Spinal Cord (Gross Structure, Functions of Afferent and Efferent Nerve Tracts, Reflex Activity)

Gross structure

A long cylinder of nerve tissue, the spinal cord has subtle enlargements at the cervical and lumbar (lumbosacral) levels. In the enlarged segments, you'll find the brachial and lumbar plexuses. As you can see from the above image of a neonatal pig's brain and spinal cord, the spinal cord and spinal roots are encapsulated by the dura mater. The segments of the spinal cord are each paired with spinal nerves. There are a series of rootlets that come together to form the dorsal and ventral spinal roots. Specifically, spinal ganglions are found distally on each dorsal root.

There are eight cervicals, 13 thoracics, seven lumbar, three sacral, and five caudal segments in the canine spinal cord. Meninges surround the spinal cord and its roots, which are housed in the vertebral canal. The epidural space is a space between the wall of the vertebral canal and the dura mater of the spine that contains a variable amount of fat. Subarachnoid space, filled with cerebrospinal fluid, surrounds the spinal cord within the dura mater. The spinal cord and membrane are connected by denticulate ligaments. The dorsal spinal roots and ventral spinal roots are united to form spinal nerves that leave the vertebral canal via intervertebral foramina. Foraminal spaces exist between adjacent vertebrae and where the intervertebral disc joins them.



Since the spinal cord and vertebrae are growing at differing rates, the majority of spinal cord segments are positioned cranially to the vertebrae corresponding to them. However, the length of spinal segments varies from individual to individual in our domesticated mammals. In particular, segments between C3 and T2 become shorter. At the thoracolumbar junction, segments extend within nominally corresponding vertebrae, and then they elongate. As segments shorten, a terminal filament of glia is formed at the end of the cord. This term refers to the cone-shaped region between the spinal enlargement and glial filament that surrounds the lumbosacral enlargement.



Because spinal nerves exit the canal at nominally corresponding intervertebral foramina, spinal roots must extend when spinal cord segments are displaced cranially. Cauda equina (horsetail) is a term that describes the caudally running spinal roots that run to intervertebral foramina in the sacrum and tail. When the cauda equina is damaged, it affects the viscera of the pelvis and the tail. Cattle epidural anesthesia (disrupting the conduction of spinal roots with anesthetic in the epidural space) is common during obstetrics.



Spinal cord segments can be palpated and seen on a radiograph unlike spinal segments, which cannot. This makes it clinically useful to know where spinal cord segments are about vertebrae.It was the C6-7 intervertebral disc that generated the cervical enlargement; the thoracolumbar junction stemmed from the C6-7 intervertebral disc, and the sacral segment originated from the C6-7 vertebral cone. (The termination of the spinal column occurs about one vertebra more caudally in small dogs (fewer than 7 kg))

Nervous tract functions in afferents and efferents

Afferent nerves

It is the function of afferent nerves to transmit sensory impulses from external organs such as the skin, viscera, and muscles to the central nervous system. Sensory responses can be triggered by a wide range of things, including pain, moving stimuli, vibrations, temperatures, light, and even noxious signals from nociceptors. Besides being able to conduct signals that could potentially cause tissue damage, nociceptors are special afferent nerves. Various organs are covered with afferent nerves.

Example -

• The afferent nerves located in the airways control the tone of autonomic neural pathways in the airways, coughing, and breathing patterns.
• Additionally, cardiac afferent nerves are responsible for cardiac muscle contraction.
• Additionally, cardiac afferent nerves are responsible for cardiac muscle contraction.

Efferent nerve

Both skeletal and smooth muscle is controlled by motor neuron efferent nerve fibers. The central nervous system (CNS) communicates effects and actions via motor-like impulses transmitted to peripheral organs.

Reflex activity

Involuntary responses to stimuli are called reflexes. The survival instinct is composed of reflexes. Most of the common reflexes are a result of all the knowledge we have acquired through years of training and experience. Anything and everything could trigger such a reflex, such as abruptly withdrawing a hand in response to an extremely cold or hot object. Such actions are referred to as reflex actions. There is a subtle connection between instinct and reflex. There is something to be considered about the fact that everyone's instincts vary based on their past experiences. Reflexes are reactions that are triggered by these instincts. It can sometimes be difficult to tell whether a pan is hot or not. As a result, instincts are quite different from reflexes. As reflex action has enabled rapid reactions to certain situations in which organisms' lives may be at stake, reflex action has played a vital role in ensuring their survival from an evolutionary perspective

Action of neuron

Two types of neurons dominate this pathway, afferent nerves (receptors) and efferent nerves (effectors or exciters).
The following events are briefly described:

• In the beginning, receptors detect a stimulus or sudden changes in the environment, and instincts perform a role again. A sensory organ receives the response.
• Once the relay neuron receives the sensory neuron's signal, it sends it to the sensory neuron.
• Following this, a relay neuron sends information to the motor neuron.
• Furthermore, motor neurons send signals to effectors.
• Effectors produce immediate responses, such as the pulling away of the hand and knee-jerk reactions.

Following the above explanations, it can be concluded that as soon as the afferent neuron receives a signal from the sensory organ, it sends the impulse to the Central Nervous System via a dorsal nerve root. Signals are sent from the CNS to the effectors by efferent neurons. Thus, a reflex action is triggered. The signals that trigger reflex actions do not travel to the brain; instead, they are directed to synapse in the spinal cord, resulting in an almost instantaneous reaction.

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