Spinal Cord

The spinal cord constitutes the central nervous system (CNS) along with the brain.

Terminologies

Nerve: A group of neurons outside the CNS

Tract: A group of neurons within the CNS

Protections to the spinal cord

As we know that CNS is a delicate system, therefore considerable protections are available to the spinal cord:

• Vertebral Column
• Meninges
• Cerebrospinal fluid (CSF)
  

Vertebral column

The spinal cord is protected by the vertebral column. The spinal cord is positioned in the vertebral canal (made by stacking vertebral foramina).

• 7 cervical vertebrae (cervic- = neck) in the neck region.
• 12 thoracic vertebrae (thorax = chest) posterior to the thoracic cavity.
• 5 lumbar vertebrae (lumb- = loin) supporting the lower back.
• 1 sacrum (SA-krum = sacred bone) consisting of five fused sacral vertebrae.
• 1 coccyx (KOK-siks = cuckoo) having four fused coccygeal vertebrae (kok-SIJ-ē-al).

Meninges

Three layers of connective tissue cover the CNS and protect the spinal cord

• Dura mater
• Arachnoid mater
• Pia mater

For details refer to https://dpsru.blogspot.com/2021/06/brain.html

Cerebrospinal fluid (CSF)

CSF is a clear, colorless liquid that transports nutrients and oxygen to neurons and neuroglia from blood.

For details refer to https://dpsru.blogspot.com/2021/06/brain.html

External Anatomy of the Spinal Cord

Shape:                 Roughly oval and flattened slightly anteriorly and posteriorly.

Length in adults: From the medulla oblongata to the second lumbar vertebra's superior                                border (42 to 45 cm).             

Max. diameter:    Approximately 1.5 cm in the lower cervical region.

In newborn infants, it extends to the third or fourth lumbar vertebra.

Superficially, two clear enlargements are observed:

1. Cervical enlargement
2. Lumbar enlargement

The cervical enlargement extends from the fourth cervical vertebra (C4) to the first thoracic vertebra (T1). Nerves to and from the upper limbs ascend from this region.

The lumbar enlargement extends from the ninth to the twelfth thoracic vertebra. Nerves to and from the lower limbs arise from this region.

Conus medullaris is the tapering end of the spinal cord that reaches the level of the intervertebral disc between the first and second lumbar vertebrae.

Filum terminale (arises from the conus medullaris) is an extension of the pia mater that fuses with other meninges and fastens the spinal cord to the coccyx.

Spinal nerves (31 pairs) are the paths of communication between the spinal cord and specific regions of the body.

• 8 pairs of cervical nerves (C1-C8)
• 12 pairs of thoracic nerves (T1–T12)
• 5 pairs of lumbar nerves (L1–L5)
• 5 pairs of sacral nerves (S1–S5)
• 1 pair of coccygeal nerves (Co1)

Two bundles of axons, called roots, connect each spinal nerve to the spinal cord through rootlets.

The posterior (dorsal) root and rootlets contain only sensory axons.

Each posterior root has a swelling, the posterior (dorsal) root ganglion, which contains the cell bodies of sensory neurons.

The anterior (ventral) root and rootlets contain axons of motor neurons.

Internal Anatomy of the Spinal Cord

Fig. 1: A transverse section of the spinal cord

A transverse section of the spinal cord reveals regions of white matter that surround an inner core of the gray matter. 

The gray matter on each side of the spinal cord is subdivided into regions called horns. 

• Posterior (dorsal) gray horns
• Anterior (ventral) gray horns
• Lateral gray horns

The posterior (dorsal) gray horns contain axons of sensory neurons as well as cell bodies and axons of interneurons. 

The anterior (ventral) gray horns contain somatic motor nuclei.

The Lateral gray horns are present between the posterior and anterior gray horns (present only in thoracic and upper lumbar segments of the spinal cord). The lateral gray horns contain autonomic motor nuclei that regulate the activity of cardiac muscle, smooth muscle, and glands.

