Posts Tagged ‘ basilar ’

Special Senses – Hearing

Introduction

The ear is the organ of hearing (and balance). Specially adapted receptor cells located in the inner ear are able to react to sound waves. The ear can be divided into three main parts, these are:

  • External ear
  • Middle ear
  • Inner ear

The external ear can be further broken down into:

  • The pinna – This is the funnel shaped cartilage on the external surface of the ear, it helps to guide sound waves into the ear drum. It leads directly to the external auditory canal/ external auditory meatus.
  • The external auditory meatus – Acts as a canal for the sound waves, carrying them to the tympanic membrane (ear drum).

The middle ear can be further broken down into:

  • The tympanic cavity – An air filled cavity, lined with a ciliated columnar mucous membrane which opens out into the Eustachian tube.
  • The tympanic membrane – Commonly known as the ear drum, they provide a boundary between the external and middle ear. The membrane is thin which allows it to convey sound waves (vibrations) to the auditory ossicles.
  • The auditory ossicles – These are the three small, flexible bones in the middle ear which are responsible for transmitting sound waves to the inner ear. The individual bones are named below (including common names):
    • Malleus (hammer)
    • Incus (anvil)
    • Stapes (stirrup)

The inner ear can be further broken down into:

  • The bony labyrinth – Consisting of the bony cochlea, bony vestibule and the bony semicircular canals. They contain perilymph which flows around the outside of the membranous labyrinth.
    • Oval Window – A thin membrane which links the bony labyrinth to the middle ear, the stapes transmits vibrations to the oval window, these vibrations are carried to the cochlea.
    • Round Window – Allows the release of pressure formed by sound waves transmitted to the cochlea by the oval window. Also dissipates sound waves, to prevent the prolonged sensation of a sound due to sound waves reverberating in the ear.
    • The membranous labyrinth – A system of interlinking tubes, containing endolymph. Specialised sensory receptors designed to respond to sound waves lie within. The  membranous labyrinth also consists of three parts:
      • Membranous cochlea – A spiral shaped tube filled with endolymph that contains sensory receptor cells which detect sound. At the end of each receptor is a sensory hair, the nerve fibres from these hairs go on to contribute towards the vestibulocochlear nerve (VII).
      • Membranous vestibule – Connects with the cochlea and semicircular canals, it contains specialised sensory receptors involved in balance when still.
      • Membranous semicircular canals – Three endolymph filled canals containing sensory receptors involved in balance when moving.

The Cochlea

The cochlea (the spiral, snail shaped structure of the inner ear) is responsible for detecting sound and allowing it to be perceived with volume and frequency. The cochlea has two large perilymph filled canals, an upper and a lower canal, separated by a small endolymph filled cochlea duct which runs almost the entire length of the cochlea. The upper canal is known as the vestibular canal and the lower canal is known as the tympanic canal.

The floor of the endolymph filled cochlea duct (basilar membrane) houses the organ of Corti. This organ contains hair cells which project into the cochlea duct. The hair cells act as mechanoreceptors and detect sound wave vibrations. The hairs are able to do this because sound waves entering the cochlea cause the basilar membrane of the cochlea duct to vibrate. As a result, the hairs bend and because they are mechanoreceptors, they are able to initiate action potentials by depolarisation.

Each hair cell contains a bundle of ‘hairs’ which project from its surface. Vibration of the basilar membrane moves the hairs, causing them to bend against the surrounding fluid and the tectorial membrane. The tectorial membrane is a membrane which lies just above the hairs of the hair cell and remains fixed in that position.

The process of perceiving sound is as follows; sound waves which have entered the ear eventually reach the stapes. The stapes vibrates on the oval window which produces a pressure wave in the perilymph of the upper vestibular canal. This pressure wave travels along the vestibular canal to the ‘apex’ (tip) of the cochlea duct. The cochlea duct ends just before the apex, allowing the pressure wave to travel back the way it came to the round window except by means of the tympanic (lower) canal. The energy of these pressure waves causes the basilar membrane of the cochlea duct to vibrate, stimulating the hair cells.

Species Difference

  • Fish & Other Aquatic Animals
    • Fish have no outer ears, only inner ears with no direct fluid connection to their environment. This is due to the easier detection of sound pressure waves in a water environment.
    • Instead, fish have neuromast receptors running along their lateral lines each of which is composed of a group of hair cells. The hairs are surrounded by a protruding jelly-like cupula.
    • The hair cells in the lateral line are similar to the hair cells inside the vertebrate inner ear, suggesting that the lateral line and the inner ear share a common origin.
    • Amphibians/ Reptiles
      • Amphibians/ reptiles have a more familiar hearing system when compared to mammals, although again they have no outer ear they do have the middle and inner ear.
      • They have an exposed tympanic membrane, from here the pathway continues very similar to mammals, via the small bones into fluid filled channels in the inner ear.

