Neuropathology

Back to Home Page by Robin Johnson
 * NEUROPATHOLOGY**

Unless otherwise noted, all information is obtained from: McCance, Kathryn L. and Huether, Sue E. (2006). Pathophysiology: The Biologic Basis for Disease in Adults and Children. Fifth Edition; Elsevier Mosby, Philadelphia, PA.


 * __STRUCTURE AND FUNCTION__**

Structurally, the nervous system has two parts:
 * **Central Nervous System (CNS):** contains the brain & spinal cord, that is enclosed within the cranial vault and vertebrae.
 * **Peripheral Nervous System (PNS):** composed of cranial & spinal nerves.

PNS has two pathways:
 * **afferent** - ascending; carries sensory impulses toward the CNS
 * **efferent** - descending; innervate effector organs, such as skeletal, cardiac, and smooth muscle, as well as glands, by transmitting motor impulses away from the CNS.

PNS can be divided into the somatic & autonomic nervous system. the autonomic nervous system can be further divided into **sympathetic** (responds to stress by mobilizing energy stores and prepares the body to defend itself) **& parasympathetic** (conserves energy and the body's resources).
 * **somatic** - motor and sensory pathways regulating voluntary motor control of skeletal muscle
 * **autonomic** - motor and sensory components and is involved with regulation of the body's internal environment through involuntary control of organ systems.


 * __CELLS OF THE NERVOUS SYSTEM__

Neurons -** specialized cells that share many of the same metabolic activities and constituents as other types of cells. They scan the environment, integrate many systems at higher cognitive levels and initate body response to maintain homeostasis.
 * fuel source is predominantly glucose
 * CNS starts out with more neurons that it needs and the ones that aren't involved in functional systems die
 * has three components: cell body (soma), dendrites and axons
 * dendrites - carry nerve impulses toward the cell body
 * axons - carry nerve impulses away from the cell body; the initial segment of the axon has the lowest threshold for stimulation (action potential begins here)
 * typical neuron has only one axon
 * insulated by the myelin sheath; layer of lipid material
 * Schwann cell forms and maintains the myelin sheath and the nodes of Ranier form the spaces on either sides of the Schwann cells
 * classified on the basis of the number of processes extending from the cell body: (1) unipolar, (2) pseudounipolar, (3) bipolar and (4) multipolar.
 * three types of neurons: sensory, associational and motor
 * http://media.nasaexplores.com/lessons/04-034/images/neuron.gif


 * Supporting cells** - neuroglial cells of CNS & Schwann cells of PNS, provide structural support and nutrition for neurons, increase speed of nerve impulses and help with processing and storing information.
 * Neuroglia ("nerve glue") is the greatest classification of cells that support the neurons of the CNS. They make up 1/2 of the total brain and spinal cord volume.
 * There are different types of neuroglia cells and they each have specific functions. For example: **astrocytes** fill the spaces between neurons and surround blod vessels in the CNS; **oligodendroglia** deposit myelin within the CNS; **ependymal** cells line the CSF-filled cavities of the CNS and **microglia** remove debris in the CNS.

__**NERVE INJURY AND REGENERATION**__ //Wallerian degeneration// occurs in the portion of the axon distal to the cut when an axon is severed. the myelin sheath reform into Schwann cells that line up in a column between the cut and the effector organ. Nerve regeneration depends on many factors. The closer to the cell body of the nerve, the greater the chances that the nerve cell will die and not regenerate. A crush injury allows recovery more fully than a cut injury; cut nerves often form connective tissue scars that block or slow regeneration.
 * 1 - characteristic swelling appears
 * 2 - neurofilaments hypertrophy
 * 3 - myelin sheath shrinks and disintegrates
 * 4 - axon portion degenerates and disappears


 * __NERVE IMPULSE__**

Neurons generate and conduct electrical and chemical impulses by selectively changing the electrical portion of their plasma membranes and influencing other nearby neurons by the release of chemicals. When the membrane potential is raised sufficiently, an action potential is generated. A action potential response occurs only when the stimlus is strong enough. This property is known as the //**all-or-none response**//. The region between adjacent neurons is called a **synapse**. Impulses are transmitted across the synapse by chemical and electrical conduction. The chemical conducting substance is called a **neurotransmitter**. Some examples of neurotransmitters are: norepinephrine, acetylcholine and dopamine. Many neurotransmitters have more than one function.

Neurotransmitters are stored on one side of the synaptic cleft and the receptor sites are on the other side; chemical synapses operate in only one direction. Therefore, action potentials are transmitted along a multineuronal pathway in only one direction.


