Spinal neurophysiology

2021-06-16 04:04 PM

Located on the inside, shaped like a butterfly, forming an anterior, posterior, and lateral horn

Structural features

The spinal cord is the part of the central nervous system located in the spinal canal. There are a total of 31 spinal segments, including:

8 cervical vertebrae (C: Cervical).

12 thoracic vertebrae (T: Thoracic).

5 lumbar segments (L: Lumbar).

5 sacral segments (S: Sacral).

1 amputation (C: Coccygeal).

Because in the process of development, the spine grows faster than the spinal cord, so the lowest part of the spinal cord is only at the level of the 1-2 lumbar intervertebral bodies (L1-L2). Therefore, when performing a lumbar puncture, to avoid damage to the spinal cord, we usually puncture at the 4-5 lumbar position (L4-L5).

Each segment of the spinal cord is composed as follows:

White matter

Located on the outside, these are the pathways that carry nerve impulses either up to the brain or down from the brain.

Gray matter

Located on the inside, it has a butterfly shape, forming an anterior, posterior, and lateral horn. Gray matter is composed mainly of neuronal bodies that play a central role in spinal reflexes.

Each spinal segment has 2 pairs of nerve roots coming out on 2 sides, each side has an anterior root which is a motor root, originating from the anterior horn; The posterior root is the sensory root, which arises from the posterior horn. These two roots will merge into the spinal nerves and pass through the respective intervertebral bodies to control movement and sensation for a certain area of ​​the body. Therefore, when the spinal cord is injured, we can rely on the movement and sensory disturbances of those areas to diagnose the location of the injury.

Functions of the spinal cord

The spinal cord governs many important reflexes and is involved in the transmission of nerve impulses from the periphery to the brain and from the brain down.

Conduction function of the spinal cord

Movement transmission:

The spinal cord conducts movement in two ways:

Tower line:

It originates from the frontal cortex (upper frontal gyrus), then descends to the spinal cord and then follows the anterior roots to control voluntary movement for the neck, trunk and extremities.

An important feature of the pyramidal line is diagonal: the pyramidal line originating from the cerebral cortex on one side will govern the movement of the other half of the body. Therefore, when the brain is damaged (tumour, trauma, haemorrhage...), we can rely on the position of hemiplegia to diagnose which side of the brain is damaged.

Outside the tower:

It originates from the subcortical motor nuclei (vestibular nucleus, red nucleus, quadrilateral brain tuber...), then descends to the spinal cord and then follows the anterior root to control automatic movements (muscle tone, reflexes, etc.) balance, coordination, etc.).

For example, The hand movement when walking is an automatic movement controlled by the extrapyramidal tract.

Conduction of sensations:

This pathway transmits all kinds of sensations from the peripheral receptors and then along the spinal cord to the brain. Includes the following routes:

Conscious Deep Sensory Line:

Starting from the sensory organs in tendons, muscles and joints (muscle rhomboids, Golgi bodies), following the posterior roots into the spinal cord, then following 2 bundles of Goll and Burdach up the cerebral cortex, giving the cortex a sense of pressure, weight, spatial position, and functional state of body parts so that the cerebral cortex can accurately regulate active movements without the need to see with the eyes.

In addition, this line also transmits subtle tactile sensations.

In Tabès disease, the two bundles of Goll and Burdach are damaged, and the patient loses a deep sense of consciousness. To correctly perform active movements, the patient must use the eyes to control, if the eyes are closed, the movements will be disturbed and easy to fall (positive Romberg sign).

Deep sensory pathways without consciousness:

It also comes from the sensory organs in tendons, muscles and joints (similar to the conscious deep sensory line), follows the posterior roots into the spinal cord, then follows the Gowers and Flechsig bundles to the cerebellum, giving the cerebellum sensory sense of muscle tone so that the cerebellum participates in the regulation of automatic movements through the extrapyramidal tract.

Tactile Pathway:

It originates from the sensory receptors in the skin and mucous membranes (Meissner corpuscles and Pacini corpuscles) and follows the posterior root into the spinal cord, then ascends to the thalamus and terminates in the contralateral cerebral cortex. This pathway conducts rudimentary tactile sensations, also known as the anterior Dejerin bundle.

And subtle tactile sensations are transmitted by two bundles of Goll and Burdach.

Pathways that transmit heat and cold sensations:

Originates from heat and cold receptors in the skin (Ruffini corpuscles, Krause corpuscles) and peripheral pain receptors, then follows the posterior root into the spinal cord, then ascends to the thalamus and ends in the cortex contralateral brain, also known as posterior Dejerin bundle.

