Spinal cord and Sensory receptors

Hey, Guys! Hope you would have been clear with the basic terms and structures of neuroscience from my previous blog Neuroanatomy-basic concept. Here in this section, we would be gaining knowledge regarding the detailed feature of the spinal cord and structures associated with it.

spinal nerves

The spinal cord is the lowest long cylindrical part of the CNS located in the spinal canal of the vertebral column. It extends from the medulla oblongata in the brainstem to the lumbar segment L1 or L2. It is around 45 cm long in adults and occupies around 2/3 rd of the vertebral column. The spinal cord has two enlargements one at the cervical level and the other at the lumbar area due to enlargements of the gray matter at this areas. The necessity to have these enlarged gray matter is to have the necessary neuronal machinery for the limb movements. Isn’t it the beauty of nervous system :).

It gives rise to 31 pairs of spinal nerves, each pair arising from a single segment of the spinal cord. Almost all the spinal nerves are mixed nerves having both sensory and motor fibers except for C1 which motor and Cx1 which is sensory.

Area of the skin innervated by the sensory fibers of a single spinal nerve is called as a dermatome. As there is overlapping of innervation of dermatomes destruction of a spinal nerve fails to produce clinically evident anesthesia in that particular dermatome.

dermatome

  • The key dermatomes are- C3-neck, C5-Deltoid area, C6-Radial forearm and thumb, the C8-Ulnar aspect of the hand and little finger, T4-T10-Nipple, T10-Umbilicus, L1-Groin, L3-Knee, the L5-Dorsal surface of the foot and great toe, the S1-Lateral surface of the foot and little toe, S3-S5-Genitoanal area.
  • Some of the clinically important muscles and their innervations are:- Biceps brachii: C5-C6, Triceps: C6-C8, Brachioradialis: C5-C7, Intrinsic muscles of the hand: C8-T1, Thoracic musculature: T1-T8, Abdominal musculature: T6-T12, Quadriceps femoris: L2-L4, Gastrocnemius: S1-S2, Muscles of the perineum, bladder, and genitals:S3-S5

You would be surprised to know that the human spinal cord has about 108 million neurons and a transverse section of spinal cord shows a butterfly-shaped central gray area consisting of neuronal cell bodies, surrounded by white matter mainly consisting of axons of the tracts. We would know more about them in the subsequent blog.

  • Sensory axons enter the spinal cord via the dorsal roots to synapse with neurons in the dorsal horn of the spinal gray matter. Large diameter fibers of dorsal column tract enter more medially and extend more deeply into the dorsal horn as compared to the smaller diameter fibers.
  • Each spinal nerve is formed from a dorsal root housing sensory axons and a ventral root carrying motor axons. The cell bodies of the primary afferent neurons lie in the dorsal root ganglion (DRG) which lies very close to the spinal cord. There is a pair of DRG for each spinal segment. Efferent neuron cell bodies lie within the ventral horns of the spinal gray area and exit the cord via the ventral root.

Dorsal and ventral horns and roots of spinal cord

Lamina

The spinal gray area can be divided into 10 columns or lamina which can be differentiated on the basis of cell size. On the transverse section of spinal cord, these appear as Rexed laminas. Each lamina has specific input and output relations. eg. nociceptors afferents synapse in lamina II, cutaneous mechanoreceptors afferents terminate in the dorsal horn, lamina VII and lamina IX has preganglionic autonomic neurons and motor neurons.

  • Lamina II and part of III make up the substantia gelatinosa, adjacent to which lies the nucleus proprius made by lamina III, IV and V. Both substantia gelatinosa and nucleus proprius are involved in pain and temperature sensation. Lamina VII has the intermediolateral nucleus which exists at T1-L2 levels. It has the sympathetic innervation and gives rise to its preganglionic efferents. Dorsal nucleus (Clarke’s column, D) is a well-defined round or oval area located at the base of the dorsal horn, in the medial part of laminae VII. In humans, it extends from C8 to L2 segments and consists of large, medium-, and small-sized neurons. Lamina IX contains the motor neurons of the anterior horn which innervates the extremities, phrenic nucleus and the spinal accessory nucleus is also located in the lamina IX.
  • Laminae VII and VIII are often included in the “intermediate gray matter” of the spinal cord. Most of the interneurons involved in the spinal reflex are located in this area.
  • Lamina X is the small gray matter area around the central canal of the spinal cord.

