The mid-brain is approximately 1.5 cm long and extends from the pons to the mamillary bodies. The ventral surface connects to the two superior cerebellar peduncles (S2.12). The roof of the mid-brain is composed of four colliculi, the superior colliculi associated with the visual system and the inferior colliculi related to the auditory system. The red nucleus is also contained within the mid-brain. The periaqueductal grey, an area of grey matter within the mid-brain, is important in the descending modulation of pain (S3.29).

The following cranial nerves emerge from the mid-brain:

• Optic (II)

• Oculomotor (III)

• Trochlear (IV)

Pons

The pons is about 2.5 cm long and is composed of transverse fibres which form ridges across the surface of the area. The middle cerebellar peduncles attach to the pons.

The following cranial nerves emerge from the pons:

• Trigeminal (V)

• Abducens (VI)

• Facial (VII)

• Vestibulocochlear (VIII)

Medulla oblongata

The medulla is about 3 cm long and forms the base of the brain stem, adjoining the spinal cord at the level of foramen magnum. The descending motor tracts passing through it form two pyramids on the ventral surface of the medulla between which is the anterior median fissure. This fissure is disrupted where the tracts cross over the midline, termed the ‘decussation of the pyramids’. The inferior cerebellar peduncle attaches to the medulla and behind it is the ‘olive’. The olive is a prominent oval swelling that marks the position of the inferior olivary nucleus (S2.12).

The following cranial nerves emerge from the medulla:

• Glossopharyngeal (IX)

• Vagus (X)

• Accessory (XI)

• Hypoglossal (XII)

Function of the brain stem

The three regions of the brain stem contain the nuclei of the cranial nerves which control a variety of vital functions (S2.10). The ascending (S2.15) and descending tracts (S2.14) also pass through the brain stem between the spinal cord and cerebral cortex. The reticular formation (S2.10) and the cerebellum (S2.12) are integrally linked with all three regions and have specialist roles related to movement.

Cranial nerves

Basic anatomy

The names, numbers and functions of the cranial nerves can be seen in Table 10.1.

Table 10.1

Names and functions of the cranial nerves

Visual system

Basic anatomy

The visual pathway

The sensory receptors of the visual system are the photosensitive rod and cone proteins of the retina. The rods and cones are most dense in the central region of the retina called the fovea. An image of the environment is focused onto the retina by alterations in the thickness of the lens. The retina converts a light pattern into a neuronal signal, which is conducted along the visual pathway to various parts of the brain to be processed (Fig. 10.1).

Figure 10.1 The visual pathway.

Function of the visual system

Binocular vision

This is defined as vision in which two eyes are used. Although there is a huge area of overlap between the visual fields of the left and right eye the eyes still view the environment slightly differently. The brain uses this slight disparity to estimate distance, allowing us to perceive 3D objects and spatial depth. If the two views are not successfully merged, the individual may develop double vision (diplopia).

Central or foveal vision

The fovea is responsible for sharp central vision necessary for any activity where visual detail is of primary importance. For example, reading and watching TV. Hence, if clarity is required, visual gaze must be centred on this region.

Peripheral vision

This aspect of vision occurs outside the centre of gaze and is weaker because the density of rods and cones outside the fovea region is much less. However, our peripheral vision is essential for fast reactions to visual stimuli in the periphery and monitoring body position relative to gravity. Our peripheral vision also allows the individual to recognize well-known structures and forms without the need to focus centrally.

Visual acuity

This is the clearness of vision and is achieved by the alteration of lens thickness and pupil size by small local muscles within the eyeball itself which focus the image at the fovea. However, acuity also relies upon coordination between head and body movements to ensure that the visual stimuli remains at fovea. This involves the ability to fix and scan.

Fixing (S3.27)

The ability to fix gaze on an object while the body is moving, e.g. when travelling on a train or in a car. The mechanism can be explained as follows. When your head turns to the right, there is a slow reflex movement of your eyes to the left, in order that they appear to remain stationary. This is achieved via the vestibulocular reflex and as the name suggests, is under the direction of the vestibular system (S2.10). The effector muscles are innervated by the cranial nerve nuclei (III and VI).

Scanning (S3.27)

The ability to scan the eyes and head in a coordinated manner to maintain the object of interest or visual stimuli in the field of vision, e.g. while reading. The relevant head/neck movements are achieved via the tectospinal and tectobulbar tracts (S2.15) and eye movements by the cranial nerve nucleui (III and VI).

Visual fields (S3.27)

Information from the temporal and nasal visual fields of both eyes reaches the occipital cortex simultaneously, via the visual pathway (Fig. 10.1).

Visual perception

Vision provides the central nervous system with information related to our body position and the environment, however how we perceive and make sense of this raw information is highly subjective. The interpretation of our surroundings is integrally linked with memory and experience and therefore is individual in nature.