The white matter of the spinal cord is organized into three broad areas called columns: 

• Anterior (ventral) white columns 
• Posterior (dorsal) white columns
• Lateral white columns 

Working of spinal cord

The spinal cord processes the sensory input and motor output in the following way:

1. Sensory receptors detect a sensory stimulus.
2. Sensory neurons deliver this sensory input into the posterior root. From the posterior root, axons of sensory neurons may advance along three possible routes (see points 3, 4, and 5 ).
3. As a sensory tract, axons of sensory neurons may ascend to the brain through the white matter of the spinal cord.
4. As a sensory tract, axons of sensory neurons may synapse with interneurons in the posterior gray horn then ascend to the brain through the white matter of the spinal cord.
5. In the case of spinal reflex, axons of sensory neurons may synapse with interneuron/somatic motor neurons in the posterior gray horn. 
6. Motor neurons provide motor output to skeletal muscles through the anterior gray horn of the spinal cord. The brain also provided motor output via descending motor tract that synapse directly or indirectly to somatic motor neurons. 
7. Upon activation, motor neurons transport the nerve impulses consequently via anterior gray horn, anterior root, and spinal nerve to skeletal muscle. 
8. Motor neurons from the spinal cord provide impulses to smooth muscles, cardiac muscles, and glands through autonomic motor neurons present in the lateral gray horn. Impulses from the autonomic motor neurons reach respective targets via spinal nerves leaving the spinal cord through anterior roots. 
9. The autonomic motor neurons from the spinal cord synapse with another group of autonomic motor neurons of the peripheral nervous system (PNS). 

Spinal Cord Physiology

The spinal cord helps in maintaining homeostasis by performing two principal functions: 

1. Nerve impulse propagation and integration of information
2. Reflexes and Reflex Arcs

Sensory and Motor Tracts

The name of a tract frequently specifies its location in the white matter, its initiation, and termination. 

For example, the anterior corticospinal tract is positioned in the anterior white column; it initiates in the cerebral cortex and terminates in the spinal cord.  

Nerve impulses from sensory receptors propagate to the brain along two main paths on each side of the spinal cord: 

1. Spinothalamic tract (sensation of pain, temperature,  itch, and tickle)
2. The posterior column (sensation of touch, pressure, vibration, and conscious proprioception) consists of two tracts:

• Gracile fasciculus  
• Cuneate fasciculus  

Two types of descending pathways convey motor output to skeletal muscles through the spinal cord: 

1. Direct (pyramidal pathways): cause voluntary movements. e.g., Lateral corticospinal, anterior corticospinal, and corticobulbar tracts
2. Indirect (extrapyramidal pathways): automatic movements and coordinate body movements with visual stimuli. e.g., Rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, and medial reticulospinal tracts

Reflexes and Reflex Arcs

A reflex is a fast, involuntary, unplanned process that happens in response to a certain stimulus. Two types of reflex are there:

1. Inborn
2. Acquired or learned

Inborn reflexes are available by birth such as drawing the hand away from a hot surface even before its temperature is felt. 

Acquired reflexes are learned during life. For example, while learning to drive one acquires many reflexes, and slamming on the brakes in case of an emergency is one of the acquired reflexes.

Categorization of reflexes based on integration center 

Spinal reflex: integration occurs at the gray matter of the spinal cord. E.g., patellar reflex (knee jerk).

Cranial reflex: integration takes place in the brainstem. E.g., eyes tracking the sentence while reading.

Other ways of categorizing reflexes

Somatic reflexes: involve skeletal muscles as effector organs

Autonomic (visceral) reflexes: involve smooth muscle, cardiac muscle, or glands as effector organs

Reflex Arc

Reflex arc (reflex circuit) is the pathway followed by nerve impulses to produce a reflex action in response to a stimulus. A reflex arc includes the following five functional components 

1. Sensory receptor 
2. Sensory neuron 
3. Integrating center
4. Motor neuron
5. Effector

Depending on the number of neurons involved in the reflex arc, it could be of the following type

• Monosynaptic reflex arc (one synapse)
• Polysynaptic reflex arc (more than one synapse)

Four important somatic spinal reflexes: 

1. Stretch reflex
2. Tendon reflex,
3. Flexor (withdrawal) reflex 
4. Crossed extensor reflex

The Stretch Reflex 

A stretch reflex leads to the contraction of a skeletal muscle (the effector) in response to the stretching of the same muscle.

This type of reflex occurs via a monosynaptic reflex arc. 

