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Principles of Neuroscience 101: The Spinal Cord - Have Some Backbone

Foreword


The nervous system is at the centre of who we are. Emotion, memory, consciousness, and the ability to move and regulate body functions are all controlled by the nervous system, whether we are aware of it or not. Neuroscience is the study of the structure and function of the nervous system. By studying neuroscience, we can gain an understanding of behaviour, movement and cognition, as well as a wide range of conditions that affect the nervous system such as addiction, schizophrenia and neurodegenerative disorders. The Principles of Neuroscience 101 series delves into the structure and function of the nervous system and how the brain, spinal cord and peripheral neurons interact to regulate body functions and determine how we perceive the world. This article series aims to cover the story of the nervous system from the beginning: neural development from embryo into adulthood, and the anatomy and physiology of the brain and spinal cord to understand where signals come from in the nervous system. The role of neurons and neurotransmitters in transmitting electronic signals throughout the body will be detailed, and finally how all of these factors come together to allow the nervous system to process sensory inputs to help us perceive and understand the world around us.

This 101 series is divided into six articles including:



Principles of Neuroscience 101: The Spinal Cord - Have Some Backbone


Having a backbone is what makes vertebrates vertebrates. It supports the upper body and gives it structure and stability, but beneath the surface, the spine is even more essential and supports some of the body’s most important functions. The spine creates a two-way pathway between the brain and the rest of the body. This is a complex structure made up of several components and has a diverse range of functions. The spine consists of the spinal cord (tube of nervous tissue), the vertebral column (bones) and the meninges (layers of tissue between the vertebrae and spinal cord). While one of the functions of the spine is to allow the body a wider range of motion, this structure also houses and protects the delicate spinal cord from external forces. Neuronal impulses from the periphery are conducted through the spinal cord to the brain, and vice versa. The spinal cord also generates reflexes for automatic and immediate responses to certain stimuli to protect the body from injury. In this article, the total anatomy of the spine will be covered, including the spinal cord, vertebral column and meninges that together make up the backbone.


Evolution of the Spinal Cord

The spinal cord is phylogenetically older than most of the CNS and is the key characteristic of vertebrate animals (Galbusera, 2018). In vertebrates, locomotion was first made possible through movement of the spine. After a few million years of evolution, limbs developed and improved movement. Eventually, humans became bipeds while other apes continued to walk on their knuckles, like gorillas and chimpanzees. However, Cotter and colleagues (2011) have demonstrated that bipedalism may have degraded human bones and made them more porous and weaker than other vertebrates. This study showed that the vertebrae of adult humans are significantly weaker and thinner than the vertebrae of other apes, accounting for body mass. It has been proposed that this is due to the fundamental changes to the musculoskeletal system to allow for bipedalism to occur in humans.


Although there are many advantages to humans for being bipedal, an upright posture has caused high impact loads to be imposed on the lower limbs while moving, particularly during the heel-strike phase of walking. It has been suggested that bones in the lower limbs, such as the distal femur and proximal tibia, are more porous than other bones so that they can better absorb this impact. Furthermore, the cranial and caudal sides of the vertebral body are larger in bipeds. With ageing and the onset of osteoporosis, these weaker, larger vertebrae are vulnerable to spontaneous fractures. Although the musculoskeletal changes that enabled bipedalism in humans are associated with vertebrae weakening, it has been suggested by Cotter (2011) that these may be evolutionary adaptations to enable humans to be bipeds.


Figure 1: Evolution of the spine in vertebrates (Caputo & Kortsha, 2018).
Anatomy and Function of the Spinal Cord

The spinal cord is a continuation of the central nervous system (CNS) and is the only part of the CNS located outside of the cranium. It is encased by the bones of the vertebral column for protection. Together, the spinal cord and the vertebral column make up the spine, or backbone (Jacobson & Marcus, 2011). This vertebral column is made up of a series of individual and interconnected bones called vertebrae [Figure 2]. There are a total of 33 vertebrae divided into five sections: the cervical region, which contains seven vertebrae, the thoracic region, which contains 12 vertebrae, the lumbar region which contains five vertebrae, the sacral region (sacrum), which contains five vertebrae, and the coccyx, that contains three or four vertebrae. The vertebrae of the cervical, thoracic, and lumbar regions are interconnected by cartilage, known as intervertebral discs. The sacrum and coccyx, however, are actually made up of fused vertebrae, and so do not contain cartilage discs. The last bone of the vertebral column is the coccyx, which is also known as the tailbone (Moore et al., 2013).


