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P & A Notes Nervous System Part 2

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P & A Notes--Nervous System Part 2

Nervous System pt. II

 Brain and spinal cord both are held in cavities.  The cranial cavity and the vertebral canal.
 Both are protected by a membrane covering, the meninges.
Meninges- made-up of 3 layers, dura mater, arachnoid mater, and pia mater.
 Dura mater- out layer. Actually is the internal periosteum of the surrounding bone, and in some places it extends between the lobes of the brain to help support and protect these lobes.
 In the spinal cord it isn’t attached to the bone, it surrounds the cord and extends past the end of the cord in a blind sac.
 Between the dura mater and bone is an epidermal space which is filled with connective tissue and fat- providing a pad for the spinal cord.

 Arachnoid mater- Middle layer, mesh like.  Between it and the pia mater is the subarachnoid space which contains cerebrospinal fluid.  

 Pia mater- Thin, is attached to the surface of brain and cord.  Contains many blood vessels which nourish the underlying cells.
(Meningitis- inflammation of meninges and is a serious childhood infection)

Spinal Cord: Begins at the foramen magnum and terminates near the inter vertebral disk separating the 1st and 2nd lumbar vertebrae.
Is made-up of 31 segments, each having a pair of spinal nerves that emerge from it.  
A cervical enlargement in the neck area gives off nerves to the arms.  A lumbar enlargement in lower back gives off nerves to the legs. (note diagram of cord pg. 232)

(diagrams pg. 230 & 233)
-Note the 2 grooves- anterior median fissure and a posterior median sulcus.
Function: Conducts nerve impulses and is the link to spinal reflexes.  Has a 2 way communication system.  Sensory neurons go to brain (ascending tract).  Motor neurons come from brain and out to effectors (descending tract).

Brain- Made-up of about one hundred billion neurons.  Divided into 3 parts:
1.)Cerebrum- largest.  Center of sensory and motor function,
            memory and reasoning.
2.)Cerebellum- centers of coordination of voluntary muscle
3.)Brain stem- contains nerve pathways connecting various parts of
            the nervous system and it regulates some visceral activities.

Cerebrum: Divided into a right and left cerebral hemisphere.  These are separated by dura mater, but are connected deep inside by nerve fibers called corpus callosum.
-Notice the ridges of the cerebrum- convolutions.
-Notice the grooves -shallow grooves, sulcus.
                                Deep grooves- fissure
  *Longitudinal fissure- R. and L. hemisphere
  *Transverse fissure- separates cerebrum and cerebellum

Some sulcus divide the cerebrum into lobes.
Frontal lobe: Anterior portion of C.H., its posterior boundary is the central sulcus.
Parietal lobe: Just posterior to the frontal lobe.
Temporal lobe: Below the frontal, is separated from it by lateral sulcus.
Occipital lobe: Back portion of brain. -No distinct boundary between it and parietal.
Insula: Is deep in the lateral sulcus and is covered by parts of frontal, parietal and temporal lobes.

-The cerebral cortex is a thin layer of gray mater around its outer edge (unmyelanated neurons- cell bodies). Under this is white mater (myelanated neurons), make-up the bulk of the cortex.
 Nerve fibers connect the cell bodies of the cortex to other parts of the nervous system often by way of the corpus cullosum.
-Deep in C.H. are masses of gray matter called basal ganglia / nuclei, which help in he control of muscular activity. (pg 238)
-Cerebral cortex is divided into motor, sensory and association areas.
 Motor area- posterior end of frontal lobe, just anterior to the central sukus.  Nerve fibers in this tract cross from one side to the other in the brain stem. (Right C.H. controls skeletal muscle on left side, left C.H. vice versa).
 Sensory areas- Areas that interpret impulses coming in from sensory receptors.  Feelings from skin-anterior parietal lobe/ sight-posterior occipital/ hearing- temporal lobe/ smell- deep in cerebrum/ taste-near base of central sulcus along lateral sulcus.
*These fibers all cross over- right C.H. interprets impulses from left side of body.
Association areas- Involved with memory, reasoning, verbalizing, judgment, emotions, and analysis of sensory experience.
 -Areas of parietal lobe- understanding speech and choosing words to express oneself.
 -Areas of temporal lobe- involved with memory of visual and auditory patterns. (reading, understanding, speech)
 -Areas of occipital lobe- visual recognition of objects. (people/things)
 -General interpretative area- posterior end of lateral sulcus. Role in complex thought processing

Ventricles- Interconnecting cavities and are continuous with the central canal of the spinal cord.
   -4 ventricles
   -largest is the lateral ventricles/1st & 2nd
These ventricles are continuous with the central canal of the spinal cord and also have openings leading to the subarachnoid space of the meninges.  All of these areas contain cerebrospinal fluid.  This fluid is produced by the specialized capillaries of the choroid plexuses.  Most of these structures seem to be in the lateral ventricles.  
This fluid surrounds the brain and spinal cord and is responsible for absorbing shock. The fluid also provides a pathway for wastes to reach the blood.

