IMED1001 - ANS 1 (Week 12)

Peripheral Nervous System

- Sensory (Afferent) Division is split into Somatic Sensory and Visceral Sensory Neurons

- Motor (efferent) division is split into autonomic NS (involuntary) and Somatic NS (Voluntary)

Peripheral NS Diagram

- somatic NS controls muscle, Autonomic NS controls everything else

- sympathetic NS has enteric NS

Role of ANS in vegetative functions

- essential homeostatic control mechanism

- regulates fundamental states and life processes (known as vegetative functions)

- examples of vegetative functoins: Heart rate, BP, body temp, digestion etc.

- animals with their ANS blocked cannot survive on their own (have to be kept warm and stress-free)

- we have drugs that can block "ganglia" (processing centres of NS) which renders you useless

Features of Autonomic Reflex

- visceral reflex arc

- Unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectorsX

Baroreceptor Reflex

- for blood pressure

1. blood pressure detected by arterial stretch receptors

2. afferent neuron carries signal to CNS

3. efferent signals travel to the heart;

4. heart then slows, reducing blood pressure

- example of homeostatic negative feedback loop

- its parasympathetic because its slowing down the heart (thats why vagus nerve is used)

Feedback Loop for Baroreceptor

DIAGRAM ON SLIDE 8

Autonomic vs Conscious (Examples)

- Purely autonomic: blood pressure, kidney function, digestion

- Linked to consciousness: temperature, hunger, thirst, defecation, urination

Autonomic Reflexes with conscious control

TWO POINTS OF CONTROL:

- AFFERENTs go to both conscious and subconscious centres

- EFFERENTS (ANS controls automatic functions (secretion, smooth muscle and somatic NS controls behavioural override (skeletal muscle)

Spinal Cord Reflexes:

- defecation and micturition reflexes are integrated in spinal cord

- We control these functions because of our control over skeletal muscle sphincters; if the spinal cord is damaged, the smooth muscle of bowel and bladder is controlled by autonomic reflexes built into the spinal cord

- its kind of like an override. if we didnt have it for urination and defecation we would never have control. the body just stuck a skeletal muscle in these areas so we can control it

Autonomic reflexes with conscious control (examples)

spinal cord reflexes in urination and defecation:

- Internal sphincter: smooth muscle - Autonomic Control

- External sphincter: skeletal muscle - Somatic Motorneuron Control

Neural Control of Defecation

- sympathetic stimulation causes contraction, parasympathetic stimulation causes relaxation

Neural Control of Urination

- micturition reflex

- afferents sense stretch (300-400mL)

- detrusor and internal urethral sphincter muscle is innervated by the sympathetic (storage) and parasympathetic (to urinate) nerve fibres from spinal cord

- external urethral sphincter muscle is under voluntary control, and as such is innervated by the somatic nervous system

Control of Autonomic Function

ANS REGULATED BY SEVERAL LEVELS OF CNS

- Hypothalamus (major visceral motor control centre): nuclei for primitive functions - hunger, thirst, sex (master gland)

- Midbrain, pons and medulla: nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes

- Cerebral cortex has an influence (anger, fear, anxiety): powerful emotions influence the ANS because of the connections between our limbic system and the hypothalamus.

- we sort of have a conscious control of the unconscious

Respiratory and Cardiac Centres and Blood Vessels

RESPIRATORY CENTRES: respiratory centre and pneumotaxic centre

CARDIAC CENTRES: cardioacceleratory and cardiac slowing

BLOOD VESSELS (BP): Vasomotor

- dont need to know how to label the diagram, just need to know that they exist

Tone

we refer to someone as having a sympathetic or parasympathetic tone

Fight or Flight Response

GET MORE OXYGEN AND FUEL TO YOUR MUSCLES (PREPARING FOR FLIGHT):

- bronchodilation, increased HR and Cardiac Output, vasodilation at muscles, breakdown of stored glucose (liver, muscle) and fat (adipose tissue) increases plasma [glucose] and [fats], blood vessel constriction (increased BP drives blood to where you need it)

- shuts down renal/hepatic/gastrointestinal tract blood flow and function, peripheral and visceral vasoconstriction

COOL DOWN:

- lots of sweat

PREPARING FOR FIGHT:

- Same as flight, plus you:

- see beter (dilate pupils)

- Increased strength and skill (increased muscle tone, increased mental activity)

- Lose less blood (peripheral vasoconstriction, increased clotting response)

Parasympathetic System - Rest and Digest

- slows down heart rate

- increased glandular (secretion) and muscular (peristalsis) activity in the gut, increased blood flow to the gut to pick up all the goodies

- increased sexual arousal, eye ciliary muscle constriction (all the better to see nearby faces)

- pupillary constrictions (sleep easier)

Easiest Way to Antagonise - Dual Innervation

- organs innervated by both parasympathetic and sympathetic divisions (antagonistic effect: oppose eachother)

- however some of them arent dual innervated

- Autonomic tone: normal background firing rate -> balance between the two divisions (Parasympathetic tone maintains smooth muscle tone in intestines and holds resting heart rate down to around 70-80 beats per minute. Sympathetic tone keeps most blood vessels partially constricted and maintains blood pressure

- e.g if we increase sympathetic tone, we get vasoconstriction, if we decrease sympathetic tone we get vasodilation. its not dual innervation, its the same nerve that is like a "switch"

Heart Rate - Dual Innervation

- antagonistic effects - oppose eachother

- May be exerted through dual innervation of same effector cells (heart rate decreases (parasympathetic) or heart rate increases (sympathetic)

Eyesight - Dual Innervation

- exerted because each division innervates different cells

- Pupillary dilator muscle (sympathetic) dilates pupil

- Constrictor pupillae (parasympathetic) constricts pupil

- stereotyped = both eyes dilate or constrict at once (dilation of one eye is indicative of CNS trauma)

Some exceptions to dual innervation

- synergistic effects: two divisions act on different effectors to produce a unified overall effect (Parasympathetic NS increase salivary serous cell secretion, Sympathetic NS increase salivary mucous cell secretion)

- Not all organs receive dual innervation:

- sweat glands receive only sympathetic neurons

- blood vessels (arterioles and veins) receive only sympathetic nerve fibres, except penis and clitoris

- sympathetic tone (vasomotor tone): increase in firing frequency means vasoconstriction, decrease in firing frequency means vasodilation

- needs alpha receptors, otherwise effect won't happen in blood vessels

Kidney and Liver Bloodflow Control

- SNS stimulation causes potent vasoconstriction

- reduced renal and hepatic blood flow

- diverts blood away from kidneys and liver

- no vasodilator fibres

Summary

DIAGRAM ON SLIDE 31