Perception of Frequency

The ear is able to distinguish between different frequencies of sound. Similar to wavelengths of electromagnetic scale (e.g. visible light), sound waves also have different variations in frequencies. In terms of audible sounds, the lowest (infra) sound detectable is 10Hz (1Hz being one vibration a second). This is an infrasound meaning it is not detectable by humans. Elephants however are able to hear sounds at this very low frequency. On the opposite end of the scale (ultrasound), bats are able to hear sounds with as high a frequency as 100kHz. To put things in to perspective, a humans hearing range is 20Hz – 20kHz.

The ear is able to detect these different frequencies due to an adaptation by the cochlea. The basilar membrane of the cochlea duct varies in stiffness and thickness. The result is that different frequencies of pressure waves cause different parts of the basilar membrane to vibrate; this therefore stimulates different hair cells. High pitch sounds (e.g. 18kHz) are detected near the stiff, thin base of the basilar membrane (near the oval window) and low pitch frequencies (e.g. 1kHz) are detected near the loose, thick end of the basilar membrane (near the apex of the cochlea duct).

Because of the way the basilar membrane vibrates, nerves which transmit the raw sound wave information to the brain each have their own characteristic frequency – the frequency at which they generate action potentials most frequently. These afferent nerves are located along the length of the cochlea – nerves located near the apex will have a characteristic frequency (the frequency at which action potentials are generated most readily) which is low (e.g. 1kHz) whereas nerves located near the oval window will have much higher characteristic frequencies.

There are two problems with this however. It is difficult for the basilar membrane to vibrate at very low frequencies e.g. 10Hz, if the basilar membrane is not vibrating then the hairs do not detect movement and the nerves are not stimulated – no sound. On the opposite end of the scale, very high frequencies cause vibrations so fast the neurones are not able to generate action potentials fast enough to keep up.

A process called phase-locking helps to detect the lower frequencies. This is where groups of neurones instead of generating action potentials as normal, they generate them at the frequency of the sound wave. For example, a 60Hz sound wave will cause a group of neurones to fire 60 times in a second.

At higher frequencies, when the nerves simply cannot fire fast to phase lock (e.g. 100kHz) the brain is able to simply detect that the very thinnest part of the basilar membrane is vibrating and interpret this as a sound. This is known as tonotopy.

Perception of Volume

Volume is also determined by the hair cell mechanoreceptors. In terms of waves, a louder sound has greater amplitude (height) but the same frequency (cycles per second). This is mimicked along the basilar membrane of the cochlea, louder sounds cause greater vibration of the membrane – making it rise and fall with greater height.

The greater displacement of the membrane causes the hairs of the hair cells to brush against the tectorial membrane more vigorously. This generates more action potentials in the sensory neurones as the brain perceives a louder sound.

Transduction of Sound Wave Signals

As you know, sound waves are ultimately detected by the hairs of the mechanoreceptor hair cells. The bending of the hairs must be converted into the generation of action potentials by sensory neurones which synapse with the hair cells before the information can be processed in the brain.

When a sound wave causes a certain part of the basilar membrane to vibrate (location depending on its frequency) the hairs bend against the tectorial membrane. As they bend mechanoreceptors cause ion channels in the hair cells to open/close depending on which direction they bend.

The initial direction of bending (stimulus applied) opens the ion channels causing an influx of ions and depolarisation of the mechanoreceptor. The depolarisation causes an increase in the amount of neurotransmitter released across the synapse with sensory neurone. This in turn increases the frequency at which the sensory neurones generate action potentials.

As the hairs bend the other way (stimulus removed), the ion channels close. The closure of the ion channels causes the hair cell to hyperpolarise. This reduces the amount of neurotransmitter released by the cell and the synapsing sensory neurone reduces the frequency at which action potentials are generated.

Auditory Pathway

The auditory pathway shows how sound waves are eventually processed by the brain. So far we have discussed from the sound entering the cochlea (via the external auditory canal) to hair cells of the organ of Corti.

From the hair cells, the majority of information travels along large, fast, myelinated nerves. The axons of these nerves join together and form the auditory nerve (part of the vestibulocochlear nerve [VII]).