Blood-brain barrier: Is a separation of the blood and cerebrospinal fluid in the central nervous system. The cells which make up the walls of the capillaries surrounding the C.N.S.  are sealed together at their edges by what is called tight junctions. Secondly, these capillaries are enclosed by the flattened ‘end-feet’ of astrocytes, which also act as a partial, active, barrier. The tight junctions and the astrocytes prevent cells and large molecules from moving from the blood into the brain tissue. This helps the blood–brain barrier become very effective in protecting the brain from many common bacterial infections. Thus, infections of the brain are very rare. Antibodies and antibiotics are too large to cross the blood–brain barrier. So, infections of the brain that do occur are often very serious and difficult to treat.
  An exception to the bacterial exclusion are the diseases caused by spirochetes, such as Borrelia, which causes Lyme disease, and Treponema pallidum, which causes syphilis. These harmful bacteria seem to breach the blood–brain barrier by physically tunneling through the blood vessel walls.
 Cells of the barrier actively transport metabolic  products such as glucose and amino acids across the barrier into the spinal fluid. Metabolic wastes must be move out of the spinal fluid to the blood.
Brain stem:
Made-up of the diencephalon, mid brain pons, medulla oblongata.
-Diencephalon-surrounds the 3rd ventricle
 Two-parts- thalamus and hypothalamus.
  Thalamus-- receives sensory impulses (except smell) and sends them to the appropriate part of the C. cortex.
  Hypothalamus-- helps to keep homeostasis by regulating organs.  Is a link between the nervous system and endocrine system. (regulates- heart rate, movements and secretion of guts, stimulates pituitary to release hormones).
The diencephalon also includes an area called the limbic system- responsible for emotional feelings. (fear, anger, pleasure, sorrow) survival mechanism.
 Pineal gland is also in this portion of the brain.  It functions more as an endocrine gland.  It stimulates the hypothalamus to produce its releasing factor.

Mid brain- Between diencephalon and pons.  Serves as a reflex center for certain reflexes (moving eyes when you turn head. Turning head to hear better.)

Pons- the rounded bulge on brain stem.
 Relays impulses to medulla oblongata and cerebrum.
Medulla oblongata- extends from foramen magnum to the pons. This is where the nerve fibers cross over.

-Certain areas function as control centers:
  -cardiac center, speed/slow heart rate.
  -vasomotor center causes vasoconstriction or vaso dilation of the blood vessels.
  -respiratory center, function with pons to regulate breath rate and depth.
Cerebellum- just below the occipital lobe of the cerebrum.
 -is also made-up of 2 hemispheres.
 -mostly white matter with a thing layer of gray matter (cereballar cortex) on outside.
---Functions- reflex, coordination of skeletal muscles.
It receives information from your tendons, muscles and joints making us aware of the condition of our muscles then we can inhibit or stimulate the appropriate groups.

Peripheral nervous system
-includes the cranial nerves.
-theses are divided into the somatic system-- the fibers connect the C.N.S. to the skin or skeletal muscles. Autonomic system-- connects the C.N.S. to visceral organs.

Cranial nerves:
-most are mixed nerves
-some do come from special sense organs. (eyes, nose)
-others are involved with activities of muscles and glands, have mainly motor fibers.
*Nerves that are mainly sensory fibers have their neuron cell bodies coated on outside brain found in groups ganglia.
-Motor cell bodies are in the gray matter of the brain.

Autonomic nervous system-
Functions without conscious effort. (smooth muscle, cardiac muscle, most glands)
-It is made-up of 2 sections the sympathetic and parasympathetic subdivisions.
 An organ is supplied with nerve fibers from both subdivisions.  Each subdivision activates some organs and each inhibits others.
*The sympathetic N.S. is responsible for getting the body ready for emergency situations.  The parasympathetic N.S. helps to maintain the normal state- restful state.

-A.N.S. involves two neurons:

-Somatic nervous system has one neuron between C.N.S. and effector.

Spinal nerves- are al mixed
                       - 31 pairs-- 8 pair cervical, 12 pair thoracic, 5 pair lumbar, 5 pair sacral, 1 pair coccygeal
-Spinal cord ends between the 1st and 2nd lumbar vertebrae--so lumbar, sacral and coccygeal nerves go past the end of the cord.
-Each spinal nerve emerges from the cord through two short branches (roots).
-the dorsal (sensory) root  has an enlargement on it called the dorsal root ganglion (contains cell bodies of sensory neurons).
-The ventral (motor) root has its cell body in the gray matter of the spinal cord.  The two roots unite to form a spinal nerve.

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