The anatomy of the nervous system

The anatomy of the nervous system

Anatomical Directions and Planes of Section

Anatomical directions help us locate structures in the nervous system.

Common directional terms must be established before undertaking a description of the nervous system. The anatomical directional terms may become confusing due to a 90-degree bend in the neuraxis (an imaginary line that runs the length of the spinal cord to the front of the brain) of humans. Comparing the use of the terms between a four-legged animal and a human is a very useful tool to minimize confusion.

1. Rostral / anterior structures are located toward the head within the body region or the front of the skull within the head region.

2. Caudal / posterior structures are located toward the tail (feet in humans) within the body region or the rear of the skull within the head region.

3. Dorsal / superior structures are located toward the back within the body region or the top of the skull within the head region.

Ventral / inferior structures are located toward the belly within the body region or the bottom of the skull within the head region.

5. In humans, the dorsal parts of our brain form a 90-degree angle with the dorsal parts of the spinal cord.

6. Midline imaginary line that divides the body into two equal halves

7. Ipsilateral structures are on the same side of the midline, and contralateral structures are on opposite sides of the midline.

My left arm and left leg are ipsilateral.

8. Structures near the midline are medial, and structures away from the midline are lateral.

My heart is medial to my arms, whereas my ears are lateral to my nose.

9. In limbs, proximal structures are closer to the body center, and distal structures are farther away.

Anatomists make particular cuts or sections in the nervous system in order to view the structures in two rather than three dimensions.

Coronal sections divide the brain from front to back (parallel to the face) in a vertically cut plane as if from ear to ear.

Sagittal sections are parallel to the midline and give us a “side” view of the brain in a vertically cut plane as if from the front to back of the head. A special sagittal section cut direction on the 3 midline of the brain is called a midsagittal section (divides brain into 2 approximately equal halves).

This is a common view used to describe the corpus callosum, the brainstem and midbrain structures, and the ventricle system.

Horizontal / axial sections divide the brain from top to bottom in a plane that is parallel to the floor in a human standing upright.

Protecting and Supplying the Nervous System- the brain is one of the most protected organs in the body→ the bony skull protects the brain from all but the most serious blows; the skull bones are not fully mature until about 18 months

The Meninges- membrane layers that cover the NS

Three layers of meninges protect the central nervous system: the dura mater (outermost layer, composed of leather-like tissue that follows the outlines of the skull bones); found in CNS & PNS),

the arachnoid (the middle layer, more delicate layer, looks like a spider web in cross section, CNS only),

the pia mater (innermost layer, nearly transparent membrane, found in CNS & PNS);

between the arachnoid & pia mater layers is the subarachnoid space (filled with cerebrospinal fluid)

Meningitis- meninges become infected; viruses, bacteria & fungi can lead to meningitis; results from infected blood from another part of the body traveling to brain or spinal cord or ear or sinus infection or skull fracture; viruses are more common but not as dangerous as bacteria

Blood barrier protects substances from going to brain, some substances can pass though

The Cerebrospinal Fluid circulates through the four ventricles (hollow spaces within the brain), the central canal of the spinal cord, and the subarachnoid space, floating and cushioning the central nervous system.

The Cerebrospinal Fluid (CSF) can be thought of as an ultrafiltered version of the plasma found in circulating blood.

The CSF is generated by the choroid plexus primarily in the lateral ventricles, and it flows in a pattern from the left and right lateral ventricles into the medial third ventricle through the narrow cerebral aqueduct of the midbrain into the fourth ventricle between the brainstem and the cerebellum and finally into the central canal of the spinal cord and the surrounding subarachnoid space where it is absorbed

A primary function of the CSF is to protect the brain through floating the brain rather than attaching it to the skull.

Buoyancy: because the brain is immersed in fluid, the net weight of the brain is reduced from about 1,400 gm to about 50 gm. Therefore, pressure at the base of the brain is reduced.

Excretion of waste products: the one-way flow from the CSF to the blood takes potentially harmful metabolites, drugs and other substances away from the brain.

Super sagittal Sinus, from CSF

Endocrine medium for the brain: the CSF serves to transport hormones to other areas of the brain. Hormones released into the CSF can be carried to remote sites of the brain where they may act.

Hydrocephalus is the condition resulting from a blockage of CSF flow through the central nervous system. The blockages usually occur at the narrow passages in the ventricle system such as the cerebral aqueduct. These blockages are commonly associated with development, tumor growth, or swelling of the brain due to trauma *shunts used to treat all symptoms, drain excess fluid *tube)

The Blood Supply: The brain is supplied with blood through the carotid (on either side of neck) and vertebral arteries (travel up through the back of the skull) (muscular blood vessels that carry blood away from the heart)

once inside the skull, these major arteries branch to form the anterior, middle, and posterior cerebral arteries, which serve most of the brain.

The neurons of the central nervous system use large amounts of energy and thus require a constant supply of oxygen and glucose among other nutrients.

There is no storage of oxygen or glucose within the central nervous system and so an uninterrupted supply is critical. Significant neural death occurs within 3 minutes of the central nervous system not receiving any new blood supply. In the event of a cardiac arrest, effective and immediate CPR first-aid helps keep oxygenated blood flowing to the central nervous system to prevent brain damage.

The Central Nervous System (divided into the CNS [brain & spinal cord] & PNS [nerves that exit brain & spinal cord, carry sensory & motor messages to & from other parts of body])

The Spinal Cord (long cylinder of nervous tissue that extends from the medulla to the first lumbar vertebra; neurons making up the spinal cord are found in the upper 2/3 of the vertebral column; 7 running down the center of the spinal cord is the central canal, which is filled with CSF)

The spinal cord may be divided into cervical (8 segments/nerves-serve the area of head, neck, arms), thoracic (12 segments-serve most of the torso), lumbar (5 segments- serve lower back & legs), sacral (5 segments- serve back of legs & genitals) and coccygeal (1 segment).

The spinal cord segments are named according to vertebral bones surrounding the spinal cord. The incoming afferent sensory nerves and outgoing efferent motor nerves exit the vertebral column between each vertebral bone resulting in 31 discrete nerve segments. The area that is innervated (supplied) by each of the 31 spinal nerves is called a dermatome. The motor cortex and somatosensory cortex respectively located in the frontal and parietal lobes are organized in a medial to lateral fashion by ascending dermatome from the toes to the head.

Afferent info going to carry sensory towards CNS

White matter: An area of neural tissue primarily made up of myelinated axons.

Gray matter: An area of neural tissue primarily made up of cell bodies.

dorsal horns: Gray matter in the spinal cord that contains sensory neurons.

ventral horns: Gray matter in the spinal cord that contains motor neurons.