Fig. 2: The stretch reflex

The steps involved in stretch reflex are as follows:

1. Stretching of a muscle stimulates sensory receptors present in the muscle (muscle spindles). The spindles observe changes in muscle length. 
2. When stretched, a muscle spindle generates nerve impulses that move along a somatic sensory neuron and reach into the spinal cord through the posterior root. 
3. At the integrating center (in the spinal cord), the sensory neuron makes an excitatory synapse with a motor neuron. 
4. If the excitation is sufficiently strong, nerve impulses initiate in the motor neuron and move along its axon to reach the neuromuscular junction (NMJ). NMJ is the synapse between motor axon terminals and skeletal muscle. 
5. At NMJ, Acetylcholine, released by the axon terminal of motor neurons, interacts with nicotinic receptors on skeletal muscle and causes a contraction to relieve stretching.  

Ipsilateral reflex: when sensory nerve impulses enter and motor nerve impulses exit from the same side of the spinal cord. 

Reciprocal innervation: The mechanisms of a neural circuit that simultaneously promote contraction of one muscle and relaxation of its antagonistic muscle. Reciprocal innervation is crucial for coordinated body movements. 

The Tendon Reflex 

The tendon reflex controls muscle tension by producing muscle relaxation to protect the tendons from breaking under extensive force.

Though the tendon reflex is less sensitive than the stretch reflex, it can override the stretch reflex when pressure is great.

For example, this reflex makes you drop a very heavy weight. 

The tendon reflex is ipsilateral. 

Fig. 3: The tendon reflex

The steps of tendon reflex are as follows:

1. Under the influence of increased applied tension, sensory receptors in tendons become stimulated and the nerve impulses propagate toward the spinal cord through a sensory neuron. 
2. Within the integrating center (the spinal cord), an inhibitory neuron gets activated by the sensory neuron.
3. The inhibitory neuron synapse with the motor neuron and consequently inhibited the stimulation of the motor neuron through an inhibitory neurotransmitter (hyperpolarization). 
4. The motor neuron generates fewer nerve impulses and leads to muscle relaxation that relieves the tension on the tendon.

Thus, the tendon reflex safeguards the tendon and muscle from damage as a result of extreme tension. 

The Flexor and Crossed Extensor Reflexes 

When you accidentally step on a nail, a polysynaptic reflex arc forces you to withdraw the leg in response to the pain. The reflex involved is termed as flexor reflex or withdrawal reflex.

Fig. 4: The flexor reflex

The steps of flexor reflex are as follows: 

1. Sensory receptor (dendrites of the pain-sensitive neuron) becomes activated when you step on a nail or thorn. 
2. These sensory receptors depolarize the neuron and propagate the nerve impulse towards the spinal cord through the sensory neuron. 
3. At the integrating center (the spinal cord), the sensory neuron stimulates the interneurons that carry the nerve impulses to various spinal cord segments. 
4. The interneurons stimulate motor neurons in different spinal cord segments and the motor neurons propagate the nerve impulses towards the effector organ. 
5. Motor neurons release Acetylcholine in the flexor muscles of the thigh (effector organ) that cause contraction of the flexor muscle and consequently, the leg is withdrawn from the nail. 

The flexor reflex is ipsilateral. 

Intersegmental reflex arc: when several segments of the spinal cord are involved through interneurons. 

Something else also happens when one steps on a nail: one might have to shift the bodyweight to the other foot to maintain the balance or to prevent oneself from falling.  

The painful stimulus causes the withdrawal of the limb (flexor reflex) and forces to shift the weight on the other limb to maintain the balance by activating a crossed extensor reflex. 

Fig. 5: The crossed extensor reflex

The steps involved are as under:

1. The sensory receptor of pain in the right foot becomes stimulated when you accidentally place your right foot on a nail or thorn. 
2. The stimulation of the sensory receptor depolarizes the sensory neuron to propagate nerve impulses towards the spinal cord. 
3. Within the integrating center (the spinal cord), the sensory neuron stimulates various interneurons that synapse with motor neurons of the different segments of the left side of the spinal cord. Thus, the pain signals cross to the opposite side of the spinal cord through interneurons. 
4. The interneurons stimulate the motor neurons in various spinal cord segments that propagate the response impulses towards extensor muscles (effector). 
5. The motor neurons release acetylcholine in the extensor muscle that causes contraction and consequently produces extension of the left leg to maintain balance. 

Contralateral reflex arc: when sensory impulses enter from one side of the spinal cord and the motor impulses exit from the opposite side.

Reciprocal innervation in antagonistic muscles also works in both the flexor reflex and the crossed extensor reflex. 

Acknowledgments: 

The images were created on www.biorender.com

Reference:

Tortora, Gerard J., and Bryan H. Derrickson. Principles of anatomy and physiology. John Wiley & Sons, 2018.