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The Somatosensory System

Introduction

The somatosensory system comprises of ‘senses’ known as sensory modalities, these include; tactition (touch), temperature, proprioception (body position awareness) and nociception (pain). It is possible there are others, and these categories may be broken down further, for example kinaesthesia is the awareness of muscle strain/tension which is a form of nociception/proprioception.

Sensory receptors and sensory (afferent) neurones of the somatosensory system can be found from the periphery (such as the skin, muscles and organs) through to the deeper neurones of the central nervous system. Specific receptors are able to detect different stimuli; the stimulation of a receptor causes information to be sent along neurones to the corresponding area of the brain.

General Organisation of the Somatosensory System

A typical somatosensory pathway will begin with a sensory receptor (for example a mechanoreceptor which is able to detect stress/stretch in the skin – helping to form the tactile sensory modality). The stimulation of the receptor will cause information to be sent to the brain, where it will be perceived (in this example as touch). The information is sent to the brain through the spinal cord, typically three long neurones will facilitate this.

The cell body of the first neurone is located in the dorsal root ganglion of the corresponding spinal nerve. The second neuronal cell body is located in the midbrain for motor/touch sensory modalities and the spinal cord for pain sensory modalities. Neurones involved in pain sensory modalities travel to the thalamus, up the spinal cord via the spinothalamic tract.

It is at this point that the ascending neurones cross-over (decussate) to the opposite side of either the spinal cord or midbrain (depending on the sensory modality – above), typically upon entry of the structure of decussation. The axons of these neurones mainly terminate in the thalamus, but may also terminate in the reticular system or cerebellum of the brain.

In the case of touch and pain, the third neurone has its cell body located in the ventral posterior nucleus of the thalamus. The axon of this final neurone terminates in the postcentral gyrus (sometimes referred to as the somatosensory cortex) of the parietal lobe – where sensory information from different modalities is integrated.

Ascending Somatosensory Pathways

Information from sensory modalities is transmitted to the brain, via the spinal cord. These ascending neurones are able to take multiple pathways to reach their destination. These pathways can be split into three main routes.

Dorsal Column Pathway

The dorsal column pathway:

  • This pathway carries tactile and proprioception sensory modality information. Touch discrimination is owed to this pathway.
  • Sensory information arrives through the dorsal horn and is carried to the dorsal columns (which consist of the Gracile & Cuneate fasciculi)
  • The neurones synapse in the Gracile & Cuneate fasciculi of the medulla, where they decussate
  • The neurones terminate at the thalamus; they travel there along the medial lemniscus. The role of the medial lemniscus is simply to carry the neurones from the Gracile & Cuneate fasciculi of the medulla to the thalamus.

Ventrolateral Pathway

The ventrolateral pathway carries all sensory modalities (except proprioception) but is specifically involved in the propagation of pain. This pathway can be divided into two, as there are two possible tracts which the sensory modalities can take. These are the spinothalamic tract and the spinoreticular tract.

The Spinothalamic Tract

  • Nociceptors (pain receptors) detect a stimulus and neurones carry this to the spinal cord
  • These neurones head directly to the thalamus from the spinal cord, without synapsing elsewhere (via the medial lemniscus)
  • This pathway is associated with nociception such as that from thermal stimuli or from a pinprick

The Spinoreticular Tract

  • Follows the same pathway as the spinothalamic tract except the neurones synapse in the reticular formation of the medulla (primarily associated with the sleep/awake cycle)
  • From the reticular formation the neurones continue to the thalamus
  • This pathway is associated with ‘true pain’

Spinocerebellar Pathway

This pathway is associated with muscle and joint proprioceptors primarily, involving it in postural reflexes. Many neurones which travel via this pathway do not decussate, as is common in the other pathways.

After entering the spinal cord from an appropriate proprioceptor (or kinaesthesia receptor etc.), the neurones synapse in the dorsal horn and then head straight to the cerebellum.