There are slight variations in the structure of the vertebrae from region to region (Moore et al., 2013). Typically, a vertebral bone consists of a body, the neural arch, and a number of processes. Between the neural arch and vertebral body is a space known as the vertebral foramen. When stacked together in the vertebral column, the vertebral foramen of all the vertebrae forms the vertebral canal, which houses and protects the spinal cord and its blood vessels. The processes are short projections of bone that stick out from the body of the vertebra and are used by ligaments and muscles in the back to attach to the spine. The number of processes varies depending on the section of the vertebral column a particular vertebra is located in (Hall, 2007). Although the vertebrae serve a similar role to the cranium (i.e, to protect structures of the CNS), the bones of the vertebrae differ from the cranium as they are covered by fibrous connective tissue known as the periosteum, which separates the vertebrae from surrounding structures (Mancall & Brock, 2011; Mescher, 2013).

Figure 2: Sections of the spine (Unknown, n.d.).

The spine is the central axis of the body, around which the rest of the body can move. Along with protecting the spinal cord, the vertebral column has important roles in posture, supporting the upper body and providing a strong base to allow for movement of the limbs. The bones of the vertebral column are articulated to give a degree of flexibility in the spine and allow for twisting and turning of the torso (Galbusera, 2018). This is enabled by the many joints between the vertebral bones. There is a defined range of movement between two vertebrae, which can depend on the section of the vertebral column where they are found. The two vertebrae, the soft tissue and intervertebral disc between them make a motion segment, which is the functional unit of the spine. Within each motion segment, there are three types of joints: symphysis joints, left facet and right facet joints. Symphyses are joints that form between the intervertebral discs and vertebral bodies. They are amphiarthrosis joints, which means they are only slightly movable. A symphysis joint is made up of discs of fibrocartilage that connect the bony surfaces of one vertebrae to another. The right facet and left facet joints are diarthroses, which means these joints allow bones to glide over each other (Hall, 2007).


The cephalic (towards the head) end of the spinal cord extends from the brain stem through the foramen magnum (“big hole” in Latin) of the skull, while the caudal (towards the tail) end of the spinal cord tapers off into a conical structure known as the conus medullaris (Jacobson & Marcus, 2011). The conus medullaris is found where the spinal cord terminates, which is usually around the L1 vertebra, but it can occur higher at T12 or as low as the L3 vertebra (Nene, 2023). Extending from the conus medullaris is the cauda equina (“horse’s tail” in Latin) [Figure 3]. This structure contains a bundle of spinal nerves that extend towards the coccyx (Berg, 2023).


Figure 3: The cauda equina (Unknown, n.d.).

In contrast to the brain, where the grey matter surrounds the white matter, in the spinal cord, the grey matter is surrounded by an outer layer of white matter. This grey matter is made up of neuronal cell bodies and is shaped like a butterfly in the centre of the spinal cord (Jacobson & Marcus, 2011) [Figure 4]. The grey matter also contains the grey commissure surrounding the central canal of the spinal cord, which is filled with cerebrospinal fluid (CSF) (Thau, 2022). There are three different ways that grey matter can be classified: in four columns, six nuclei, or 10 laminae. The four main columns are the dorsal horn, ventral horn, lateral horn and intermediate column. The ventral (or anterior) horn is made up of somatic motor neurons, while the dorsal horns contain interneurons. The lateral horn and intermediate column contain neurons that project to the visceral and pelvic organs. Information can be processed at the spinal level in the horns of the grey matter (Thau, 2022).


The nuclei are groups of neuronal cell bodies. Marginal zone (MZ), substantia gelatinosa (SG), nucleus proprius (NP), dorsal nucleus of Clarke (DNC), interomediolateral nucleus (IMN) and the lateral motor neurons and medial motor neurons (MNs). The MZ is involved in relaying pain and temperature. The SG relays pain, temperature, and light touch sensation. These two nuclei are found at the tip of the dorsal horn. The NP is important for relaying mechanical and temperature sensations and is located in the neck of the dorsal horn. The DNC relays unconscious proprioceptive information and is found in the dorso-medial area, but only in spinal segments C8 to L3. The IMN relays sensory information from the visceral organs to the brain and transmits autonomic signals from the brain to the viscera. It is found in the intermediate column and lateral horn. The MNs contain motor neurons that innervate visceral and skeletal muscles. They are located in the ventral horn.


Figure 4: Cross-section of the spinal cord (Unknown, n.d.).

