Introduction

Postural Control is the act of maintaining control of a state of balance while performing a specific task or activity. Many factors go into Postural control, such as orientation and equilibrium [1], while there are three main feedback systems that receive information about orientation to gravity. The three main feedback systems are the visual system, the vestibular system, and the proprioceptive system. These three systems receive input from ascending pathways and project this information about orientation and equilibrium to many higher structures, such as the cerebellum or basal ganglia. Information is then projected to descending pathways to adjust balance and posture to the changing internal and external environments. Balance plays a huge role in postural control, and is controlled largely in part by these three systems.

Functional Anatomical Review

Visual System

The visual system contains the eye, where stimuli enters through the lens, through the aqueous humor, through the lens and the vitreous humor, and is projected back onto the fovea, or the focal point, of the retina. Information then travels past the photoreceptors onto the optic nerve, crosses at the optic chiasm, and travels back to the lateral geniculate nucleus in the thalamus by way of the optic tract. Information is then sent to the visual association cortex and the visual cortex.
In order to move the eye, there are 6 muscles that are used. There are the four rectus muscles: the superior, lateral, inferior and medial, and there are the two oblique muscles: the superior and inferior, that aid in moving the eye in response to visual stimuli. These muscles are innervated by the oculomotor nerve, the abducens nerve, and the trochlear nerve.
VisualPathways-thumb-500x486-51886.jpg(1)
Vestibular system

The organs of the vestibular system lie primarily within the inner ear. There are three semicircular canals and two otolithic organs.
The three semicircular canals are oriented at roughly a 90-degree angle from one another, one being anterior, one being posterior and one lateral. The orientation of these canals to one another helps for them to work together as the head moves in any direction. Each semicircular canal contains one ampulla. Within each ampulla is a projection of hair cells contained within a viscous fluid called endolymph. As the head moves, the endolymph within the ampulla flows in the opposite direction of the head motion, causing the hair cell to bend and exhibit either an excitatory or inhibitory firing response. The response is excitatory or inhibitory depending This information about the heads angular acceleration then travels out through the ganglion of Scarpa to the vestibular nuclei.
The otolithic organs are the Utricule, which is oriented horizontally with hair cells projecting vertically, and the Saccule, which is oriented vertically with hair cells projected horizontally. The otolithic organs give information about head tilt and orientation to gravity. On top of the hair cells of the otolithic organs is also a viscous fluid where the otoconia, or crystals, sit. When the head is tilted, the otoconia shift which causes the fluid to shift, moving the hair cells. The hair cells then send an excitatory or inhibitory firing response through Scarpas ganglion (vestibulo-cochlear nerve) to the vestibular nuclei. From the vestibular nuclei information goes to the abducens nerve and the oculomotor nerve, as well as the vestibulospinal tracts.
external image ga2--inner-ear_full.jpg(2)external image da6c40ff9.gif.png(3)


Proprioceptive/ Somatosensory System

There are two main proprioceptive organs that the body uses in order to relay information about orientation of body parts in space and with respect to one another, and they are called Golgi Tendon Organs (GTOs) and Muscle Spindles. The somatosensory system is used in order to pick up information from the surroundings, and in the case of postural control and standing it would mostly be pertaining to the amount of pressure put on the different areas of the feet. This pressure intake will give the body information about stance and balance.
Golgi Tendon Organs are in series with muscle fibers and lie within the tendon of the respective muscle. When the muscle contracts and the tendon pulls, the GTO fires and sends information to the dorsal column of the spinal cord about the tension of the muscle via 1b afferents.
The Muscle spindles are a set of intrafusal (lying within the muscle) collagen fibers found lying parallel to the body of the muscle. Around the intrafusal fibers are 1a and 2 afferents that carry information about the length and the velocity of the muscle stretch to the alpha motor neurons within the dorsal column of the spinal cord.


Golgi (1).gif[4]




Input and Output Pathways

Information about the environment and the body’s relation to the environment enters in through sensory organs. These sensory organs include (mainly) the eyes, the organs of the inner ear (semicircular canals and otoliths), and the muscle spindles and Golgi tendon organs.

Input pathways (used by Proprioceptive Organs (muscle spindles and GTOs))