Segmental Organisation

The spinal cord can be divided into sections by which part of the body it serves; cervical (head/ immediate upper body & arms), thoracic (trunk), lumbar (lower back/legs) and sacral (hind). Each of these sections is then made up of 5-12 nerve pairs each serving a smaller sub section of the body/skin; they send sensory information to the brain from their corresponding section.

  • Cervical Nerve Pairs – 8
  • Thoracic Nerve Pairs – 12
  • Lumbar Nerve Pairs – 5
  • Sacral Nerve Pairs – 5

This is significant diagnostically, because the deratomes (small section of skin served by a spinal nerve pair) are served by a specific spinal nerve pair. This means pain deriving from a deratome (or area of skin) if located, can be tracked back to its spinal nerve source.

For example, a human with pain in the skin of the abdomen (Thoracic nerve 12 [Th12]) could point out this pain to a doctor. The pain would be a symptom of possible damage to Th12 and further action could be taken.

Reflexes

Certain sensory modalities such as nociception provide information which needs to be responded to rapidly, using the example of nociception the information received may be that a hot object is causing tissue damage and requires the removal/release of the object quickly. This type of action is usually processed without involvement of the conscious brain and is known as a reflex.

Comparing a conscious response to reflex:

Conscious Response


Reflex Response


Reflexes offer the chance to act quickly by using local processing in the spinal cord – without the need for information to travel to the brain, thus saving time. However there is another type of reflex (sometimes called a long loop, compared to a simple reflex – short loop) called an inter-segmental reflex. This type of reflex looks more like a conscious response, yet the conscious brain is still not involved, so it is deemed a reflex. The processing is done in the brainstem or a separate spinal cord segment, the complete pathway is as follows:

An example of this type of reflex is the ‘Tonic-Neck’ reflex; the reorientation of the head (and thus neck) causes a reflex repositioning of the body and limbs to accommodate the new posture.

Receptors

So we have discussed the transmission of somatosensory signals, but what about their detection? As said earlier, receptors found all over the periphery of the body (e.g. skin, muscle, and organs) detect specific stimuli and transmit the information to the brain, but there are multiple types of receptors available to detect the different stimuli.

Mechanoreceptors

Two key attributes of a mechanoreceptor are the size of its receptive field and the speed at which its fibres adapt. The receptive field is important for discriminating from where a stimulus arises. A small receptive field has better discrimination than a larger one. Typically smooth skin has a small receptive field.

The speed at which fibres adapts concerns how quickly the receptors become desensitised to a stimulus. Rapidly adapting fibres will quickly become desensitised and stop generating action potentials to a stimulus (they may fire action potentials when the stimulus is stopped), whereas slow adapting fibres generally continue to fire action potentials during the length of exposure to the stimulus.

Meissner’s Corpuscle

Found in smooth skin, these mechanoreceptors have a small receptive field and rapidly adapting fibres. They are said to perceive fluttering stimuli.

Pacinian Corpuscle

Found deep within all types of skin, these mechanoreceptors have a large receptive field and rapidly adapting fibres. They are able to perceive vibrations.

Merkel Discs

Found in all types of skin, fairly shallow. These mechanoreceptors have small receptive fields and slow adapting fibres. They are able to perceive pressure.

Ruffini Corpuscle

Found deep within all types of skin, these mechanoreceptors have large receptive fields and slow adapting fibres. They are able to perceive stretching.

Free Nerve Ending

Common receptors for temperature and nociception, they are able to express different types of receptors; mechanical, thermal nociception and polymodal nociception (slow burning pain from chemicals, temperature etc.)

Stimulation of Receptors

The majority of somatosensory receptors are modified ion channels, which when stimulated allow the influx of ions and depolarisation which results in the generation of an action potential and its transduction.

Mechanoreceptors like those listed above; require some sort of mechanical stress e.g. stretching, to stimulate them. This causes the shape of the receptor to distort and opens the ion channel.

Chemoreceptors and thermoreceptors are stimulated by their corresponding stimulus, either directly or by the binding of the (chemical) stimulus to the receptor or a protein linked to the receptor. Again, stimulation leads to depolarisation and action potential generation.