Anterior or the ventral horn mainly consists of motor neurons whose activity maybe influenced by the various inputs either from the descending tracts, interneurons or the afferent sensory neurons forming the spinal reflex pathway. You would be surprised to know that these neurons located in the ventral horn are having a somatotopic representation. Interestingly at various location of the CNS such thalamus, cortex cerebellum, we have a somatotopic organization of our body parts.

In the ventral horn neurons innervating the flexor muscles lie dorsally to those innervating the extensor and the neurons innervating the hand muscles lie lateral to those innervating the trunk. The whole purpose of this type of arrangement is to be inline with the descending tracts associated with the different group of muscles. e.g. corticospinal and rubrospinal tract innervating the flexor muscles are located dorsally in the lateral funiculus.As evident now the ventral horn consists of two group of neurons:

  • Medial group – Neurons from this group innervate the muscles of the axial skeleton and the abdominal and intercostal musculature.
  • The lateral group of neurons are mainly located in the cervical and lumbosacral enlargements and innervate the muscles of the limbs.

Meninges

  • Brain and spinal cord are invested by three connective tissue layers known as the meninges.
  • Just over the brain and spinal cord is the covering known as pia mater above which is the arachnoid mater.
  • The space between these layers is filled with cerebrospinal fluid (CSF) and is known as subarachnoid space.
  • Through this space runs the blood vessels, branches of which enters the brain or spinal cord. passive exchange of water and solutes across the pia mater keeps the equilibrium between CSF and the brain or spinal cord.
  • The outermost tough layer is the dura mater which contains the venous sinuses into which arachnoid villi project.
  • Space between dura mater and arachnoid mater is called as subdural space.

meninges in spinal cord

Receptors

For the somatosensory system, sensory receptors are the modified primary afferent nerve endings which get depolarized on the application of threshold stimulus, whereas in other sensory systems receptors are specialized cell type which forms synaptic connections with the afferent neuron. On stimulation, the sensory neurons release neurotransmitters which affect the afferents.

Sensory receptors are classified into 1. Mechanoreceptors, 2. Thermoreceptors, 3. Nociceptors

  • Mechanoreceptors are the receptors found in the skin, muscles, joints, and viscera. Skin mechanoreceptors are classified into slowly adapting (SA) and rapidly adapting (RA) and also classified as type I and type II, distinguished by the location and receptive fields (RF).
  • Thermoreceptors are the receptors found in the skin. They are the free naked nerve terminals of afferents nerves of small diameter. They sense the change in the temperature and slowly adapting and tonically active. They have small terminals and small RF. These thermoreceptors do not respond to noxious temperatures.
  • Nociceptors They are the receptors for noxious, pain-producing stimuli. They are present everywhere in the body. Anatomically they are the free nerve endings of small diameter afferents. They have been further classified based on the stimulus they respond.
    1. Mechanical nociceptors- stimulated by intense mechanical force.e.g- pain in viscera due to excessive distention.
    2. Thermal nociceptors respond to temperature either greater than 45°C or less than 5°C, also respond to intense mechanical stimuli.
    3. Polymodal nociceptors- present in the skin and respond to puncture, extreme temperature and to molecules released during tissue damage such as K+, H+, bradykinin, serotonin, and histamine. Conduction is slow as the afferents are C-type fiber, pain from the viscera, muscle, and tooth are perceived by these receptors.
    4. Itch receptors- respond to histamine released from mast cells and belong to C-type fiber.

mechanoreceptors-types

The diagram above illustrates the different types of receptors which can be further made lucid to remember by the classification table below.

receptor classification table

Meissner corpuscles are dendrites encapsulated in connective tissue which respond  to changes in texture and slow vibrations. Merkel cells are expanded dendritic endings, and they respond to sustained pressure and touch. Ruffini corpuscles are enlarged dendritic endings with elongated capsules, and they respond to sustained pressure. Pacinian corpuscles consist of unmyelinated dendritic endings of a sensory nerve fiber encapsulated by concentric lamellae of connective tissue that give the organ the appearance of a cocktail onion. Free nerve endings are not specialized receptors rather are naked or unmyelinated nerve endings, located  around hair follicles throughout the glaborous and hairy skin as well as deep tissue. These respond to pain or temperature sensation.

In this picture below we can see the schematic representation of the terminations of the primary afferent neurons into the various lamina of the dorsal horn of the spinal gray area.

lamina of spinal cord, fibers terminating

So, we have by now gained the knowledge about the anatomy of the spinal cord and the receptors which form the initial level of the sensory system. Let’s take our journey little up vis the highway system of CNS called as “Tracts”  in the next blog. See you there!.

 

Posted in Neuroscience.

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