Vision and movement

The visual system plays an important part in movement and balance as it provides us with:

• A reference point to the vertical (visual proprioception) and spatial awareness

• The relationship between our body parts and our perception of midline during movement

• A comparison of our own self-motion and the movement of external objects.

However, we can of course balance without vision and vision can be unreliable. For example, our visual system is not always accurate when differentiating between self-motion and the movement of an external body. This may be observed when stationary in a car/bus/train and a vehicle pulls off alongside. Often, this can be perceived as self-motion and you may react accordingly, when actually it is the external object that is in motion.

Vestibular system

Basic anatomy

The vestibular nuclei of the 8th cranial nerve are found within the pons and are the integrating centre for the vestibular system.

Input to the vestibular system

The sensory receptor of the vestibular system is the vestibular apparatus housed in the labyrinth of the inner ear – the labyrinth being made up of an outer bony labyrinth and an inner membraneous labyrinth full of endolymph. The labyrinth is divided into three sections, all of which function by translating movement of sensory hairs into electrical impulses:

Cochlear: which is associated with hearing and will not be discussed further here

Vestibule: known as the static labyrinth

Semicircular canals: known as the kinetic labyrinth.

Vestibule

The vestibule consists of two organs, the utricle and saccule. In the wall of the utricle and saccule is a thickened region of specialized epithelium, called a ‘macula’ (Fig. 10.2).

Figure 10.2 The structure of a macula.

The macula consists of two types of cells:

Supporting cells: Each supporting cell has a sensory neuron connection (vestibular branch of the vestibulocochlear nerve VIII). These cells secrete a thick gelatinous layer called the ‘otolithic membrane’ on top of which is a dense layer of calcium carbonate crystals called ‘otoliths’. This characteristic is highly relevant to the receptors function.

Hair cell bundles (Fig. 10.2): Each supporting cell holds a bundle of hair cells, stereocilia (40–80/bundle) and kinocilium (1/bundle). Each stereocilia is physically connected to the kinocilium so that when it is moved, mechanically gated ion channels are opened allowing the movement of ions. This creates a graded potential in the sensory neuron. The cell is hyperpolarized when the stereocilia moves away from the kinocilium and depolarized with movement in the opposite direction.

The macula is divided into two halves with all of the hair bundles within one-half of the macula orientated in the same way. This ensures that a body movement, which leads to a graded potential in one hair bundle cell is likely to do the same in all the other cells and hence, guarantee an action potential in the sensory neuron. The hair bundles in the other half of the macula are all orientated in the opposite direction producing the opposite response and allowing the whole organ to be responsive to all relevant directions of movement. The macula orientation in the other side of the brain is the same, the relevance of this can be seen later in this section.

Semicircular canals

There are three semicircular canals which lie at 90° to each other. At the base of each canal is a thickened region termed the ‘ampulla’. Inside the ampulla is an elevation of specialized cells called the ‘crista’ (Fig. 10.3).

Figure 10.3 (A,B) The structure of a crista.

The crista consists of two types of cells:

Supporting cells: The supporting cells are similar in structure to those of the macula. They produce a gel which overlies the hair bundles like a cap. This is termed the ‘cupola’ and it is much deeper than the otolithic membrane of the macula so that it covers the full width of the ampulla. This means that the endolymph within the membranous labyrinth cannot pass the cupola and it is this feature that is highly relevant to its function.

Hair cell bundles: The hair bundles are in the same arrangement as for the macula and are all orientated in the same direction within one canal. However, each canal has a different orientation so that the semicircular canals can respond to movement in any plane. The orientation of the left and right sides of the brain is also different with each contralateral equivalent aligned in the opposite direction. The relevance of this will be explained below.

Output from the vestibular system

The vestibular system primarily works subcortically and has few synapses, therefore it responds very quickly, which is vital in terms of balance. From the vestibular apparatus, information passes along the vestibular nerve (VIII) and is processed at the vestibular nucleus in the pons. From here efferent connections go to the:

Cerebellum (S2.12) for processing and execution of smooth co-ordinated movement

Cranial nerves (III and VI) to initiate the vestibulo-ocular reflex

Descending tracts to initiate the appropriate muscle response. The muscle activation via the vestibulospinal tract will be specific to need but involving the neck, proximal limb and trunk muscles (lateral and medial vestibulospinal tract).

Function of the vestibular system

Vestibule

The static labyrinth (utricle and saccule) detects linear movement by way of its sensory organ, the macula. The utricle detects linear movement in a frontal plane and the saccule in a sagittal plane.

The linear movement may be a result of gravity, e.g. head tilting or bending over to pick up an object from the floor or linear translation such as the acceleration and deceleration experienced when walking or travelling by car. In either case, the linear movement causes changes in the macula structure and consequently, a muscle action in terms of a balance response.

