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The Story

Our hero is at a dull office party, and has been stuck standing and talking to the chief accountant for the last 15 minutes. Out of the corner of his eye he notices the arrival of the next round of sausage rolls over at the catering table. Time to make an exit.

He makes his excuses and embarks on a journey across the room.

After a couple of steps he notices that somebody is backing up without watching where they are going – our hero is forced to take evasive action.

He smells someone smoking – boy does he hate that!

Oh dear the new office boy has had too much to drink and is tripping over, our hero manages to catch him, and help him to his feet.

Finally at the table he reaches down and scoops up a sausage roll: ouch it is too hot to hold! He dunks it in some tomato sauce, and bring it to his mouth. Now into the mouth: boy does that taste good!

Our hero had several neural activities going on during his epic journey. In order to describe them this paper shall first look at the motor (efferent) pathways, and then the sensory (afferent) pathways.

The Motor (Efferent) Pathways

A very important part of our story is movement. In order to achieve movement signals are sent from the primary motor area of the cerebral cortex to stimulate contraction of appropriate muscles. There are two general efferent neural pathways involved in movement direct and indirect.

The direct pathways (pyramidal) are responsible for precise voluntary movements. The simplest pathway consists of two sets of neurons, upper motor neurons (UMNs) and lower motor neurons (LMNs). UMNs (first order neurons) descend from the motor cortex and through the cerebrum. In the medulla they form bulges known as the pyramids. The UMNs terminate in the nucleus of the cranial nerves or in the anterior grey horn of the spinal cord forming three distinct tracts. The LMNs (second order neurons) innervate the muscles of the face and head from the cranial nerves or extend from the spinal cord to muscles throughout the body. They synapse either directly with the UMNs or via association neurons.

The tracts associated with the direct motor pathways include:

Lateral corticospinal – which comes into use when our hero makes precise voluntary movements with his hands and feet. For example, when avoiding people in the room and when picking up the sausage roll;

Anterior corticospinal – which came into use when our hero moved his axial skeleton. For example, when he turned to leave his boring conversation or when he helped the office boy off the floor;

Corticobulbar – which is used to control voluntary eye movements, tongue, neck, chewing, facial expression, and speech. This pathway was active when our hero was in conversation, when eating, and throughout his effort to cross the room.

These pathways are presented in detail in figure 1.

Figure 1 Tracts that form the direct motor pathways.

The indirect motor pathways (extrapyramidal) follow a more complex route through several structures, including the motor cortex, basal ganglia, limbic system, thalamus, cerebellum, reticular formation, and nuclei in the brain stem. There are five major spinal cord tracts associated with the indirect pathways. These include:

Rubrospinal – which controls precise movements of the hands and feet. Picking up the sausage roll required the use of this tract;

Tectospinal – which moves the head and eyes in response to visual and audio stimuli. The initial sighting of the sausage rolls, the initial detection of someone walking backwards, and the initial fall of the office boy would have all triggered responses from this tract;

Vestibulospinal – which helped our hero stay upright by maintaining balance (see equilibrium).

Lateral reticulospinal – act to facilitate walking by activating flexor reflexes, inhibiting extensor reflexes, and decreasing muscle tone in muscles of the axial skeleton and proximal portions of limbs. All necessary for the act of walking and movement.

Medial reticulospinal - act to facilitate walking by activating extensor reflexes, inhibiting flexor reflexes, and increasing muscle tone in muscles of the axial skeleton and proximal portions of limbs. All necessary for the act of walking and movement.

These pathways are presented in more detail in figure 2.

Figure 2 Tracts that form the indirect motor pathways.

It should be noted that the lower motor neurons (second order neurons) receive both excitatory and inhibitory signals from both pathways, the final response is a sum of all the signals.

Sensory (Afferent) Pathways

Feedback, from afferent neural pathways, is provided to the brain by a number of sensors throughout the body. Sensory information includes:

Cutaneous sensations – touch, pressure, vibration, and temperature;

Proprioceptive sensations – which provide awareness of body positions and movements;

Special senses – taste, smell, hearing, sight, and equilibrium.

The sensations of touch result from stimulus of crude and discriminative tactile receptors in or just below the skin. Crude receptors result in the perception that something has touched the skin but with no knowledge of location. Discriminative receptors allow the brain to pin-point the location of contact. Receptors associated with touch include: Meissner’s corpuscles, hair root plexuses, Merkel discs, and end organs of Ruffini.