While the nuclei classify spinal cord regions based on location, grouping the spinal cords into laminae takes both location and function into account. The laminae are layers of the spinal cord, numbered I-X [Figure 5]. The first six laminae (I to VI) are found within the posterior grey column. Lamina VII is in the intermediate column, VIII is in the medial anterior grey column and lamina X, known as the grey commissure, makes up the area around the CSF-filled central canal, which is lined with ependymal cells (Khan, 2023). Each lamina plays a distinct role. Lamina I responds to noxious and thermal stimuli and sends information to the brain via the contralateral spinothalamic tract. Lamina II is involved in the sensation of noxious and non-noxious stimuli and modulating sensory input by sending information to Lamina III and IV. Input from this lamina helps the brain’s depiction of sensations being painful or benign. Lamina III projects proprioception and light touch information. Lamina IV relays non-noxious sensory information and contributes to the cortical processing of these signals. Lamina V transmits nociceptive information to the brain through the ascending contralateral and spinothalamic tracts. It also receives descending signals from the brain through the corticospinal and rubrospinal tracts. Lamina VI is made up of small interneurons and sends signalling information through the ipsilateral spinocerebellar pathways to the brain (Diaz & Morales, 2016).


The interneurons of lamina VI are major contributors to spinal reflexes as this lamina receives sensory information from muscle spindles which can prompt a muscle reflex reaction. Lamina VII is a large spinal layer with a structure that changes slightly in different regions of the spinal cord. It receives signals from laminae II-VI and from the visceral organs and sends motor signals to the viscera. Lamina VIII also varies throughout the spinal cord but is seen in the cervical and lumbar enlargements, where it helps modulate motor signals to the skeletal muscle. Lamina IX contains groups of motor neurons that innervate skeletal muscle. Lamina X, the grey commissure, surrounds the central canal. Through lamina X, axons can cross from one hemi section of the spinal cord to the other (Diaz & Morales, 2016).


Figure 5: Spinal laminae (Kyranou & Puntillo, 2012).

Surrounding the grey matter is the white matter, which is made up of axons. The ascending and descending pathways are the bundles of axons found in the spinal white matter. They connect the spinal cord to the brain. Ascending pathways carry messages to the brain while descending pathways carry messages from the brain to the periphery. The neural pathways in the spinal cord are bilateral, which means that each one runs through either side or hemi section of the spinal cord, and to either side of the cerebral hemisphere (Jacobson & Marcus, 2011; Waxman, 2009). In the nervous system, tracts are neural pathways that run between the brain and spinal cord. Ascending tracts transmit sensory signals from the periphery to the brain, while descending tracts carry motor signals from the brain to the periphery (D’Mello & Dickenson, 2008). Tracts follow a specific naming system where the first part of the name is where the tract originates, and the second half is where the tract terminates. For example, the spinothalamic tract originates in the spinal cord and terminates in the thalamus (Waxman, 2009).


As the spinal cord is so essential to the movement and functioning of the body, it must be protected. In addition to the bones of the vertebral column, the spinal cord is also physically protected by meninges [Figure 6]. The meninges are the three layers of membranes that encase the brain and spinal cord to protect them from the bones surrounding them, the skull or vertebral column, respectively. The meninges also support the blood vessels and form cavities for CSF to flow through. The cranial and spinal meninges are continuous and contain the same three layers: dura mater, arachnoid mater and pia mater (Mancall & Brock, 2011). The dura mater is the outermost meningeal layer and is composed of dense irregular connective tissue. Between the dura mater and the periosteum of the vertebral column, there is a cavity, known as the epidural space, which contains loose connective tissue and adipose tissue. The epidural space is where local epidural anaesthesia (e.g., lidocaine) may be injected for surgeries or for pain management (Patestas & Gartner, 2013). By injecting an anaesthetic directly into the spinal epidural space, the roots of the nerves at this vertebral level are anesthetised, resulting in highly localised pain relief. An epidural can be performed at the level of any vertebra, depending on the area of the body that requires anaesthesia (Hernandez, 2022).


Figure 6: Spinal meninges (Unknown, n.d.).

The next layer is the arachnoid mater, which gets its name as its structure is similar to that of a spiderweb. The space between the arachnoid mater and the dura mater is called the subdural space and is very narrow, containing only CSF. The innermost layer of the meninges is the pia mater, which closely envelops the spinal cord. The two inner meninges, the arachnoid mater and pia mater meningeal layers, are known as the leptomeninges. The cavity between them is called the spinal subarachnoid space, which extends from the conus medullaris. The spinal subarachnoid space contains the major blood vessels and CSF also passes through this cavity. As the cranial and spinal subarachnoid spaces are continuous, this provides an enclosed system for CSF circulation throughout the CNS (Patestas & Gartner, 2013). Traumatic head injuries can result in a subdural haemorrhage, which occurs when blood collects in the subdural cavity. However, the most common post-trauma bleeding is a subarachnoid haemorrhage. Symptoms include a sudden, intense headache, dizziness, confusion and even coma (Swann et al., 1984).


