Animations & videos from the books
Serratus anterior
From: An Atlas of Equine Anatomy with human comparative anatomy: The Thoracic Limb
The muscle structure and function of the equine serratus ventralis thoracis is very like our serratus anterior, which reaches from the far side (medial border) of our scapula, forwards around our ribs, to similarly stabilise the shoulder on the ribcage.
From:
An Atlas of Equine Musculoskeletal Anatomy for Physical Therapists, with comparative human anatomy. Volume 1: The Thoracic Limb.
FIG. 4.158: Serratus anterior, human. (l) gif of serratus anterior, image credit: Anatomography.
Cutaneous trunci in action
From: An Atlas of Equine Anatomy with human comparative anatomy, The Thoracic Limb
The cutaneous muscles are really fine, with a structure almost like thick skin! They lie within the superficial fascial layer, just deep to the dermis of the skin.
They have strong fascial connections to the pectoralis ascendens; but their prime role, is in performing the twitch reflex.
The twitch reflex, so important for repelling pesky flies, can actually be a problem for sensitive horses - in girth tightening and general saddle fitting and even with leg aids.
Like all muscles they develop with use, and can be clearly seen through a thin summer coat.
From:
An Atlas of Equine Musculoskeletal Anatomy for Physical Therapists, with comparative human anatomy. Volume 1: The Thoracic Limb.
FIG. 4.235: Cutaneous trunci and cutaneous brachialis muscles fulfilling a vital role in fly repelling. Slow motion movie can be watched here.
Note the demarcation of the muscle clearly seen through the thin summer coat.
Signal propagation along the axon
From: An Atlas of Equine Anatomy with human comparative anatomy, The Thoracic Limb
If enough neurotransmitter release is stimulated, so that sufficient positive ions flow into the receiving axon, a polarising charge will be generated across the first part of the cell membrane, and an action potential is triggered.
Normally the cell membrane is at a resting potential where, due to the high concentration of Na+ ions outside the cell, the outside of the cell membrane is positively charged, and the inside of the cell is negatively charged.
The action potential causes a short section of cell membrane to become depolarised, opening voltage-gated sodium channels that allow Na+ ions to flood into the cell. This results in that section inside the axon becoming positively charged.
Charged particles like Na+ ions, can’t normally cross the cell membrane, even with such a large concentration gradient, because it consists of a phospolipid bilayer.
Because the next section of the axon is still negatively charged, the positive charge of the action potential is attracted into this next section and so the charge, and therefore the signal, travels along the axon.
From:
An Atlas of Equine Musculoskeletal Anatomy for Physical Therapists, with comparative human anatomy. Volume 1: The Thoracic Limb.
FIG. 6.18: Electrical charge of a nerve impulse travelling along the cell membrane of an unmyelinated axon. Image: Laurentaylorj