or excess of Ach
- Drugs raise levels of DA or block actions of Ach (atropine drugs)
Schizophrenia - Ach levels = low, DA levels = High
Mania - Ach levels = low, Glu, NA & DA = high
Dementia - Ach levels = low, Monoamines levels = high
What are the social drugs that work on the CNS?
Drugs acting on the CNS are used more than any other type of agent
In addition to their therapeutic uses, drugs such as
- Caffeine (stimulate)
- Alcohol (depressant)
- Nicotine (stimulate)
- Are used socially to provide a sense of wellbeing
Central drugs of produce dependence with continued use, and many are subject to strict legal controls
What are clinical drugs that work on the CNS?
Anaesthetics
E.g. halothane, propofol, ketamine, thiopentone
Analgesics
E.g. Codeine, morphine, oxycodone, pethidine
Antianxiety, sedative and hypnotic
E.g. Diazepam, phenobarbitone, Chloral hydrate
Psychotropic agents
E.g. imipramine, lithium
