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Involvement of Autonomic Nervous System and Stress

Mammary Nerves
& Stress

Cows walking to the barn.

The autonomic nervous system is part of the central nervous system. It mainly controls viseral function. The autonomic nervous system is made up of two types of nerves, the parasympathetic nerves and the sympathetic nerves.

Parasympathetic nerves: The neurotransmitter of parasympathetic nerves is acetylcholine.

There is no parasympathetic innervation in the mammary gland.

Sympathetic nerves: The neuroendocrine components of sympathetic nerves are epinephrine and norepinephrine. Epinephrine (adrenaline) is primarily from adrenal medulla. Norepinephrine is a neurotransmitter from peripheral nerves and nerves in the brain. Norepinephrine also can from the adrenal medulla.

The effect of sympathetic nerves on milk ejection depends upon the type of neurotransmitter receptor:

- alpha-receptors are vasoconstrictive.
- norepinephrine can stimulate milk ejection via brain alpha-receptors.
- norepinephrine can inhibit milk ejection via brain beta-receptors.
- it is the location of these receptors that is the most important thing.

Most sensory receptors (neurons) are located in the teat. There are pressure-sensitive neurons around the cisterns and the large ducts.

*** There is no direct innervation of alveoli or myoepithelial cells!

If you electrically stimulate the cut end of the mammary nerves, you do not get milk ejection.

Norepinephrine and epinephrine can inhibit oxytocin-induced contraction of myoepithelial cells.

Stressful stimuli will inhibit milk ejection. This occurs via epinephrine or norepinephrine derived from the adrenal gland or the sympathetic nerves by the following mechanisms :

  • Norepinephrine reduces myoepithelial cell contractial response to oxytocin; this is a direct inhibition at the myoepithelial cell level.
  • Norepinephrine decreases mammary blood flow (amount of oxytocin to the gland); this is an inhibition at the mammary tissue level.
  • Norepinephrine reduces oxytocin release from the pituitary; this is an indirect effect mediated by inhibition of oxytocin release at the hypothalamic level.

In the bovine species norepinephrine is the primary catecholamine. Injections of norepinephrine into cows which increase blood levels to 2 to 5X above normal will cause a decrease of milk yield by 10%. Oxytocin is not altered.

Emotional disturbances can cause inhibition of the CNS part of the milk ejection reflex. This may especially occur after calving in the first-calf heifer. Inject of oxytocin may be needed to remove milk because failure to remove the milk will result in reduced yield through lactation. This may need to be continued for a time.

Other Mechanisms of Milk Ejection

Myoepithelial cells will also contract in response to vasopressin (ADH or antidiuretic hormone). Vasopressin has about 1/5 the oxytocic activity of oxytocin. It probably is not of physiological significance in milk ejection.

Visual or auditory stimuli can cause milk ejection. Milk ejection is a condition response.

Stimulation of the genital tract such as vaginal distention causes release of large amounts of oxytocin.

The Mechanical Tap Stimulus does not involve oxytocin. It will occur under anesthesia or denervation of the udder. It is not inhibited by epinephrine. Kneading or butting of the udder by the young may elicit this response. This may involve distortion of the alveolar structure or the myoepithelial cell structure, resulting in milk ejection.

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