Pressure sensations are detected by end organs of Ruffini (type II cutaneous mechanoreceptors) and lamellated corpuscles. The latter are located in subcutaneous tissue, around joints, tendons and muscles, and in various other parts of the body.

Thermal reception has been associated with free nerve endings called thermoreceptors. There are separate thermoreceptors that respond to warm and cold stimuli.

Proprioceptive sensors include: muscle spindles which detect changes in muscle length and tension; golgi tendon organs found in the muscle tendon junction and detect changes in force of contraction and tension; and joint kinesthetic receptors which detect joint movement.

Special senses are discussed below.

The afferent pathways consist of three sets of neurons: first, second, and third order. First order neurons carry signals from receptors to either the brain stem or spinal cord. Second order neurons carry signals from the spinal cord and brain stem to the thalamus, they cross-over (decussate) to the opposite side of these structures before reaching the thalamus. Third order neurons connect the thalamus to the somatosensory areas of the cortex where sensation is perceived.

There are five major sensory tracts these include:

Posterior column – which is used to transmit tactile and proprioceptive sensations. This pathway helped our hero, amongst other things: to identify the sausage role by touch (stereognosis); take evasive action from falling bodies (conscious proprioception, kinesthesia); and use the correct force to lift the office boy of the floor (weight discrimination);

Lateral spinothalamic – transmits thermal and pain sensations. This helped our hero sense the heat in the sausage rolls;

Anterior spinothalamic – transmits sensations of tickle, itch, crude and poorly localised touch and pressure;

Posterior spinocerebellar – transmits subconscious proprioception from the trunk and lower limbs of one side of the body to the same side of the cerebellum. This enabled our hero to walk smoothly, stay balanced, and coordinated;

Anterior spinocerebellar – plays a similar role to the posterior spinocerebellar, but follows a different route.

These pathways are presented in more detail in figure 3.

Figure 3 Tracts that form the sensory pathways

Special Senses & Activities

The main neural pathways used by our hero are discussed above. The following are additional senses that would have been used in his effort to cross the room.

Equilibrium

Equilibrium or balance plays an important role in the story. There are two types of equilibrium: static and dynamic. In static equilibrium body position is maintained (mainly the head) relative to the force of gravity¹. In dynamic equilibrium balance is maintained whilst moving.

The neural pathways for both static and dynamic equilibrium are the same. The receptor organs for equilibrium are found in the ear and include the saccule, urticle and semicircular ducts. Collectively they are called the vestibular apparatus. Signals from the equilibrium sensors in the inner ear are sent through the vestibular branch of the veistbularcochlear (VIII) nerve. These reach the cerebellum and the motor area of the cerebrum and make adjustments to muscle tension in order to stay upright. Afferent and efferent pathways are summarised in figure 4.

Figure 4 Summary of afferent and efferent pathways involved in equilibrium

Sight

Being able to see in order to negotiate obstacles is an important part of the trip across the room. The neural pathways associated with sight are discussed in question 3 (a.).

Speech

A conversation plays a part in the sequence of events. In order to speak, thoughts, generated in the cerebral cortex, are passed into the motor speech area. Impulses, are used to control parts of my face, tongue, and respiratory system producing audible words. The neural pathways used in speech are presented in figure 5.

Figure 5 Neural Pathways associated with speech.

Hearing

Listening to conversation is also a part of the story. The neural activities associated with hearing are described in question 3 (b.).

Salivation

At the first sight of the sausage rolls our hero starts to salivate. The sight of the sausage rolls stimulates our hero’s memory and the thought rolls down from his cerebral cortex to the common integrative centre. Triggering an autonomic response resulting in stimulation of the parotid salivary gland via the glossopharyngeal (IX) nerve, and the stimulation of the GIT via the vagus (X) nerve. The neural pathways are presented in figure 6.

Figure 6 Neural pathways for salivation.

Smell

Smell is a chemical sensation which is triggered by molecular interaction with between 10 and 100 million olfactory receptors located in the nasal cavity. The afferent pathway for olfaction is presented in figure 7.

Figure 7 Neural pathway for olfaction.

Taste

The gustatory sensations or taste are closely associated with olfaction. Taste receptors are located inside structures called taste buds that line the surface of the tongue, and are found on the soft palate, pharynx, and larynx. The neural pathways for taste are presented in figure 8.

Figure 8 Neural pathway for taste sensation.

References

1. Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8^th Edition, Harper Collins, NY, 1996.