Sensory Pathways

What is sensation ?

It is the conscious or subconscious awareness of changes in internal or external stimulus.

What is the Sensory Process ? How much steps ? What are the steps ?

1. Stimulus activates a receptor.

2. Sensory receptor converts stimulus to nerve impulse.

3. Impulse is sent to the brain via. ascending afferent pathways.

4. Signal is then integrated in the brain producing a sensation.

What are the three characteristics for sensation we have learned ?

1. Perception: conscious awareness and interpretation of sensation (e.g. knowing a raindrop has fallen on your skin).

2. Sensory modality: Different types of sensations (touch, pain, vision). Each neuron carries one sensory modality depending on the receptor that it is in contact with. 

3. Adaptation: A feature of particularly the special senses. Neurons involved with smell, taste, sight, hearing and touch are all involved in sensory adaptation. E.g. when you are coming out of the cinema, it will take time for your eyes to adjust to the difference in light.

What is the classification of the senses ?

1. General senses (divided into 2 sensations):

a) Somatic Sensation (muscles, joints, skin): Pain sensations, Thermal sensations (warm, cold), Tactile sensations (touch, pressure, vibration), Proprioception (conscious awareness of the position, location and movement of a body part).

b) Visceral Sensation (sensation coming from the viscera - internal organs): Pain, fullness, cardiac rhythm and respiration.

2. Special Senses: Hearing, taste, vision, smell, balance 

How are sensory receptors classified ?

1. Location / position in the body.

2. Type of stimulus they detect

If you were classifying them based on location / position in the body can you give one example ? List the other two 

Exteroceptors: located at or near the body surface, relay information from the external environment such as touch, vibration, thermal, pain, visual, smell, auditory sensation.

2. Interceptors

3. Proprioceptors

Interceptors: Location ? Function ? Is the impulses produced felt ?

Interceptors are deep in the body. They are located in the blood vessels, nervous system, and the visceral organs. They relay information from the internal environment. Impulses produced are not felt cosnciously but may be felt as pain / pressure.

Proprioceptors: location ? function ?

• Located in the muscles, tendons, joints and inner ear.

• Provide information about body poistion, muscle length and tension, position and motion of joints and equilibrium.

Classification of the receptors based on the type of stimulus they detect ? Photoreceptors ?

Photoreceptors detect light as it shines into the retina if the eye,

Mechanoreceptors ?

Will detect mechanical stimuli such as touch, pressure, vibration, proprioception, hearing and equilibrium, also the stretching of blood vessels and visceral organs ( such as the dilation of blood vessels).

Thermoreceptors ?

detect changes in temperature

Osmoreceptors ?

detect the osmotic pressure of body fluids.

Chemoreceptors ?

Detect chemicals in the mouth (taste), nose (smell) and body fluids (changes in chemical composition of body fluids).

Nocireceptors ?

Respond to painful stimuli caused by physical or chemical damage to tissue

Somatic sensory pathways ? How many pathways ? What information is it carrying ? How is the sensory information travelling ?

• There is 3 pathways.

• It carries somatic sensation from the body to the brain.

• Sensory information is travelling to the brain via. 3 ascending spinal tracts.

What are the names of the 3 pathways ? What is the difference ?

1. Posterior column-medial pathway

2. Anterolateral / Spinothalamic pathway

3. Spinocerebellar pathway

Describe 1st order neurons ? Location of cell body ? Projection of axons ?

• Cell body is located in the dorsal root ganglion of the spinal nerve.

• Axons will project from the somatic receptors to the brainstem or spinal cord

2nd order neurons: Projection from where to where? Axons? What does this mean ?

• From the spinal cord/brainstem to the thalamus or the cerebellum

• All the axons of the 2nd-order neurons will decussate (cross over) before ascending to the thalamus.

• This means that all somatic sensory information will reach the brain on the contralateral (opposite) side of the sensation.

What about 3rd order neurons ?

Axons from the third order neurons will project from the thalamus to the somatosensory cortex.

What does our first somatic sensory pathway be referred to ?

1. Posterior column medial lemniscus pathway

2. Posterior dorsal column medial lemniscus pathway

3. The dorsal columns for short

What sensory information does the posterior column medial lemniscus pathway relay ?

Discriminative touch, weight discrimination, vibration and proprioception.

Posterior column-medial lemniscus pathway: 1st order neurons, where are the cell bodies located ? Where do the axons project from and to ? What side of sensation ? What happens as these axons ascend ?

• The cell body is located in the dorsal root ganglion of the spinal nerve.

• Axons project from the spinal cord to the medulla oblongata on the ipsilateral side of the sensation.

• As these axons ascend they combine to form the dorsal columns (where it gets its name), there are two sets of these ascending axons. 1 forms the fasciculus cuneatus and the other forms the fasciculus gracilis.

• The fasciculus cuneatus relays info from the upper thorax (above T6), the posterior head, neck and upper limbs

• The fasciculus gracilis relays info from the lower part of the body, the lower thorax, abdomen, pelvis and lower limbs.

What about the 2nd order neurons: location of cell body ? Axons ? What they are referred to ? On what side of the brain ? What does this mean ?

• Cell body is located in the gracile and the cuneate nucleus of the medulla oblongata.

• Axons decussate (cross over) at the level of the medulla and are referred to as the internal arcuate fibres. As they ascend to the thalamus they are referred to as the medial leminiscus (where it gets its name).

