Stress lecture 1

• Controls & monitors the 'internal environment' (internal organs or viscera- heart rate, GI tract, salivation, adjustments in blood flow (blushing)). 
• ANS nerves synapse with visceral organs, mediated by smooth muscle- not striated skeletal muscle

Structure

• Divisions
  • Sympathetic branch- (fight or flight)
  • Parasympathetic branch- times of relaxation (rest and digest)
    • Divisions have a push-pull arrangement- work in antagonist fashion- activity in one branch means a decrease in the other branch. 
• Effectors
  • Dorsal route ganglion CNS relays: ANS feedback to CNS via junctions in the spinal cord, informing the brain about the body.  

• Multitude of connects allows ANS to control locally individual organs by monitory local info.
  • E.g. salivation to food is a local event. 
  • Local heart rate adjustments made in response to blood pressure changes- by way of the sensory loop via CNS.  

Command Neurons

• An overall control mechanism that ensures the ANS responds to situations correctly.  
• Global response needed when acute stressor or emergency occurs to:
  • Excite sympathetic branch
  • Inhibit parasympathetic 
  • Integrate multiple actions at the same time to:
    • mobilise fuel (liver)
    • Bypass digestion- send blood to skeletal muscles
    • Release adrenalin and noradrenaline from the adrenal gland. 

Hypothalamic-pituitary-adrenocortical axis (HPA Axis)

• System output= corticosteroids (stress hormones).  Humans= cortisol, animals= corticosterone.

Chain of events leading to cortisol release:

• Neurons secrete corticotropin-releasing hormone (CRH) in response to noxious stimuli, loss of control.
• CRH binds to receptors in the pituitary gland and this sends signals release of ACTH.
• Cortisol is released by the adrenal cortex (adrenalin from adrenal medulla)

The first link (bullet point) is the hypothalamus.  Nuclei within this structure respond to stimuli (internal & external) and release of CRF at the pituitary.  HPA system broadly responds to events that lead to uncertainty, threat and where there is a need for a behavioural goal.  

Main functions that hypothalamus regulates are often referred to as the 4 f's

Fear, Fighting, Feeding, Fornicating

CRH also secreted in areas of the brain involved in emotion: periaqueductal gray matter, central nucleus of the amygdala, locus coeruleus.  Effect of injecting CRH into brain regions is synonymous with stress. 

• ICV injection of CRH increases fear responding, reduces time in the centre of an arena.
  
• Increases startle response to loud noises.

Antagonists (blockers) of CRF reduce anxiety and stress

• Chronic activation of HPA indicated stress, but fleeting activation can lead to pleasurable arousal.

• The novelty might be fearful for some but arousing for others. 

• Evidence of animals working to self-administer amphetamine which elevates HPA activity & corticosterone analogues. 
  • Animal eventually die if continually exposed to a stressor after adrenalectomy--> HPA is life-saving.

Stress not always negative- increases arousal allowing us to achieve goals. Severe stress or exposure to mild stress over a long period can lead to illness.

• Stressors:
  • Animals= social conflict, predators, noise. Psychological & analogous to the human-centred definition if stress.
  • Physical= disturbances to body injury leading to blood loss, low blood sugar.
    

Term stress covers psychological & physiological triggers which bring about behavioural and physiological adaptations (coping or not coping). 

Physiological stressors:

• blood loss- corticosteroid release
• Low blood sugar- mobilise resources to seek salient stimuli to restore equilibrium
• cold

Psychological stressors

• situational/contextual that stress hormones are sensitive to
  • Cognitive challenges with sympathetic activation.  Behavioural output- 'I can't cope'. 
    
  • Loss of control in an instrumental situation (behaviour has a consequence that suddenly changes the outcome).
  • Earning smaller reward than expected

Stress arises from the chronic failure of the coping response.

Perception of stressor and degree of perceived control influences types of coping strategies & their success- perhaps why some people use a lucky charm to give a sense of control or religion?

Stress can be thought of as a feedback loop:

• Stressor- corrective action- stressor goes away- negative feedback in a closed loop. 
• Stressor- corrective action- stressor remains- open loop.

Do nothing- passive avoidance is controlled by parasympathetic activation and sympathetic inhibition. This strategy also has negative outcomes- strategy over long period considered stressful

Toates (1995)- 4 criteria for stress:

• Behaviour doesn't correct disturbance over time (open loop).
• Excessive activity in sympathetic branch and neurohormonal systems (HPA)
• Vulnerable to a certain pathology (depression, immunosuppression).
• Increased frequency of pointless behaviours (stereotypes). 

Ordinary neural & hormonal systems work successfully in a feedback loop.  Encounter stressor, the appropriate behavioural action returns HPA activity to normal - a closed loop.  Stress occurs when excitation of HPA axis and SNS is not justified by behavioural outcome, e.g. job stress. Open loop- allostatic load is high & HPA axis doesn't return to baseline, cortisol levels elevated. Long-term health suffers. 

Lipid accumulation in the bloodstream: Increased cortisol releases fuels stored as fat or glycogen.  Cortisol type of glucocorticoid.  Leads to artherosclerosis- coronary artery disease, increased risk of stroke, heart attack.

Elevations in HPA activity can damage NS and immune system. Cobb & Ross 1973- shown people subjected to long-term stressors more likely to have high blood pressure which increasingly gets worse with age. Kielcot-Glaser et al. 1995- carers of Alzheimer's suffers take longer to recover from a biopsy of the forearm in those stressed around caring.