Speed of Signal Transmission

The different receptors propagate their signals along nerve fibres which differ in the speed at which they transmit action potentials. Nerve fibres associated with pain are often much slower than those associated with touch. The different nerve fibres are classified as:

  • Aα – The fastest nerve fibre (72-120ms-1)
  • Aβ – Fast (36-72ms-1)
  • Aδ – Small, slow, myelinated fibres associated with nociception and temperature (4-36ms-1)
  • C – Small, very slow, unmyelinated fibres associated with nociception (0.4-2.0ms-1)

Pain

Pain Perception

Pain is the perception of nociception, until ‘pain’ reaches the cortex it is not pain, but nociception. It is believed that there is a ‘pain gate’ in the dorsal horn, the theory is that by preventing a ‘pain’ stimulus from passing through this gate you can prevent its perception as pain – making it a target for drugs.

Triple Response

The triple response is a phenomenon which occurs after inflammation, it results in pain – caused by irritant chemicals released after physical injury/damage. The chemicals are released onto the skin and free nerve endings. This causes nociception information to be sent to the spinal cord for processing, but the chemicals are also able to spread to other local nerves causing the release of more chemicals and more nociception transduction to the spinal cord. The overall effect is the spreading of inflammation and pain to a larger area than was originally damaged.

The Route of the Somatosensory System through the Brain

Upon reaching the brain, the majority of somatosensory information travels through the thalamus and continues further into the brain. From the thalamus, information head to the sensory cortex. Processing here allows the sensory modalities to be perceived.

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The Vestibular System

Introduction

Located in the inner ear are several small organs responsible for balance. These organs belong to the vestibular system and give humans and many other mammals a sense of spatial awareness.

The vestibular apparatus lie within the vestibule, located in the inner ear labyrinth (along with the cochlea – involved in hearing). The major structures of the vestibular system are:

  • The otolithic organs; the utricle and the saccule – Involved in detecting linear (forward and back or up and down) motions.
  • The semicircular canals – Involved in detecting rotational movements

The Otolithic Organs

As mentioned above, the otolith organs comprise of the utricle and saccule, these are simply cavities which form part of the labyrinth of the inner ear. They contain hair cells and otoliths which send signals to the brain concerning the orientation of the head.

The hair cells of the utricle and saccule project into a gel like material. Within this gel lie many small calcareous structures – otoliths. Their function is to detect motion, which they do by movement. When the head is tilted the otoliths move and press onto the hairs protruding into the gel like substance. The hair cells are receptors which send sensory information to the brain, signalling the orientation of the head when stimulated by the otoliths.

The orientation of the utricle makes it sensitive to horizontal movement and acceleration (e.g. accelerating in a car) whilst the saccule is sensitive to vertical movement and acceleration (e.g. accelerating in an elevator).

The signals received from the sensory hairs can cause corrective motor movement of the posture or eyes, which helps to keep the animal balanced.

The Semicircular Canals

The semicircular canals of the inner ear are arranged over 3 spatial planes (x,y and z) to detect rotational movement across all 3 axis. At the base of all three canals lies a cluster of sensory hair cells (as found in the saccule and utricle). In a similar manner to the otolithic organs, the hair cells project into a gel, however the gel only forms a cap over the hairs – the rest of the canal is filled with fluid. The gelatinous cap is called a cupula.

Rotational movement causes the fluid in the semicircular canals to flow, this in turn pushes on the cupula which stimulates the hair cell receptors embedded within. The stimulation of the hair cells causes impulses to be sent to the brain by sensory neurones alerting the brain that the body is engaged in angular motion.

Sensory information from the vestibular system is detected by the vestibular nuclei via the vestibular nerve (one half of the vestibulocochlear nerve VIII).

Testing Cranial Nerves

Introduction

Cranial nerves arise from the brain directly (unlike spinal nerves which arise from the spinal cord). There are twelve pairs of cranial nerves, varying in length – from supplying nearby structures of the head to the Vagus nerve (X) which is the longest nerve in the body.