The reflex function of the spinal cord

Reflection definition:

Reflexes are the basic functioning of the nervous system, which is the body's response to stimuli through the nervous system.

The spinal cord governs many important reflexes, which are called spinal reflexes.

Spinal reflex arc:

The reflex arc is the anatomical basis of the reflex, which is the path of nerve impulses from the receptor to the response organ.

A reflex arc consists of 5 parts:

Sensory part.

Return line.

Central neutral system.

Output line.

The agency responds.

Reflection can only be performed when all 5 of these parts are intact, only 1 part is damaged, the reflex will be lost.

The spinal reflex arc is the reflex arc where the central nervous system is the spinal cord.

Types of spinal reflexes:

Muscle tone reflex:

It works to maintain the muscle to always have a certain tone so that when stimulated, the muscle will contract faster and more sensitively. The receptor of this reflex arc is the muscle spindle located within the muscle fibre. When the muscle tends to relax, it will stimulate the muscle spindle, the impulse is transmitted to the spinal cord and from here there is a flow of impulses to adjust muscle tone.

Vegetative reflexes:

The spinal cord is the centre of several vegetative reflexes such as:

Sweat reflex.

Reflex defecation and urination.

Genital reflexes...

Tendon reflex:

Tendon reflex is a very important type of spinal reflex that is widely used in clinical examination to contribute to the diagnosis of several neurological diseases.

The sensory part of this reflex is the tendon, and when the tendon is tapped, the muscle contracts.

Each tendon reflex is innervated by a specific centre in the spinal cord, which consists of several consecutive spinal vertebrae. Therefore, based on the disorder of tendon reflexes, we can determine the location of the damaged spinal cord or diagnose the cause of some neurological diseases.

The following are some commonly used tendon reflexes in clinical practice:

Table: Types of tendon reflexes.

Reflex name

Stimulation site

Response

The dominant segment of the medulla

The top of the arm

Biceps tendon

Forearm contraction

C5-C6

Bone rotation

Rotary brooch

Forearm contraction

C5-C6-C7

Three heads and arms

Pillars

Forearm stretch

C6-C7-C8

Patella

Quadriceps tendon

Stretch your legs

L3-L4-L5

heel tendons

heel tendons

Stretch your feet

S1-S2

In pathological cases, the tendon reflex response will be lost, decreased, or increased more than usual.

Although the tendon reflex is essentially a medullary reflex, its response can be influenced by the CNS parts of the medulla, especially the cerebral cortex. The cerebral cortex may be dominant, causing the tendon reflexes to show dishonesty.

Therefore, in the examination, to honestly evaluate tendon reflexes, we must use the following measures to limit the influence of the cerebral cortex:

Instruct the patient to keep the limbs in a relaxed position, without muscle contraction.

Distract the patient from the examination by asking the patient to look away or ask questions while examining.

Use the Jendrasik manoeuvre when examining the lower extremity reflex: ask the patient to hook both hands together and try to pull hard while tapping to find the lower extremity reflex.

Skin reflexes:

Scratching certain areas of the skin with a slightly pointed object causes muscle contractions in the nearby area. Each skin reflex has a specific centre in the spinal cord and has the same diagnostic value as the tendon reflex.

Some of the skin reflexes commonly used in clinical examination are:

In addition, there is a very important skin reflex that is widely used in clinical practice, the soleus reflex (Babinski reflex). This reflex is not merely a medullary reflex but is closely related to the pyramidal bundle.

Here's how to do the Babinski reflex:

Scratch along the outside of the sole of the foot, starting at the heel and working around towards the big toe. Normally, the toes are drooping (no Babinski sign). If there is a phenomenon of thumbs up and other fingers spread like fan blades, the conclusion is the Babinski sign.

Babinski's sign is very important, based on this sign we can determine whether a nerve injury is central or peripheral.

When the Babinski sign is present, it is certain that the pyramidal bundle is damaged and thus this is a central lesion. Conversely, if the Babinski sign is absent, peripheral lesions are present.

However, in children under 2 years of age, it is normal to still have Babinski's sign, so it is of little diagnostic value at this age.

Table: Types of skin reflexes.

Reflex name

Stimulation site

Response

The dominant segment of the medulla

Upper belly skin

2 sides of the navel above, the outer border of the rectus muscle

The navel seems to be shrinking

T7-T8-T9

Middle belly skin

across both sides of the navel

-//-

T9-T10-T11

Lower abdomen skin

2 sides of the belly button below

-//-

T10-T11-T12

Scrotal skin Da

1/3 on the inner thigh

The skin of the scrotum shrinks, the testicles go up due to the Dartos muscle contraction

L1-L2