The ANS Flashcards
Describe the general organisation of the nervous system.
- Consists of the central nervous system (CNS) - brain & spinal cord, and peripheral nervous system (PNS).
- PNS has 2 branches:
- somatic (conscious perception and voluntary movement)
- autonomic (involuntary movement and functions)
- ANS has 2 branches:
- sympathetic (fight or flight)
- parasympathetic (rest & digest)
What is the function of the ANS?
- Regulates involuntary functions of internal organs (e.g. Heart and stomach).
- Controls smooth & cardiac muscle, and glands.
How do sympathetic and parasympathetic nerves differ?
- Site of origin from CNS
- Sympathetic: thoracolumbar nerves - originate in lateral horn of thoracic and lumbar spinal cord.
- Parasympathetic: craniosacral nerves - originate in lateral horn of medulla (cranial) and sacral spinal cord.
- Ganglia location
- Sympathetic: in paravertebral chain close to spinal cord - 1 signal can rapidly activate many effectors
- Parasympathetic: further from spinal cord, even within innervated tissue - activate single organs
- Relative length of fibre
- Sympathetic: short myelinated pre-ganglionic fibres -> long unmyelinated post-ganglionic fibres
- Parasympathetic: long myelinated pre-ganglionic fibres -> short unmyelinated post-ganglionic fibres
Which neurotransmitters are used in the ANS?
- All pre-ganglionic nerve fibres are cholinergic: release ACh and activate nAChRs on post-ganglionic cells.
- Most sympathetic post-ganglionic nerve fibres are adrenergic: release NA and activate adrenoreceptors on post-ganglionic cells.
- Some sympathetic post-ganglionic nerve fibres are cholinergic: release ACh and activate mAChRs on post-ganglionic.
- Sympathetic post-ganglionic neurones in the medulla of adrenal glands differentiate to form neurosecretory chromaffin cells. Do not project into a target tissue but release adrenaline into the bloodstream.
- Parasympathetic post-ganglionic nerve fibres are cholinergic: use ACh to activate mAChRs on post-ganglionic cells.
What type of NT receptors are found in the ANS?
- Nicotinic ACh receptors (on post-ganglionic neurones) - ligand-gated ion channels
- Muscarinic ACh receptors (in parasympathetic innervated tissues, in some sympathetic innervated tissues e.g. Sweat glands and piloerector muscles) - GPCRs
- Adrenoreceptors (in most sympathetic innervated tissues) - GPCRs
Give examples of alternative NT used in the ANS.
In some situations (often co-released with ACh or NA) non-adrenergic, non-cholinergic (NANC) transmitters are used. E.g.
- ATP
- serotonin (5HT)
- nitric oxide
- several neuropeptides inc. neuropeotide Y, vasoactive intestinal peptide & substance P
Describe the changes occurring in the sympathetic ‘fight or flight’ response.
Activate ‘survival functions’:
- Dilate pupils
- Increase heartbeat
- Relax airways
- Stimulate release of glucose
- Adrenaline and noradrenaline secretion
Deactivate non-essential functions:
- Inhibit activity of stomach, intestines, gallbladder and saliva secretion
- Relax bladder
- Promote ejaculation/vaginal contraction
Describe the changes occurring in the parasympathetic ‘rest and digest’ response.
Deactivate ‘stress’ responses:
- Constrict pupils
- Slow heartbeat
- Constrict airways
- Inhibit release of glucose
Promote digestion:
- Stimulate activity of stomach, intestines, gallbladder and saliva secretion
- Contract bladder
- Promote erection of genitals
What is the effect of sympathetic release of NA on the heart?
- positive chronotropy (beta1 in SA node)
- positive inotropy (beta1 in ventricles)
What is the effect of sympathetic release of NA on adrenoRs throughout the body?
- a1 (Gq): vasoconstriction (SM in vasculature), pupillary dilation (dilator pupillae in eye), continence (IUS of bladder)
- a2 (Gi): CNS
- B1 (Gs): increased HR (SAN) and inotropy (ventricles)
- B2 (Gs): bronchodilation (lungs)
- B3 (Gs): continence (detrusor relaxation)
What is the effect of sympathetic release of NA in the kidney?
Renin release from juxtaglomerullar cells (B1 R)
What is the effect of parasympathetic release of ACh on the heart?
Acts primarily in atria rather than ventricles as regulates rate rather than contraction force.
- bradycardia (M2 in SA node)
- decrease conduction velocity (M2 in AV node)
What is the effect of parasympathetic release of ACh on smooth muscle?
- bronchoconstriction (M3 in lungs)
- increased intestinal mobility/secretion (M3 in GI tract)
- bladder contraction (detrusor) and relaxation (trigone/sphincter) (M3 in GU tract)
- penile erection (NO generation)
- ciliary muscle and iris sphincter contraction
What is the effect of parasympathetic release of ACh on glands?
Increase sweat, salivary and lacrimal secretion (M1 and M3)
Why does the sympathetic system increase vaso-/veno-constriction in some tissues but not others?
Prioritises blood flow:
- Promotes vasodilation in the brain, skeletal muscle and heart
- Promotes vasoconstriction of BVs to skin - minimises bleeding in injured
- Promotes vasoconstriction to non-essential function organs (e.g. GI tract)
- Promotes venoconstriction to increase cardiac return