The auditory nerve initially travels through the cochlear nuclei; one nucleus located both dorsally and laterally. Information from either ear remains separate at this stage.

The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the inferior/caudal colliculus of the midbrain.

The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. It is here that sound location data from different areas becomes integrated, allowing sound direction/ distance to be interpreted.

Two ways in which sound location is distinguished include:

  • Time delay – If a sound arrives at one ear first, this is recognised in the inferior colliculus.
  • Volume Differences – Subtle differences in the volume of a sound at each ear also help to place a sound.

The Medial Geniculate Nucleus is part of the auditory thalamus and is what finally connects the inferior colliculus to the auditory cortex. It is involved in the tonotopic organisation of sound information, as well as the detection of relative intensity and duration of a sound.

The primary auditory cortex is the region of the brain that is responsible for processing of auditory information. It is located on the temporal lobe, and performs the basics of hearing; pitch and volume.

The auditory cortex is involved in tasks such as identifying and segregating auditory “objects” and identifying the location of a sound in space.

Arterial Blood Supply to the Brain

Introduction

Compared to other tissues, the brain is extremely dependent on a stable and efficient blood supply. Despite making up only 2% of total body mass, the brain requires 15-20% of total cardiac output; this makes the brain extremely sensitive to hypoxia. Any hypoxic damage caused to the brain becomes irreversible after only a few minutes.

In normal tissue, there are three typical forms of metabolites utilisable for energy, these include; glucose, fatty acids and ketone bodies. However, in the brain only glucose can be utilised, except under extreme conditions, such as starvation. During these harsh conditions for the brain, ketone bodies may be used for energy. Because of the brains dependence on glucose, hypoglycaemia will result in dizziness and confusion as the brain is starved of energy.

An overview of Trophic Support:

  • The heart pumps blood into the arterial system
  • This moves blood to the skull
  • The blood passes through a series of membranes (meninges)
  • From here it moves into:
    • Capillaries
    • Neuronal extracellular fluid
    • Cerebrospinal fluid (CSF)
    • Used blood then flows into venous sinuses (blood filled cavities between the skull & brain)
    • Draining back into venous blood
    • Moving back to the heart

The Circle of Willis

The Circle of Willis is a circular network of arteries that supply blood to the brain. It acts as a redistribution centre for blood which is supplied to the Circle of Willis; blood is brought together here and then moved to the brain. The Circle of Willis sits directly beneath the brain. The arrangement of the brain’s arteries into the circle of Willis creates redundancies in the cerebral circulation. If one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can often preserve the cerebral perfusion well enough to avoid the symptoms of ischemia (restriction of blood supply).

There are 4 routes which blood can take to reach the Circle of Willis, these are:

  1. Common carotid arteries -> internal carotid arteries (This is the most direct route)
  2. External carotid arteries -> maxillary arteries -> anatomising ramus ->internal carotid
  3. Vertebral arteries -> rete mirabile -> internal carotid
  4. Vertebral/Vertebral spinal arteries -> basilar artery

Fig. 1 – The arterial supply to the Circle of Willis, the different routes all begin by flowing in through the aorta.

The routes are colour coded to correspond with the above four routes:

  1. Black
  2. Blue
  3. Yellow
  4. Green

In effect there are only two final routes into the Circle of Willis, either:

  • Through either internal carotid artery
  • Through the Basilar artery

At the Circle of Willis, the arteries anastamose (join together) to form a ring.

Fig. 2 – The Circle of Willis, upon reaching the Circle of Willis blood is then distributed further heading towards the brain. The blood supply from the Circle of Willis is shown in Fig. 3.


Fig. 3 (below) – Blood Supply from the Circle of Willis. The lines in red show which arteries the blood leaves the Circle of Willis from, whilst the lines in pink show the arteries which blood use to enter the Circle of Willis

Blood Supply From the Circle of Willis

There are essentially 5 pais of arteries which supply the brain with blood. 4 of these are derived from the cerebral arterial circle, this is the red circle in Fig. 3, only the caudal cerebellar arteries are not derived from this ‘circle’. The five pairs of arteries which therefore supply the brain are:

  • Rostral cerebral arteries
  • Middle cerebral arteries
  • Caudal cerebral arteries
  • Rostal cerebellar arteries
  • Caudal cerebellar arteries

The terms rostral (front) and caudal (back) are interchangable with the terms anterior and posterior respectively (these terms are used in human medicine).