Cranial nerves may carry:

  • Sensory information only, i.e. information from an organ to the brain
  • Motor information only, i.e. information from the brain to an organ
  • Both sensory and motor information

Cranial nerves may also be either:

  • Afferent – Meaning to carry sensory information into the central nervous system
  • Efferent – Meaning to carry motor information away from the central nervous system
Cranial Nerve Type of Nerve Fibre Function
Olfactory (I) Sensory Carries sensory information from the olfactory bulb to the brain
Optic (II) Sensory Carries sensory information from the eye to the brain
Oculomotor  (III) Motor Enables the eye to make small, intricate movements
Trochlear (IV) Motor Supplies the extrinsic muscles of the eye
Trigeminal  (V) Both Receives sensory information from the face and supplies motor fibres involved in mastication
Abducens (VI) Motor Supplies the extrinsic muscles of the eye
Facial (VII) Both Supplies motor fibres for facial movements and receives sensory information from ‘anterior taste’
Vestibulocochlear (VIII) Sensory Carries sensory information from the vestibule (balance) and cochlear (hearing) of the inner ear
Glossopharyngeal (IX) Both Carries sensory information from posterior taste (posterior tongue and pharynx) and supplies muscle fibres of the pharynx
Vagus (X) Both Carries sensory information from the pharynx and larynx. Supplies muscle fibres of the larynx as well as; visceral motor fibres to the heart and various thoracic and abdominal organs (including the gastrointestinal tract)
Accessory (XI) Motor Supplies muscle fibres of the neck and shoulders
Hypoglossal (XII) Motor Supplies muscle fibres of the tongue

Testing Cranial Nerves

There are certain tests which can be done to ensure that a cranial nerve is working properly. The tests differ between the nerves due to their different functions. Each test usually has a reflex response which signifies that the cranial nerve is undamaged. The tests have been written primarily with animals in mind, but the majority of these are also observable in humans.

Cranial Nerve Test of Afferent Nerve Test of Efferent Nerve
Olfactory (I) A strong smell is used to test the aversion reflex. If the cranial nerve was undamaged the subject would respond to the smell
Optic (II) Avoiding creating air movement, a finger or hand is thrust towards the eye. If the optic nerve is undamaged, the subject will employ the menace reflex and close the eyelid in response to the finger/hand
Oculomotor (III) Testing eye muscles- Usually tested alongside nerves IV & VI, the movement of the eye and eyelid is observed in response to a stimulus. If this nerve is damaged, the pupils of the eye at rest point down & out

Pupillary reflex- Shining a light into the pupil of one eye should result in the constriction of both pupils

Trochlear (IV) Tested alongside nerve III & VI, if this nerve is damaged a strabismus (abnormal eye alignment) in an up & in direction will be apparent
Trigeminal (V) Touching the skin around the eye will result in the palpebral reflex (closing of the eyelids in response to the touching of the skin). If the nerve is damaged, this will not occur. Also, touching the cornea itself should result in the corneal reflex (closing of the eyelids in response to the touching of the cornea). Again this is absent if the nerve is damaged Should the efferent nerve become damaged, you will be able to observe a drooping jaw in the subject
Abducens (VI) Tested alongside nerve III & IV, if this nerve is damaged a strabismus in a medial, inward direction will be apparent
Facial (VII) The corneal reflex may be tested to check for damage to the nerve. In animals with motile pinna (external ear – not motile in humans), the handclap reflex can be tested. If the nerve is not damaged the pinna will move in response to a loud clap If the efferent nerve is damaged, drooping ears and facial paralysis may be observed. Ptosis (drooping of the eyelid) can also be observed. The menace and palpebral reflexes may be tested to check for nerve damage
Vestibulocochlear (VIII) Testing the Cochlea- The handclap reflex is tested. If the pinna do not respond, this may indicate damage to the nerve.