 * __CENTRAL NERVOUS SYSTEM__**

The **BRAIN** has three major divisions:
 * Forebrain - formed by the two cerebral hemispheres (http://www.brainexplorer.org/brain_atlas/Brainatlas_Forebrain.shtml); responsible for such things as ability to concentrate, motor movements, specific movement of muscles, etc. Cerebral impulses control function in the opposite side of the body, a phenomenon called **contralateral control**.
 * The **Telencephalon** consists of the cerebrum and basal ganglia. The lobes of the cerebrum are:
 * Frontal - motor cortex; voluntary skeletal movement, fine motor movements, eye movements.
 * Parietal - processes sensory data, interp. of tactile sensations (temp, pressure, pain, size, shape, texture & two-point discrimination) as wellas visual, gustatory, olfactory and auditory. Recognition of body parts and proprioception.
 * Occipital - primary vision center & provides interpretation of visual data.
 * Temporal - perception & interpretation of sounds and determines their source. Involved in the integration of taste, smell & balance. Reception and interpretation of speech are located in the Wernicke area.
 * The **basal ganglia** system function as the extrapyramidal system pathway and processing station between the cerebral motor cortex and upper brainstem. Contribute inpute from visual, labyrinthine and proprioceptive sources. Allows gross intentional movement without conscious thought.
 * The **diencephalon** consists of the epithalamus, thalamus, hypothalmus and subthalamus.
 * Midbrain - includes the corpora quadrigemina, tegmentum and cerebral peduncles (http://www.brainexplorer.org/brain_atlas/Brainatlas_Midbrain.shtml#image); involved in voluntary and involuntary visual motor movements, positioning the head to improve hearing, synthesizes dopamine and contains the cerebral aqueduct that carries CSF.
 * Hindbrain - includes the cerebellum, pons, and medulla (http://www.brainexplorer.org/brain_atlas/Brainatlas_Hindbrain.shtml); is responsible for both conscious and unsconscious muscle synergy and for maintaining balance and posture.


 * Brainstem** - is the pathway between the cerebral cortex and spinal cord. It consists of the midbrain, medulla oblongata, pons and the diencephelon.
 * Reticular formation** - this is a network that provides constant muscle stimulation to counteract gravity. Regulates cardiovascular and respiratory function. The reticular formation is essential for maintaining wakefulness and in conjunction with the cerebral cortex is referred to as the reticular activating system.

Lobes of the brain: http://www.stanford.edu/group/hopes/basics/braintut/f_ab11lobes.gif Cerebral cortex: http://www.brainexplorer.org/glossary/cerebral_cortex.shtml Spinal cord: portion of the CNS; multi functions; connects the brain and body. http://www.daviddarling.info/images/spinal_cord.gif Simple reflex arc: http://www.biology.lsu.edu/webfac/dlongstreth/biol12014f02/lectur96.jpg


 * __MOTOR PATHWAYS__**

There are four relevant motor pathways: lateral corticospinal, corticobulbar, basal ganglia, and vestibulospinal pathways. There are three clinically important spinal afferent pathways. They are the posterior column, anterior spinothalamic, and the lateral spinothalamic.


 * __PROTECTIVE STRUCTURES__

Cranium** - encloses and protects the brain and its associated structures.
 * Meninges** - consist of three layers: dura mater, arachnoid, and the pia matter.
 * Cerebrospinal Fluid and the Ventricular System** - clear, colorless fluid in which the intracranial and spinal cord structures float. This prevents the brain from tugging on meninges, nerve roots and blood vessels.
 * Vertebral Column** - consists of 33 vertebrae that functions to absorb shocks.

The brain derives its arterial supply from two systems: the internal carotid arteries and the vertebral arteries. These connect to form the circle of Willis. The spinal cord derives its blood supply from branches off the vertebral arteries and from branches from various regions of the aorta.
 * __BLOOD SUPPLY TO THE BRAIN__**


 * __PERIPHERAL NERVOUS SYSTEM__**

Motor and sensory nerves and ganglia outside the CNS. They carry information to and from the CNS. There are 31 pair of spinal nerves; they contain both sensory and motor neurons (mixed nerves). Dermatomes are a specific body distribution where sensory & motor fibers of each spinal nerve supply and receive information. Spinal nerves separate into ventral and dorsal roots: (ant) ventral - motor (efferent) fibers carry impulses from spinal cord to the muscles and glands of the body; (post) dorsal - sensory (afferent) fibers carry impulses from sensory receptors of the body to the spinal cord.


 * __AUTONOMIC NERVOUS SYSTEM__**

Regulates the internal environment of the body; functions over which a person has no voluntary control. Has two divisions >
 * **sympathetic** - pushes the body into action during times of physiologic & psychologic stress.
 * **parasympathetic** - functions in a complementary and a counterbalancing manner to conserve body resources and maintain day to day body function such as digestion & elimination.
 * CRANIAL NERVES:**
 * 1) Olfactory
 * 2) Optic
 * 3) Oculomotor
 * 4) Trochlear
 * 5) Trigeminal
 * 6) Abducens
 * 7) Facial
 * 8) Vestibulocochlear
 * 9) Glossopharyngeal
 * 10) Vagus
 * 11) Spinal Accessory
 * 12) Hypoglossal

The primary mechanism responsible for most of these structural changes is a decrease in the number of neurons. External features of the aging brain are decreased brain weight and size. Principal cellular changes associated with aging include dendrite structure, lipofuscin deposition, and the presence of neurofibrillary tangles, senile plaques, and Lewy bodies. The elderly experience progressive slowing of neurologic responses.
 * __AGING AND THE NERVOUS SYSTEM__**


 * __TESTS OF NERVOUS SYSTEM FUNCTION__**

There are many different tests used to test the nervous system. They include CT, MRI, MRA, PET scan, brain scan, angiography, myelography, ultrasound, electroencephalography, evoked potentials and CSF anaylsis.


 * __Click here to go to Chapter 15__**