For example, during a forward head tilt (cervical spine flexion), the macula of the saccule will respond. The forward tilt causes the heavy mass of the otoliths and the otolithic membrane to ‘fall’ forwards moving the stereocilia in relation to the kinocilium and producing a graded potential in the sensory neuron. As the orientation of hair cells is opposite within each half of the saccule, the result will be depolarization (excitation) in one-half and hyperpolarization (inhibition) in the other (S2.6).

The action potential produced travels along the vestibular nerve (VIII) and recruits or inhibits the appropriate muscles via the vestibulospinal tract (S2.15). In this case, the forward head tilt has moved the centre of gravity forwards making the body less stable. The balance reaction required will attempt to bring the centre of gravity backwards, by recruiting extensor muscles around the pelvis/hip or trunk and simultaneously reduce the activity of the flexor muscles. This is important so that the centre of gravity is not moved further forwards and balance becomes seriously compromised. Remember also that the orientation of hair bundles in one-half of the saccule is the same in both the left and right sides of the brain and therefore the response will be bilateral. This makes sense, as the response to a linear movement needs to be symmetrical.

Semicircular canals

The kinetic labyrinth (semicircular canals) detects the movement of rotation in all planes, e.g. head rotation to look over your shoulder, or a somersault on a trampoline.

As with the static labyrinth, the movement occurring results in a muscle action response as a result of nerve cells being depolarized or hyperpolarized. However, the mechanism by which the sensory organ achieves this is slightly different.

As highlighted in the anatomy section, the cupola is the full width of the ampulla and this stops the free flow of endolymph within each canal. When a rotation movement is experienced, movement of the endolymph is blocked by the cupola producing a force upon the gel structure. The response of the hair bundles embedded in the cupola is as for the macula in terms of production of an action potential in the sensory nerve.

The hair bundles within one canal are all orientated the same way so that the response is sufficient to guarantee an action potential. In the semicircular canals, the orientation of one canal is different on the contralateral side of the brain and this is important because rotation is asymmetrical in nature and therefore requires a different motor response from each side of the body for balance to be maintained.

Assessment of the vestibular system

Rehabilitation of vestibular disorders is recognized as a specialist field in physiotherapy. Therefore, if a lesion of the vestibular system is suspected, referral to a specialist unit is advised. Assessment of the vestibular system is not covered in this text.

Reticular system

Basic anatomy

The reticular formation is a complicated network of circuits which extend from the spinal cord through the medulla, pons, mid-brain, subthalamus, hypothalamus and thalamus. Although previously considered anatomically ill-defined, more recent scientific investigation has shown the reticular formation to be well organized and functionally discrete. Anatomically it can be conveniently divided into three longitudinal columns:

• The median column consisting of intermediate sized neurons

• The medial column containing large neurons (the median and medial columns are also known as the raphe nuclei)

• The lateral column mainly consisting of smaller neurons.

Afferent connections

Afferent information enters the reticular formation from most parts of the central nervous system. Most notably:

• Spinal cord (S2.13)

• Cranial nerves (S2.10)

• Cerebellum (S2.12)

• Limbic system (S2.9)

• Cerebral cortex (motor and sensory areas) (S2.7).

Efferent connections

The reticular formation projects to all levels of the central nervous system including most of the cerebral cortex. Most notably:

• Cerebellum (S2.12)

• Cranial nerves (S2.10)

• Motor pathways of the spinal cord (S2.13, 14).

Function of the reticular formation

Cardiovascular and respiratory control

The cardiorespiratory centres are now considered part of the reticular formation. In terms of respiratory function, many of the neurons from the reticular formation synapse on motor neurons that innervate the muscles of inspiration and expiration. The control of cardiovascular function is via regulation of the autonomic system (a) parasympathetic output from the vagus nerve and (b) sympathetic output from the sympathetic trunk levels T_1–5 of the spinal cord.

Modulation of muscle tone

The appropriate level of resting muscle tone is regulated by modulation of the stretch reflex (S2.13) by the descending tracts. The reticulospinal tracts in combination with the vestibulospinal and reticulobulbar tracts influence muscle tone at the level of the alpha motor neuron. The two reticulospinal tracts specifically influence extensor muscle tone. The pontine/lateral reticulospinal tract terminates at the ventral horn directly on motor neurons and enhances extensor muscle tone, whereas the medullary/medial reticulospinal tract (S2.14) terminates indirectly on motor neurons and exerts an inhibitory influence on extensor muscle tone. The antagonistic effect of these tracts allows modulation and grading to set the level of extensor muscle tone appropriately for the functional need. However, the reticulospinal tracts also work in cooperation with the vestibulospinal tracts (S2.14) to specifically maintain tone of the antigravity muscles.

The reticular formation also has a role in the motor control of facial expression related to our emotional behaviour (S2.9) via its links to the limbic system and coordination of the timing and direction of eye movements via the superior colliculus and cranial nerves.