Testing vestibular responses- In response to altering the orientation of an animal i.e. tilting the body down to face the floor slightly, the neck will self right the head so the head is facing forwards if the nerve is undamaged (tonic neck reflex). If the nerve is damaged, animals may tilt their head with the ear down on the side of lesion/damage. Further observations include nystagmus – spontaneous eye movement, moving slowly in a lateral direction and then returning with a quick eye movement. The direction of the slow movement indicates the side of the lesion/damage

Glossopharyngeal (IX) Bilateral damage to the nerve results in the loss of the gag reflex. Observations that this nerve is damaged include dysphagia – difficulty swallowing
Vagus (X) Similarly to nerve IX, lack of the gag reflex and observing dysphagia can indicate damage. Laryngeal paralysis can be observed with damage, this can cause loss of ability to speak/bark etc. and loud noises when inhaling. Other respiratory and cardiovascular anomalies may arise if damaged.
Accessory (XI)
Hypoglossal (XII) If the nerve is damaged, minor dysphagia and a drooping tongue may be observed – often drooping to the side of damage/lesion if damage is unilateral

Synapses

Introduction

When an axon ends, the action potential which it carries can no longer be transmitted by the same medium. Axons end in a structure known as a synapse, the synapse allows the action potential to be propagated by means of chemicals (as opposed to electrically charged ions). An axon which terminates on an individual muscle fibre is known as a neuromuscular junction.

The terminating end of the axon has a bulbous structure, known as the presynaptic bulb. Within this bulb there can be one of many types of chemicals known as neurotransmitters. These neurotransmitters are the chemicals which transmit the action potential across the synapse (the space between the presynaptic bulb and the post synaptic structure is known as the synaptic cleft).

The most commonly found chemical used as a neurotransmitter is acetylcholine; however others include adrenaline, serotonin and dopamine. These neurotransmitters are housed in the presynaptic bulb contained in vesicles.

The process of transmission is as follows:

  • An action potential propagated to the presynaptic bulb (caused by an earlier stimulus)
  • The action potential depolarises the plasma membrane, this opens voltage gated channels in the membrane that allow the influx of Ca2+
  • The influx of Ca2+ results in some of the neurotransmitter containing vesicles to fuse with the presynaptic bulb membrane, resulting in the expulsion of the neurotransmitter into the synaptic cleft
  • The neurotransmitter diffuses across the synaptic cleft and binds to neurotransmitter specific receptors on the postsynaptic bulb
  • These receptors are ligand-gated ion channels and once the ligand (neurotransmitter) binds to the receptor, it opens allowing Na+ and K+ to diffuse through the channel
  • The influx of Na+ and efflux of K+ ions generates an action potential in the postsynaptic bulb which is propagated further
  • The neurotransmitter is released from the receptor causing it to close
  • The neurotransmitter is recycled, initially being broken down by an enzyme in the synaptic cleft (e.g. acetycholinesterase breaks down acetylcholine into acetate and choline)
  • Some of the neurotransmitter is taken up again by the presynaptic bulb (e.g. choline is pumped back into the presynaptic bulb by a choline carrier)
  • Enzymes convert the molecule back into a neurotransmitter (e.g. choline converted into acetylcholine) and the neurotransmitter is repackaged in an empty vesicle

It is possible for the neurotransmitter to be inhibitory as well i.e. prevent the generation of an action potential, e.g. glutamate is the primary excitatory neurotransmitter of the brain whereas GABA is the brain’s primary inhibitory neurotransmitter.

Types of Post Synaptic Receptors

As mentioned in the typical transmission above, neurotransmitters bind to a post synaptic receptor to result in either the inhibition or generation of an action potential. However, there are other types of post synaptic receptors which do not bind the neurotransmitter directly. The main 3 types are given below:

