Sensory Neuroscience

Sensory neuroscience is a subfield of neuroscience that explores the anatomy and physiology of neurons that form the sensory system; vision, hearing, olfaction, taste and touch.

Central nervous system (CNS)

The central nervous system, or CNS, is the area that controls the majority of the body’s functions. It consists of the brain and the spinal cord and is a system of nerves that carry out the functions of both; the voluntary/somatic nervous system, and the involuntary/ autonomic/vegetative nervous system.

The two parts of the CNS are connected through a section of the brain called the brainstem, which runs through the spinal canal. All electrical impulses generated by the peripheral nerves (motor and sensory nerves outside of the CNS) must travel through the spinal cord in order to reach the brain.

The CNS is protected by the meninges and the cerebrospinal fluid. The meninges help to anchor the brain and spinal cord in place, preventing the brain from moving around within the skull. The cerebrospinal fluid surrounds the meninges and acts as a shock absorber for any sudden impact or injury to the brain and/or spinal cord. The fluid also provides a solution in which the brain is kept suspended, allowing its shape to be preserved, and removes waste products from the brain.

Some reflexive movements, such as pulling your hand away from a hot pan, can occur via the spinal cord itself, and do not require the involvement of the brain. This allows reflex actions to be carried out at a faster rate, hence providing protection to the body against potential damage.

Voluntary and involuntary responses

There are two main types of responses to stimuli - voluntary responses and involuntary responses. Voluntary responses are those which are produced with the involvement of conscious control. These include walking, running, jumping and eating. This type of response is controlled by the somatic nervous system, and usually involves the action of skeletal muscles.

Involuntary responses are those that occur with no involvement of conscious control, such as breathing, heartbeat, blinking and metabolic processes. This type of response is controlled by the autonomic nervous system.

Receptors

Sensory receptors perform countless functions in our bodies, mediating vision, hearing, taste and touch. They are linked to sensory neurons, which receive stimuli from the receptors and generate an electrical impulse corresponding to the specific type of stimulus, via a process known as sensory transduction. The electrical impulse can be in the form of an action potential or a graded potential.

There are many different types of receptors present in the body, which can be classified by the type of stimulus which causes a response in the receptor. Generally, sensory receptors respond to one of four stimuli:

• Chemicals (chemoreceptors)

• Temperature (thermoreceptors)

• Pressure (mechanoreceptors)

• Light (photoreceptors)

Coordination centres

Coordination centres within the body are organs that receive and process information regarding internal and/or external conditions from receptors. In the nervous system, these coordination centres are the brain and the spinal cord.

Effectors

Effectors are types of cells that produce a response to a given stimulus when specific nerve impulses from the coordination centre are received. Examples of effectors are muscle cells, which contract to produce movement.

Muscle cells are generally grouped into two types of effectors:

• Somatic effectors - comprise of striated muscle cells, such as those found in the arm

• Autonomic effectors - comprise of smooth muscles, such as those found in the iris

Both somatic and autonomic effectors receive impulses from the grey matter of the spinal cord, with differences in the specific region of grey matter from which the impulse is received. Somatic effectors receive impulses from the ventral horn, a large neuron in the ventral region of grey matter, while autonomic effectors receive impulses from the lateral horn, another large neuron in the lateral region of grey matter.

Reflexes

Within involuntary responses are another important type of response known as reflexes. These are rapidly-occurring actions in response to external stimuli, and usually serve to protect specific parts of the body from damage due to harsh external stimuli. For example, the pupillary light reflex, which is responsible for controlling the diameter of the pupil in response to the external stimuli of light, occurs to protect the pupil from being damaged as a result of exposure to a light source with a high intensity.

There are many different types of reflexes that can occur. These include:

• Myotatic reflexes (deep tendon reflexes) - cause muscle contraction after the muscle is stretched. Eg. biceps reflex, patellar reflex, ankle jerk reflex.

• Cranial reflexes - mediated by pathways in the cranial nerves and the brain. Eg. pupillary light reflex, accommodation reflex, jaw jerk reflex.

• Primitive reflexes - usually can be observed in infants but not in adults. Eg. palmar grasp reflex, sucking reflex, rooting reflex.

A reflex arc is a neural pathway that controls a reflex. In vertebrates most sensory neurons do not pass directly into the brain, but to a synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons, which remove the delay when compared to if the signals were to be routed through the brain.

The general sequence followed by a reflex arc is as follows:

stimulus → receptor → sensory neuron → relay neuron → motor neuron → effector → response

An example of a simple reflex arc is the withdrawal reflex arc, which occurs if we accidentally touch something hot. The steps involved are as follows:

1. Thermoreceptors (receptors) in the skin detect the change in temperature.

2. The stimulus is converted into an electrical impulse by a sensory neuron.

3. The sensory neuron transmits the electrical impulses to a relay neuron across a synapse.

4. The relay neuron transmits the electrical impulses to the motor neuron across another synapse.

5. The motor neuron conveys the electrical impulses to flexors (effectors) in the arm.

6. The flexors cause muscles in the arm to contract, leading to the hand being moved away from the heat source.

References

• Sensory neuroscience (2008). Available at: https://en.wikipedia.org/wiki/Sensory_neuroscience (Accessed: 16 October 2020).

• Reflex actions - The nervous system - GCSE Biology (Single Science) Revision - BBC Bitesize (2020). Available at: https://www.bbc.co.uk/bitesize/guides/zkdnb9q/revision/3 (Accessed: 16 October 2020).

• Sensory neuron (2017). Available at: https://en.wikipedia.org/wiki/Sensory_neuron (Accessed: 10 November 2020).

• 12.2A: Classification of Receptors by Stimulus (2018). Available at: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/12%3A_Peripheral_Nervous_System/12.2%3A_Sensory_Receptors/12.2A%3A__Classification_of_Receptors_by_Stimulus (Accessed: 7 November 2020).

• Somatic nervous system (2015). Available at: https://en.wikipedia.org/wiki/Somatic_nervous_system (Accessed: 7 November 2020).

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• The Nervous System | Boundless Psychology (2020). Available at: https://courses.lumenlearning.com/boundless-psychology/chapter/the-nervous-system/ (Accessed: 7 November 2020).

• Pupillary light reflex (2020). Available at: https://en.wikipedia.org/wiki/Pupillary_light_reflex#:~:text=The%20pupillary%20light%20reflex%20(PLR,to%20various%20levels%20of%20lightness%2F (Accessed: 7 November 2020).

• Biga, L. et al. (2020) "13.1 Sensory Receptors", OpenStax/Oregon State University, p. Available at: https://open.oregonstate.education/aandp/chapter/13-1-sensory-receptors/ (Accessed: 10 November 2020).

• work?, H. (2016) "How does the nervous system work?", Institute for Quality and Efficiency in Health Care (IQWiG), p. Available at: https://www.ncbi.nlm.nih.gov/books/NBK279390/ (Accessed: 10 November 2020).

• Myotatic Reflex - an overview | ScienceDirect Topics (2020). Available at: https://www.sciencedirect.com/topics/immunology-and-microbiology/myotatic-reflex#:~:text=Myotatic%20Reflexes-,Myotatic%20reflexes%20are%20stretch%20reflexes%20that%20cause%20muscle%20contraction%20after,on%20cerebral%20input%20for%20function. (Accessed: 12 November 2020).

• Spinal Cord Anatomy (2020). Available at: http://courses.washington.edu/pbio375/spinal-cord/sc-anatomy.html (Accessed: 28 November 2020).

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