As stated in the functional anatomy review, the Golgi tendon organs send information about the tension of a muscle, while the muscle spindles send information about the muscle length and the velocity of the stretch. This is a vital part of the feedback and feedforward systems in controlling balance and posture. The muscle spindles have afferents called 1a afferents, while the Golgi tendon organs have afferents called 1b afferents. The afferents are named differently because of how heavily myelinated their axons are. The 1a and 1b afferents carry information from the muscle spindles and Golgi tendon organs respectively to the Dorsal Column Medial Lemniscal pathway, which I will talk about in further detail now.
The Dorsal Column Medial Lemniscal pathway, or DCML, carries information about discriminative touch and proprioception to the level of the cerebral cortex [7]. Information travels through the 1a or 1b axon to the ventral rootlets, through the ventral horn, and synapses on the 1st order neuron in the dorsal column. Information ascends ipsilaterally, and gets pushed more medially as it ascends. Above the midthoracic area, the dorsal column is divided into two fascicles called the Cuneate and Gracilis. The Cuneate fascicle contains fibers from vertebrae T7-T12, while the Gracilis fascicle contains fibers from C7-T6. These two fascicles terminate on two different nuclei, called the Cuneate nucleus and the Gracile nucleus, that are located in the lower medulla. These nuclei are called the 2nd order nuclei. These 2nd order neurons decussate and project contralaterally toward the Posterolateral Thalamus where the fibers synapse through the internal capsule to the primary somatosensory cortex. The primary somatosensory cortex is organized somatotopically, so information from the proprioceptive organs is sent there respective to where is originated [7].

Output Pathways

Rubrospinal

The Rubrospinal Pathway is the main lateral descending pathway that descends from the red nucleus. The fibers that come from the red nucleus decussate almost immediately and descend contralaterally through the brainstem. The Rubrospinal pathway influences the tone of the upper extremities flexor muscles. This pathway also gives rise to collaterals from the cerebellar nuclei and the vestibular nuclei. [6]

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Tectospinal

The Tectospinal Pathway originates in the superior colliculus and crosses over the opposite side of the brainstem, where it projects only as far as the cervical spinal segments where the fibers synapse with interneurons. The Tectospinal pathway has facilitory affect on neck and upper shoulder and trunk muscles. The superior colliculus receives information from the retina and visual cortex, so the job of the Tectospinal pathway is the orient the head and neck to visual stimulus. [6]

Reticulospinal

The Reticulospinal Tract receives extensive input from the cortex and vestibular nuclei, and the pontine and the medullary regions of the tract project to the spinal cord.
The Pontine Reticulospinal Pathway originates in the Pontine tegmentum, and descends ipsilaterally. It receives its information from the vestibular nuclei, and terminates on interneurons. This pathway primarily has an excitatory affect on proximal muscle groups, influences muscle tone and provides important information to the gamma motor neurons.
The Medullary Reticulospinal Pathway originates in the medial Medullary formation and projects bilaterally to synapse on interneurons and alpha motor neurons. The Medullary Reticulospinal pathway receives information from the cerebral cortex. This pathway primarily influences the distal musculature and inhibits muscle tone. It counteracts the antigravity muscles, thus counteracting the activity of the Pontine Reticulospinal pathway.
The Reticulospinal pathway and the two tracts that it contains work antagonistically in activities such as gait. [6]

external image reticulospinal_tract1321847115422.png(6)

Vestibulospinal (lateral and medial)

The Vestibular nuclei receive input from the Vestibular labrynth, cerebellum, reticular formation, visual system and proprioceptors. Two pathways stem from the Vestibulospinal tract: the Lateral and Medial Vestibulospinal pathways.
The Lateral Vestibular pathway originates in the lateral Vestibular nuclei where information comes from the otolithic organs, the utricle and saccule. The fibers from the nuclei descend ipsilaterally where they terminate on interneurons, alpha motor neurons or gamma motor neurons of extensor muscles in the cervical and lower lumbar segments of the spinal cord.
The Medial Vestibular pathway originates primarily in the medial vestibular nuclei where information comes from the semicircular canals. The fibers descend through the brainstem and descend bilaterally to the cervical or upper thoracic regions of the spinal cord. They synapse with interneurons and alpha motor neurons that innervate extensor muscles of the neck and back. The tract facilitates extensor motor neurons and inhibits flexor motor neurons.
The work of the vestibulospinal pathways is postural control of the head and neck by use of the vestibulocollic reflex, which is the contraction of the dorsal neck muscles when the head tilts forward without the neck bending. [6]
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Posture and Balance

Posture and Balance are directly dependent on the integration of visual, vestibular, and proprioceptive information acting together to give the body proper commands based on the body's relation to the environment. In normal individuals, these systems all work in harmony in order for the body to maintain an "automatic" control system so that the mind pays minimal conscious attention to balance and posture [5]. In fact, we have many unconscious reactions to the changes in our environments that make adjustments to our body's position to gravity. The unconscious reactions are said to be a part of our sensory feedback and feedforward systems that constantly supply our body with information based on our body's relation to the environment so that we may fine tune our movements as a reaction [4]. In those individuals with visual, vestibular, or muscular impairments, balance and posture are also impaired, and this raises the risk for falls. In order to rehabilitate those with any of these impairments one must have an understanding of all of the systems that underly postural control [1]. There are many Balance and Posture issues that come with old age, and many disease related or incident related issues, such as from Parkinson's Disease and stroke, respectively. In fact, the risk of developing problems in one or more of the sensory, motor, or adaptive brain components of balance increases with age as the body is exposed to degenerative or infectious diseases, or the effects of injuries accumulated over a lifetime [5]. Many exercises to improve posture are related to improving core strength, as this becomes decreased with old age.