  • Ligand-Gated Ion Channels: (Fast – 0.1ms delay) also known as ionotropic receptors, these are the type of postsynaptic receptors spoken about earlier. They require the binding of a specific neurotransmitter, the binding of this neurotransmitter results in a conformational change causing the channel to open and allow the passage of ions. They are associated primarily with excitatory neurotransmitters such as acetylcholine and glutamate.
  • Metabotropic Receptors: (Slow – 10ms delay) unlike ionotropic receptors, these receptors are not directly associated with an ion channel. Instead they indirectly link with ion channels by means of signal transduction mechanisms, often G-proteins. Neurotransmitters will bind to the metabotropic receptor, resulting in the activation of these secondary messenger signal transduction mechanisms. The result is the opening of an ion channel and from this point, the process continues as in an ionotropic receptor.
  • Electrical Synapses: (Very Fast – <0.1ms delay) unlike either the ionotropic or metabotropic receptors, there is no association with an ion channel. Instead gap junction like channels pass directly through the presynaptic bulb and synaptic cleft heading directly into the postsynaptic bulb. This means the action potential can head straight through into the next axon without having to stop. This type of system is extremely fast and very uncommon. It is found in only a few specialised systems within the body.

Summation

The process of action potential propagation across synapses can vary. Most post synaptic terminals have a threshold which must be met in order for the action potential to be propagated. Therefore if a weak action potential arrives and is sub-threshold, the action potential ends there. It is not transmitted any further. There is an exception to this – summation.

A post synaptic structure such as a motor neurone may synapse with more than one presynaptic sensory neurone. If this is the case, then it is possible for summation to occur. Summation is when two sensory neurones fire simultaneous sub-threshold action potentials. Normally, as they are sub-threshold, they would not be propagated any further, but as there are two, the effect stacks in the process known as summation.

The above form of summation is known as spatial summation – the action potential of two neurones combine to allow threshold to be reached.

There is another form of summation however, temporal summation. Temporal summation is possible from just one sensory neurone. It works when a neurone is able to generate two action potentials in short succession. By the time the second action potential reaches the synapse, neurotransmitters from the previous action potential have not yet unbound from the postsynaptic receptors. This means the effects from both action potentials stack and threshold is reached.

Special Senses – Taste

Introduction

The tongue is the organ of taste. Perception of taste is also known as gustation and in a similar manner to smell it depends on chemoreceptors to detect specific chemicals from the environment. Specialised epithelial cells (Taste cells) detect these chemicals (tastants). In aquatic animals there is no distinction between olfaction and gustation. Insects use sensory hairs on their feet to taste. Taste probably evolved as a means for animals to detect whether what they were about to consume should be ingested or rejected.

In mammals, taste cells are clustered into taste buds which are scattered around the tongue and mouth. These taste buds are located very slightly beneath the surface of the tongue, a taste pore allows tastants to access the taste bud.

Taste buds are further localised onto raised clusters visible on the tongue, these are known as papillae. Differently shaped papillae are located at different sites on the tongue.

Taste Transduction

Mammals are able to detect the four conventional tastes (sweet, sour, bitter & salty) as well as ‘umami’, which is considered a savoury taste.

The sensory receptor cells (taste cells) which make up the taste buds contain smaller receptors in their plasma membranes. These receptors are specific to one of the above tastes.

When a tastant arrives at one of these receptors, it binds to it, resulting in a secondary messenger cascade. The receptor is coupled with a G-protein and the binding of a tastant activates the G-protein, beginning the secondary messenger process within the sensory receptor cell.

The G-protein eventually activates phospholipase C which in turn generates a secondary messenger known as IP3. IP3’s role in this secondary messenger system is to open IP3 gated calcium channels on the endoplasmic reticulum of the cell.

Opening of these channels causes Ca2+ to leave the endoplasmic reticulum and enter into the cytoplasm of the cell. Ca2+ also acts as a secondary messenger; it causes sodium channels to open allowing an influx of Na+ ions.

The sudden influx of Na+ ions causes the sensory receptor cell (taste cell) to depolarise, this depolarisation results in the local release of neurotransmitters (as taste cells do not have axons). This excites local sensory neurones which generate action potentials in response.