Spinal Nerves

The spinal nerves carry signals to and from the body and brain to allow humans to perceive sensations [Figure 7]. The brain also sends information to move the limbs through these nerves. There are 31 pairs of spinal nerves, including eight cervical pairs, 12 thoracic pairs, five lumbar pairs, five sacral pairs, and one pair of coccygeal spinal nerves. These nerves emerge from the spinal cord at these points to innervate the body structures (Jacobson & Marcus, 2011). Spinal nerves are named according to the region of the spine they emerge from. For example, T1 is the first pair of spinal nerves from the thoracic region, while L3 is the third pair from the lumbar region (Jacobson & Marcus, 2011). Each nerve innervates specific regions of the body by exiting the spinal cord through the intervertebral foramina in the vertebral column at its designated location (Thau, 2022).


Figure 7: Spinal nerves (McDonald et al., 2014).

The cauda equina gets its name from the many spinal nerves that emerge from this structure, which resembles a horse’s tail. The lumbar sympathetic, sacral somatic and sacral parasympathetic nerves continue downward from the spinal cord within the cauda equina (Nene, 2023). These nerves carry signals to innervate the lower limbs and pelvis, as well as receive some afferent signals from the periphery, such as vibration, proprioception, pain, and temperature (Berg, 2023).


Sensory Functions of the Spinal Cord

Sensory information from the body’s periphery is first projected to the spinal cord before it is transmitted to the brain. There are three orders of neurons involved in transmitting these signals. When a stimulus, such as touch, activates sensory receptors in the body’s periphery, this information is then carried from the site of stimulation by sensory neurones called primary afferent fibres. These are the first-order neurons. The bodies of these neurones are in the spinal or dorsal root ganglion. The axons of these neurons then project the signal to the posterior horn of the spinal cord grey matter. Here, first-order neurons synapse with the second-order neurons. These neurons ascend upwards through the spinal cord and synapse with the third-order neurons in subcortical brain structures, for example, the thalamus. These neurons transmit the signal to the somatosensory cortex of the cerebrum (Thau, 2022; Waxman, 2009). Once the afferent neurons carry sensory information from the periphery to the dorsal horn of the spinal cord, these primary afferent fibres form synapses with intrinsic spinal dorsal horn neurones to first process the information at the spinal level (Waxman, 2009). The signal is then carried further to the higher centres of the brain by spinal projection neurones along ascending pathways. There is a high concentration of projection neurons found in lamina I of the spinal cord (D’Mello & Dickenson, 2008). Once the signal reaches the brain, non-noxious and noxious signals can be perceived (Waxman, 2009). The grey matter’s ventral horns, containing motor neuron axons, control peripheral movement and regulate involuntary and voluntary reflexes (Thau, 2022).


Figure 8: Spinal cord pathways (Unknown, n.d.).

Stimuli include external and internal physical changes that cause an afferent input to the nervous system. They are not always accompanied by sensory experience or a physical response. A noxious stimulus can cause pain or damage to tissue, but not always. Non-noxious stimuli do not cause pain. Furthermore, non-noxious stimuli can be used to reduce noxious stimuli responses. For example, a sharp pain (noxious stimulus) and be eased by rubbing (non-noxious stimulus) the affected area (Cerveró & Merskey, 1996).


A key function of the spinal cord is in nociception (sensing pain). When a painful stimulus occurs, the information is first sent to the spinal cord, and from there the sensation is relayed onwards to the brain. When the brain receives this sensory information, it sends modulatory signals back to the spinal cord to modulate pain, as well as signals to other brain regions that can perceive the sensation of pain (D’Mello & Dickenson, 2008). Pain transmissions follow particular pathways to reach the brain. Firstly, the primary afferent fibres that receive sensory information (Aβ-, Aδ-, and C-fibres) transmit a signal of a painful stimulus from the point of occurrence in the periphery and transmit the signal, via the dorsal root ganglion (DRG), to the spinal cord’s dorsal horn. Nociceptive specific (NS) cells and wide dynamic ranges (WDRs) cells in the dorsal horn of the spinal cord receive this information. NS cells receive inputs from Aβ fibres, which exclusively transmit touch signals, while WDRs receive inputs from all types of sensory fibres and so can receive pain signals relating to heat and sharp pain, as well as touch. The NS cells project this information to the parabrachial area (PB) and periaqueductal grey (PAG) in the brain and affect the limbic system. This, in turn, activates descending pathways from brainstem nuclei which then modulate spinal processing. WDR cells mostly project to the thalamus, through what is known as the spinothalamic tract. The thalamus then projects to cortical regions involved with pain perception, including the primary and secondary somatosensory cortices, and the insular, anterior cingulate, and prefrontal cortices (D’Mello & Dickenson, 2008).