Summary

To obtain proper balance and posture, one must have properly working visual, vestibular, and proprioceptive systems that work together to maintain these things on a highly subconscious level. Feedback and feedforward information entering in and being processed by these systems work together in critiquing movement subconsciously. Deficits in any of these pathways or organs within these systems will lead to trouble with posture and balance.


Glossary
Posture: A position of the body or of body parts.
Balance: A state of bodily equilibrium; the ability to maintain the equilibrium of the body.
Equilibrium: A condition in which all acting influences are canceled by others, resulting in a stable, balanced, or unchanging system.
Proprioception: The unconscious perception of movement and spatial orientation arising from stimuli within the body itself.
Interneurons: A nerve cell existing completely in the nervous system that acts as a link between sensory and motor neurons.
Alpha Motor Neuron: Innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating muscle contraction.



(all terms taken from http://medical-dictionary.thefreedictionary.com/)

Quiz

1) (T or F) According to image 1, images that hit the temporal portion of the retina cross at the optic chiasm.

2) The Rubrospinal pathway affects the tone of which of the following:

a) upper extremity flexors
b) upper extremity extensors
c) lower extremity flexors
d) lower extremity extensors

3) (T or F) The Lateral Vestibulospinal pathway receives informationg from the semicircular canals

4) (T or F) Posture and Balance should not have to be conscious actions in a normal individual.

5) The Dorsal Column Medial Lemniscal pathway carries information about:
a) proprioception
b) pain and temperature
c) discriminative touch
d) a and b
e) a and c
f) b and c

6) What kind of information is carried by the Golgi Tendon Organs and the Muscle Spindles, respectively.

7) Name a pathway that could affect the posterior neck muscles of somebody who lost balance and was falling forward? What affect would this pathway have on those muscles (excitatory or inhibitory)?

8) Why are the semicircular canals oriented in such angles to one another? What is/are the angle(s)?

9) Name the pathway, its origin, if/where it decussates, and where it terminates. Also name what kind of information it carries and how the pathway uses the information.



Sources

[1] Postural orientation and equilibrium: what do we need to know about neural control of balance to prevent falls? http://www.cs.cmu.edu/~cga/legs/chamr1.pdf

[2] Coordination of posture and movement http://ptjournal.apta.org.proxy2.noblenet.org/site/subscriptions/

[3] Neuroscience Online " http://neuroscience.uth.tmc.edu/s3/chapter05.html

[4] "Sensorimotor Integration in Human Postural Control" http://jn.physiology.org/content/88/3/1097.long#sec-1

[5] "Setting the Standard for Balance and Mobility" http://www.resourcesonbalance.com/clinical_info/BalanceControl.aspx

[6] Neuroscience Online "Spinal Reflexes and Descending Motor Pathways" http://neuroscience.uth.tmc.edu/s3/chapter02.html

[7] Neuroscience Online "Somatosensory Pathways" http://neuroscience.uth.tmc.edu/s2/chapter04.html

[8] Chapter 11 "The Control of Posture" http://cw.tandf.co.uk/neurophysiology/sample-material/Neurophysiology-5E_Chapter-11.pdf

[9] http://medical-dictionary.thefreedictionary.com/


Images

(1) http://myweb.rollins.edu/jsiry/Visual_Cortex.html

(2) http://www.balancemaryland.com/sites/all/themes/balance/assets/images/ga2--inner-ear_full.jpg

(3) http://www.ib.cnea.gov.ar/~redneu/2013/BOOKS/Principles%20of%20Neural%20Science%20-%20Kandel/gateway.ut.ovid.com/fulltextservice/ct%7B06b9ee1beed594190674f1983457a7dd32af6a0d5a4c9892~47/da6c40ff9.gif.png

(4) http://www.missionbeachvolleyball.com/Plyometrics/Golgi.aspx

(5) http://what-when-how.com/wp-content/uploads/2012/04/tmp14105_thumb.jpg

(6) http://classconnection.s3.amazonaws.com/938/flashcards/82938/png/reticulospinal_tract1321847115422.png

(7) http://what-when-how.com/wp-content/uploads/2012/04/tmp14106_thumb.jpg


Quiz Answers: 1) F 2) A 3) F 4) T 5) C
6) The Golgi Tendon Organs carry information about the tension of the muscle, while the Muscle Spindles carry information about the length and velocity of the muscle.
7) The Tectospinal pathway, excite the flexors to keep the head oriented upright with respect to gravity.
8) The canals are oriented at 90 degree angles in respect to one another, and this is so they can work in pairs with one another to pick up information about the body's movement with respect to the environment.