Gray matter

GRAY MATTER WHITE MATTER SPINAL NERVES 

Each spinal segment contains millions of neurons within the gray matter. Functionally, four types of neurons exist  

The cell bodies of second-order sensory neurons are found in the dorsal horn. These neurons receive input from afferent neurons (first-order sensory neurons) entering the CNS from the periphery of the body through the dorsal 

The cell bodies of somatic motor neurons are found in the ventral horn. The axons of these neurons exit the CNS through the ventral root of the spinal nerve and innervate skeletal muscles. The two types of motor neurons located in the ventral horn are  

The spatial organization of the cell bodies of the motor neurons follows a proximal-distal rule. Motor neurons that innervate the most proximal muscles (axial muscles of the neck and trunk) lie most medially in the gray matter. Motor neurons innervating the most distal muscles (wrists, ankles, digits) lie most laterally in the gray matter. 

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Franklin TR, Acton PD, Maldjian JA, et al Decreased gray matter concentration in the insular, orbitofrontal, cingulate, and temporal cortices of cocaine patients. Biol Psychiatry 51 134-142, 2002... 

In multiple sclerosis, which is a demyelinating disease, there is loss of both phospholipids (particularly ethanolamine plasmalogen) and of sphingolipids from white matter. Thus, the lipid composition of white matter resembles that of gray matter. The cerebrospinal fluid shows raised phospholipid levels. 

Cheeran MC, Hu S, Sheng WS, Rashid A, Peterson PK, Lokensgard JR (2005) Differential responses of human brain cells to West Nile virus infection. J Neurovirol 11 512-524 Cudrici C, Ito T, Zafranskaia E, Niculescu F, Mullen KM, Vlaicu S, Judge SI, Calabresi PA, Rus H (2007) Dendritic cells are abundant in non-lesional gray matter in multiple sclerosis. Exp Mol Pathol 83 198-206... 

Nucleus A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord. 

The inhibition of two cholinesterase activities in blood can also be used to confirm exposure to certain organophosphate ester compounds. Red blood cell acetylcholinesterase is the same cholinesterase found in the gray matter of the central nervous system and motor endplates of sympathetic ganglia. Synonyms for this enzyme include specific cholinesterase, true cholinesterase, and E-type cholinesterase. Plasma cholinesterase is a distinct enzyme found in intestinal mucosa, liver, plasma, and white matter of the central nervous system. Synonyms for this enzyme include nonspecific cholinesterase, pseudocholinesterase, butyrylcholinesterase, and S-type cholinesterase (Evans 1986). Nonspecific cholinesterase is thought to be a very poor indicator of neurotoxic effects. 

The gray matter, which contains the cell bodies of neurons, is on the outer surface of the cerebrum and forms the cerebral cortex. The white matter, composed of the myelinated axons of neurons, is found underlying the cortex in the core of the cerebrum. These axons are bundled together according to function and organized into units referred to as tracts. The three types of tracts in the cerebrum are ... 

Figure 6.2 Frontal section of the brain. The cerebrum is composed of two types of tissue internal white matter and external gray matter which forms the cerebral cortex. Embedded within the cerebral hemispheres are other masses of gray matter, basal ganglia, and thalamus. The ventricles are filled with cerebrospinal fluid (CSF).

The cerebellum (Latin, little brain) is part of the hindbrain and is attached to the dorsal surface of the upper region of the brainstem. Although it constitutes only 10% of the total volume of the brain, it contains more than half of all its neurons. Its surface consists of a thin cortex of gray matter with extensive folding, a core of white matter, and three pairs of nuclei embedded within it. 

Explain the function of the gray matter of the spinal cord... 

Describe the location and function of each of the four types of neurons found in the gray matter of the spinal cord... 

The gray matter is composed of nerve cell bodies and unmyelinated intemeuron fibers. The location of the gray matter in the spinal cord is opposite to that of the brain. In the brain, the gray matter of the cerebrum and the cerebellum is found externally forming a cortex, or covering, over the internally located white matter. In the spinal cord, the gray matter is found internally and is surrounded by the white matter. 

GRAY MATTER WHITE MATTER SPINAL NERVES... 

A cross-sectional view of the spinal cord reveals that the gray matter has a butterfly or "H" shape (see Figure 7.1). As such, on each side of the spinal cord the gray matter is divided into three regions ... 

Interneurons are found in all areas of the spinal cord gray matter. These neurons are quite numerous, small, and highly excitable they have many interconnections. They receive input from higher levels of the CNS as well as from sensory neurons entering the CNS through the spinal nerves. Many intemeurons in the spinal cord synapse with motor neurons in the ventral hom. These interconnections are responsible for the integrative functions of the spinal cord including reflexes. 

Upon entering the spinal cord, the first-order sensory neurons may enter the gray matter and may then synapse with one or more of the following neurons ... 

Alternatively, the first-order sensory neurons may initially enter the white matter of the spinal cord. In this case, the axons of these neurons may ascend the cord to the medulla or travel up or down the cord to a different spinal segment. Upon reaching its destination, the axon then enters the gray matter of the spinal cord and synapses with one or more of the neurons discussed previously. 

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

An example of the mechanism of the withdrawal reflex is illustrated in Figure 7.4. When a painful stimulus activates a sensory receptor on the right foot, action potentials are transmitted along the afferent neuron to the spinal cord. By way of divergence, this neuron synapses with several other neurons within the gray matter of the spinal cord ... 

Endorphins are found primarily in the limbic system, hypothalamus, and brainstem. Enkephalins and dynorphin (in smaller quantities) are found primarily in the periaqueductal gray matter (PAG) of the midbrain, the limbic system, and the hypothalamus. These endogenous substances mimic the effects of morphine and other opiate drugs at many points in the analgesic system, including in the dorsal horns of the spinal cord. 

Figure 8.2 The endogenous analgesic system. The three major components of the endogenous analgesic system include the periaqueductal gray matter in the midbrain nucleus raphe magnus in the medulla and pain inhibitory complex in the dorsal horns of the spinal cord. This system causes presynaptic inhibition of pain fibers entering the spinal cord. The binding of enkephalin to opioid receptors on the pain fibers prevents release of the neurotransmitter, substance P. As a result, the pain signal is terminated in the spinal cord and does not ascend to higher centers in the CNS.

The efferent pathways of the ANS consist of two neurons that transmit impulses from the CNS to the effector tissue. The preganglionic neuron originates in the CNS with its cell body in the lateral horn of the gray matter of the spinal cord or in the brainstem. The axon of this neuron travels to an autonomic ganglion located outside the CNS, where it synapses with a postganglionic neuron. This neuron innervates the effector tissue. 

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