Figure 9: Sensory responses (McDonald et al., 2014).

There are several mechanisms that are used to modulate spinal cord outputs during nociception transmissions and can either increase or decrease neuronal activity in the dorsal horn. These mechanisms can be excitatory or inhibitory and include local interneurons, N-methyl-D-aspartate receptor activation, and descending signals coming from the brain (D’Mello & Dickenson, 2008). In cases of nerve injury, inflammation conditions and chronic pain, these excitatory and inhibitory mechanisms could become altered. This usually causes dorsal neurons to have heightened responses to incoming nociceptive signals, which is known as central sensitisation and results in increased pain transmission outputs from the spinal cord to the brain (D’Mello & Dickenson, 2008).


Spinal Reflexes

As the spinal cord evolved earlier than other CNS structures, many functions that are essential to life are mediated by the spinal cord, while higher functions are controlled by the brain. Reflexes are an important feature as they help the body avoid injury. For example, rapidly removing the hand from a hot surface is a reflex that prevents us from getting burned. They are mediated by the spinal cord as they require rapid responses to protect the body from harm. Reflexes involve the stimulation of a receptor followed by the activation of a series of connected neurons of a reflex arc that ends by innervating an effector muscle. Depending on how many neurons are involved, reflexes can be monosynaptic or polysynaptic (Waxman, 2009). The patellar, or knee-jerk, reflex is a monosynaptic stretch reflex mediated at the spinal level and is often tested by doctors by tapping a hammer just between the patella (kneecap). In a healthy person, there should be an immediate response to this tap that causes the lower leg to kick out (Lazar, 2022) [Figure 10].


Figure 10: Knee-jerk reflex (Unknown, n.d.).

The knee-jerk reflex occurs when the patellar tendon is struck with an appropriate force. This causes the quadriceps femoris muscle in the upper leg to suddenly stretch, activating the muscle spindle. Muscle spindles are receptors found within skeletal muscles that detect changes in the length of the muscle, such as muscle stretching. When the muscle spindle is stretched, it emits a signal that directly projects to alpha motor neurons in the lumbar region of the spinal cord, usually L3 or L4. The signal is processed within the spinal cord, and interneurons transmit the response directly to efferent motor neurons to innervate the quadriceps femoris muscle and cause it to contract. When the quadricep femoris contracts this rapidly, it pulls the other muscles in the leg with it, resulting in the lower leg kicking out (Ginanneschi, 2015). The stretch reflexes evolved to prevent the muscles from becoming overextended and tearing. When the muscle spindle is stretched and the stretch reflex is activated, the spinal response to pull the muscle back is received very rapidly and the muscle is protected (Jacobson & Marcus, 2011). Testing the knee-jerk reflex helps doctors detect L3 or L4 root injuries. This method can determine if there are any issues with nervous system functionally and is also used as part of diagnosing neuromuscular disorders (Ginanneschi, 2015).


Conclusion

The spinal cord is a tube of nervous tissue that extends from the foramen magnum in the cranium to the conus medullaris. The anatomy of the spine is extremely varied and complex, and there are differences in the structure of both the spinal cord and vertebral bones between the regions of the spine. The spinal cord is a key component of the CNS and has many functions, including relaying sensory information from the sensory organs to the brain, transmitting motor commands from the brain to the periphery, innervating localised areas through the spinal nerves and conducting reflex reactions to keep the body functioning smoothly. To better protect the spinal cord vertebrae bones, layers of connective tissue called the meninges and cerebrospinal fluid all contribute to shielding the spinal cord from harm. The vertebral column of bones that surround the spinal cord has functions outside of protecting this delicate structure, including supporting the upper body, bearing weight, supporting movement of the limbs and allowing flexibility and movement of the torso. Although most spinal cord functions are controlled by the brain, a major function of the spinal cord is in reflex signalling relays, which occur independently from the brain. All vertebrate animals depend on their spine for transmitting nerve impulses and without it would be unable to perform even basic movements or receive any somatosensory inputs from the periphery.



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