• On the contralateral side of the sensation - this means that the left somatosensory cortex will receive information from the right of the brain and vice versa.

What about the 3rd order neurons ? Where do there axons project to and from ?

• From the posterior ventral nucleus of the thalamus to the somatosensory cortex.

What is multiple sclerosis ?

It is an autoimmune disorder where the immune system mistakenly attacks and destroys myelin. This leads to a loss of sensation in the fingers, hands and lower limbs. This can cause a decrease in dexterity and ability to identify objects from touch alone.

What does the anterolateral / spinothalamic pathway relay information about ?

• pressure

• pain

• thermal sensation

• non-discriminative touch

1st order neurons: cell bodies are located where ? Axons projecting ? What side of brain ?

• The cell body is located in the dorsal root ganglion of the spinal nerve

• axons project to the dorsal horn of the spinal cord on the ipsilateral side of sensation.

2nd order neurons: cell body is located where ? What happens the axons ? Where ?

• Cell body is located in the dorsal horn of the spinal cord.

• The axons decussate (cross-over) at the entry level of the spinal cord and ascend the lateral (temp & pain) and anterior (crude touch &pressure) to the spinothalamic tracts.

• The axons either ascend in the lateral spinothalamic tract or the anterior spinothalamic tract all the way to the posterior ventral nucleus of the thalamus where it synapses with the 3rd order neurons.

3rd order neurons: axon projection ? Somatosensory cortex ?

• Axons project from the thalamus to the somatosensory cortex.

• Because of the decussation, the left somatosensory cortex receives information from the right and vice versa.

What is Syringomyelia ? Tractotomy ?

Syringomyelia is a condition in which the central canal of the spinal cord becomes enlarged, interfering with the lateral spinothalamic tract. People suffering from this condition often lose sensation in pain and temperature. So you will find them often accidently hurting themself

Teratomy - a surgical procedure where the lateral spinothalamic tracts are cut.

Spinocerebellar pathway: What is different about this pathway ? What does it not involve ?

• It is a two neuron pathway 

• It does not involve the thalamus.

What information does it send ?

Send proprioceptive information from the muscles and the joints to the cerebellum for control of posture and coordination of movement.

1st order neurons: Where is the cell body located ? Where do the axons project ? Where does it relay info from ?

• Cell body is located in the dorsal root ganglion of the spinal nerve.

• Axons project to the dorsal horn of the spinal cord

• Relay proprioceptive information from the golgi tendons of organs (muscle tension), spindle fibres (muscle length) and teh tendon capsules

2nd order neurons: where is the cell body located ? Where do they ascend ? Via what ? 

• The cell body is located in the dorsal horn of the spinal cord.

• Ascend to the cerebellum via the anterior and posterior SCT.

• The difference between the two is that in the anterior, the axons decussate, whereas in the posterior, they remain ipsilateral

What is Friedreich’s Ataxia ?

It is a neurodegenerative condition that specifically affects the spinocerebellar tracts. The tracts will be disrupted and degenerate over time. It will cause incoordination of the upper limbs, individual will have wide gait and will eventually become wheelchair-bound.

What pathway are we going to look at now and what does it focus on ?

We are now going to look at a pathway that carries sensory information from the head and neck to the brain. This involves cranial nerves that are involved in general sensory (Trigeminal Nerve (V), Facial Nerve (VII), Glossopharyngeal Nerve (IX) and Vagus (X) Nerve).

3 other cranial nerves carry sensory information but they are involved in special senses (Optic, Olfactory, VestibuloCochlear)

What does the Trigeminal Nerve receive ?

Receives general sensory information from most areas of the head.

• Touch, pain, pressure and thermal sensation from the face, scalp, cornea, oral and nasal cavities, such as gums, teeth, paranasal sinuses.

Where does the facial nerve receive sensory information from ?

• The anterior 2/3 of tongue , the floor of the mouth and palate.

• Cutaneous (skin) of the external ear.

Where does the glossopharyngeal nerve receive sensory information from ?

• The posterior 1/3 of tongue and pharynx (important for swallowing and gag reflex)

• Eustachian tube of the inner ear (equalises air pressure)

Where does the vagus nerve receive sensory information from ?

Muscles of the throat - larynx, pharynx, oesophagus, sensation from external ear.

What do these 4 cranial nerves do ? Via what ?

These 4 cranial nerves send sensory information to the head and neck via. trigeminal sensory system.

Trigeminal Sensory System: Location ? Cell body is located where ? Projects from ?

• Located in the cranial nerves themselves NOT the spinal nerves.

• Cell body is located in the ganglia of the cranial nerves

• Projects from periphery to the main sensory trigeminal nucleus (CN V) or the spinal trigeminal nucleus (CN V, VII, IX, X)

2nd order neurons: Where is the cell body located ? What happens the axons ? Where do they project to ?

• Cell body is located in the main sensory trigeminal nucleus or the descending spinal trigeminal nucleus.

• Axons (touch/pressure) will decussate/cross over ascending ventral trigeminal leminiscus to the thalamus

• Axons (pain) will be bilateral to form reticular formation for processing excruciating pain

3rd order neurons: Where do the axons project from ?

• Project from the thalamus to the somatosensory cortex and the intralaminar thalamic nuclei.