The blood from different arteries emerging from the Circle of Willis tend to supply different parts of the brain:

  • The medial surface of the brain is mainly supplied by:
    • Rostral cerebral arteries
    • Caudal cerebral arteries
    • Whilst the lateral surface of the brain is mainly supplied by:
      • Middle cerebral arteries

Species Differences

Retia Mirabilia

A rete mirabile is present in sheep, goats, swine, ox and dogs and is located in the venous cavernous sinus. The cat has a rete mirabile present also, however it is located extracranially. Retia Mirabilia are absent in the rat and rabbit.

Arterial Blood Supply to the Circle of Willis – Cats

During embryonic development in the cat, the internal carotid artery degenerates. In other species the internal carotid artery is typically a major artery in terms of direction of blood flow (see fig. 1). The maxillary artery compensates and becomes the major supply of blood to the Circle of Willis. Additional supply is also aided by a greater developed pharyngeal artery. This degeneration of the internal carotid also occurs in adult sheep, cows and pigs.

Variation of the Circle of Willis

The Circle of Willis’ structure can differ between species; this has even been observed between different breeds of dogs. The typical example is that some dogs completely lack a rostral communicating cerebral artery altogether.

Supply of Arterial Blood to the Cortex

The route of supply of blood to the brain may also differ between species:

  • Human & Dog – The Circle of Willis receives blood by both the internal carotid and the basilar arteries. (Fig. 1) This means blood supply to the forebrain mainly originates from the internal carotids. Caudal areas of the brain are typically supplied with vertebral arterial blood.
  • Cats, Sheep & Pigs – The internal carotid is much less important; the maxillary artery supplies the Circle of Willis via the anatomising ramus. In this situation the basilar artery carries blood away from the Circle of Willis (unlike in man). This means most blood supply to the forebrain and midbrain is derived from the maxillary artery.
  • The Ox – Blood again flows away from the Circle of Willis via the basilar artery. Blood enters the Circle of Willis via maxillary and vertebral artery pathways, which are well mixed -meaning all areas of the brain, are supplied with blood from mixed maxillary and vertebral arterial origins.

Arterial Supply to the Spinal Cord

Like the brain, the spinal cord requires an equally rich supply of arterial blood, the general arrangement:

  • Ventral spinal artery – a large, broad artery which runs the length of the ventral surface of the spinal cord
  • Dorsal spinal arteries – a pair of smaller arteries which run parallel to the ventral spinal artery along the dorsolateral surface of the spine
  • These arteries are joined by various anatomising arteries which form an arterial ring between the ventral spinal and dorsal spinal arteries.

Regulation of Arterial Blood Supply to the CNS

Should levels of CO2 or O2 alter in the blood, certain regulatory methods are in place to aid the return to normal levels. Hypoxic conditions occur when oxygen levels are low, whilst anoxic conditions occur when there is no oxygen. There are three main regulatory methods:

  • Chemical autoregulation – In all systems, blood flow is locally regulated. When O2 tension falls or pCO2 increases local vasodilation occurs. This results in increased blood flow (Increased oxygen delivery and CO2 removal). The situation is similar for the brain within the restrictions of the inelastic skull.
  • Sympathetic control – A sympathetic drive constricts blood vessels (vasoconstriction) which causes hypoxia. Vasodilation occurs as a result which increases the volume of the brain within the skull causing pressure and pain.
  • Myogenic autoregulation – As pressure increases within cerebral arteries, muscle responds by constricting to prevent increased blood flow.

The Blood Brain Barrier

To protect the brain tissue, a blood-brain barrier is in place. This prevents unwanted substances crossing into brain tissue freely; certain transport systems are in place to selectively transport required substances.

Glucose and ketone bodies do not freely pass through the blood-brain barrier they are instead specifically transported across the barrier. It is believed than even molecules such as water require selective transport across the blood-brain barrier, water is moved across by aquaporin transport proteins.

Structure

In simple terms the brain capillaries are surrounded by ‘astrocyte feet’. These astrocytic feet serve to regulate the blood brain barrier in some way.

Differences between a regular capillary tissue boundary and the blood-brain barrier:

  • The capillaries are not fenestrated, but bound together with tight junctions. This prevents simple diffusion of substances between the gaps as would happen outside the CNS.
  • Pericytes site alongside the capillaries, these may be involved with capillary proliferation.
  • Astrocytic feet surround the capillaries and their role appears to be in the formation and maintenance of the blood-brain barrier complex.