The axons of these neurones bundle together to form part of the facial nerve (VII), the glossopharyngeal nerve (IX) or the vagus nerve (X). Which one depends on the origin of the axon, anterior taste is carried by the facial nerve (VII) and posterior taste is carried by the glossopharyngeal nerve (IX). The throat/ epiglottis are dealt with by the vagus nerve (X).

All three cranial nerves eventually reach the gustatory nucleus of the ‘nucleus of the solitary tract’ in the medulla. From here the information can be sent to one of three places, each of which solicits a different response:

  • The gustatory cortex (via the thalamus) – Here taste is integrated and perceived as a taste.
  • The brainstem – From here reflexes associated with taste may be activated e.g. salivation
  • The Limbic system (especially the hypothalamus) – Emotional responses to taste (e.g. pleasure) can be elicited from here.

Special Senses – Smell

Introduction

The sensation of smell is also referred to as olfaction. The sensory cells of olfaction are chemoreceptor neurones with a direct connection to the brain. The organ responsible for smell is the olfactory organ (‘nose’), also responsible for olfaction is the vomeronasal organ. Odour molecules, which stimulate the olfactory cells, can be either present in gases or dissolved in vapour droplets. Molecules present in gas are detected by the main olfactory organ, whilst those dissolved in liquid are detected by the vomeronasal organ.

The important structures of the olfactory organ are as follows:

  • An external pair of ‘nares’ (nostrils) which act as the entrance to the organ.
  • An internal pair of ‘nares’ located just before the nasopharynx.
  • A richly vascularised anterior nasal mucosa, involved in heat exchange.
  • Turbinate bones, which refer to any of the spongy bones of the nasal passages. The turbinates divide the nasal airway into air passages, responsible for forcing inhaled air to flow in a steady, regular pattern around the largest possible surface of cilia and climate controlling tissue.
  • The sensory cells of the olfactory organ, the olfactory cells which are ciliated chemoreceptors.

Vomeronasal Organ

Located ventrally to the nose, this small organ is able to detect dissolved odours (e.g. pheromones) this makes it important for signalling related to sexual behaviour. The vomeronasal organ is present in many animals including humans and although functionality in animals has been shown there is still debate over whether such functionality is exhibited in humans.

Similarly to the olfaction organ, the vomeronasal organ contains receptors whose neurons possess axons which travel from the vomeronasal organ to the brain. The difference is that the neurons from the olfactory organ travel to the olfactory bulb of the brain whilst vomeronasal neurones travel to the accessory olfactory bulb.

Transduction Pathway of Signals

The pathway of transduction begins in the epithelial lining of the olfaction organ. Chemoreceptor neurones project from the epithelium into the mucosal layer of the organ. The projections are ciliated which increases the surface area of the receptors.

Lining the plasma membrane of the cilia are smaller receptors, these receptors are responsible for detecting individual odour molecules or odourants. The odour receptors are odorant specific, so different odours bind to different receptors. There are a few thousand, different odour receptors.

The odour receptors are G-protein coupled, so when an odour receptor binds its specific odourant the G-protein is activated and a secondary messenger cascade begins.

In the majority of these secondary messenger systems (a relay system where the activation of a receptor causes many changes in the cell, eventually leading to the desired outcome), the enzyme adenylyl cyclase is eventually activated. This enzyme converts ATP to cAMP. The production of cAMP opens Na+/Ca2+ permeable ion channels in the plasma membrane.

The influx of Na+/Ca2+ ions causes the chemoreceptor to depolarise. This causes the chemoreceptor to generate action potentials which are sent directly to the olfactory bulb of the brain.

The unmyelinated axons of the chemoreceptors bundle together in small groups, they then travel through the olfactory foramina, (Small holes in the cribriform plate [part of the skull which makes up the roof of the nasal cavity]) to the olfactory bulb.

These axons synapse with mitral cells in the olfactory bulb. From here the bundled axons from the mitral cells travel directly to the olfactory (piriform) cortex without first heading to the thalamus.

Here odours are processed; typically one smell is made from many different odourants. The different odourants are detected individually and eventually pieced back together to form the perceived, one smell.

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.