The Nervous System

1
The Nervous System

• Chapter 7

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Functions of Nervous system

• Sensory input Nervous system receives
  information from environment (inside and outside
  of the body)
• Integration It analyses the information and
  responds to the information. How?
• Motor output By activating muscles or glands

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Structure and Function of Nervous Tissue

• Consists of two major types of cells
• Neuron nerves cells involved in generating and
  conducting nerve impulses
• Glial cells nourish, support and protect nerve
  cells

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Neurons or Nerve Cells

• Consists of 3 parts
• Cell body
• Metabolic center of the body
• contains nucleus and nucleolus
• cytoplasm filled with rough ER (Nissl bodies),
  Golgi, and other organelles.
• Primary site for neurotransmitter synthesis
• Neurofibrils provide shape,
• made of Intermediate filaments

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Neurons or Nerve Cells

• Dendrites
• cytoplasmic extension extend from cell body
• Conduct nerve impulse towards cell body
• They are generally branched

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Neurons or Nerve Cells

• Axon
• arise from cell body
• Each neuron contains single axon
• The place where axon originates from the cell
  body is called axon hillock
• Cytoplasm is called axoplasm, cell membrane is
  called axolemma.

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Axon

• Axons end in axonal terminals
• Axonal terminals contain vesicles with
  neurotransmitters
• Axonal terminals are separated from the next
  neuron by a gap
• Synaptic cleft gap between adjacent neurons
• Synapse junction between nerves

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Neurons or Nerve Cells

• Axon
• Conduct nerve impulses away from cell body
  towards another neuron or muscle
• Axon can be myelinated or nonmyelinated
• Myelination (Myelin sheath) helps in conducting
  action potential at a faster rate

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Classification of Neurons

• Nerve cells are classified into different
  types based on structure or function
• Structural Classification Nerve fiber
  arising from cell body differentiates into
• axon and dendrite
• Unipolar cell body gives off two branches from
  the same point, one axon and second dendrite like
  structure

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Classification

• Bipolar Neurons with one dendrite and one axon
• Multipolar Contains many dendrites and one axon
  .

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Classification

• Functional Classification
• Sensory or afferent neurons send impulses from
  organs to central nervous system(CNS).Generally
  unipolar
• Interneuron receive impulse from sensory neuron
  and relays it to motor neuron within CNS. Present
  within the brain or spinal cord. Generally
  multipolar
• Motor or efferent neurons send impulses from CNS
  to organs.Generally multipolar

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Neuroglia or Glia Cells

• Different types of CNS glia cells
• Astrocytes
• Ependymal cells
• Microglia
• Oligodendrocytes
• Schwann cells

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Neuroglia or Glia Cells

• Astrocytes regulate chemical composition of
  brain fluid.
• Helps in forming blood brain barrier.
• Connect nerve cell to capillaries
• Ependymal cells lines the cavities of brain and
  spinal cord and
• circulate cerebrospinal fluid around CNS
• Microglia macrophages of CNS and
• Dispose of debris, dead cells and bacteria

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Neuroglia or Glia Cells

• Oligodendrocytes form myelin sheath around axon
  of neuron
• in CNS
• Schwann cells form myelin sheath
• In PNS
• On the axon, schwann cells are separated, by gaps
  called nodes of ranvier
• Help in nerve cell conduction

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Nerve Impulse Transmission

• Resting neurons maintain a difference in
  electrical charge across their cell membranes.
• The inside of the resting neuron is negatively
  charged, the outside is positively charged.
• Due to unequal distribution of Na and K and
  other charged molecules

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• The sodium-potassium pumps(membrane proteins)
  actively transport sodium out of the cell and
  potassium in.
• Three Na are pumped out for every two K pumped
  in.
• The cell has more Na on the outside and more K
  on the inside.
• Thus outside is positive and inside is negative.

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• The Neuron Membrane at Rest
• Resting membrane potential contains -70
  millivolts inside the cell membrane

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The Stimulated Neuron (action potential)

• On nerve stimulation,
• i.e,inside is positive and outside is negative
• Due to opening of Na-K pump and
• allows Na to pass freely into the cells.
• Resulting in depolarization .
• The membrane potential is now gt40mV.
• This phenomenon is called action potential.

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• This action potential moves along the cell like a
  wave.
• Speed impulse flow is directly proportional to
  diameter of the nerve cell
• The membrane restores the resting potential very
  quickly (in less than 7 milliseconds).
• This is called repolarization

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Neuron to Neuron Communication The Synapse

• Synaptic cleft gap between adjacent neurons
• Synapse junction between nerves or between
  nerve and a muscle
• Into the synaptic cleft chemical messengers
  called neurotransmitters are released

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Neuron to Neuron Communication The Synapse

• neurotransmitters are synthesized in the cell
  body and
• Transported through axon into axon terminals
• Different types
• Acetyl choline neuromuscular junction
• Norpeinephrine, dopamine, serotonin

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Synapses

• Synapse between an axon terminal of a nerve cell
  and a muscle is known as
• neuromuscular junction

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Rules for stimulation of nerve impulses

• Neuron require a threshold activation potential
  to be stimulated.
• If the activation potential is below threshold,
  it will not be stimulated.
• This requirement is called all and none law

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How are impulses processed

• They produce their effect on the peripheral
  nervous system
• Ultimately resulting in performance of a
  specific function by the body

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The Reflex Arc

• Reflex rapid, predictable, and involuntary
  responses to stimuli
• Reflex arc direct route from a sensory neuron,
  to an interneuron, to an effector

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 Reflex Arc

•  A complete pathway through the nervous system
  from stimulus to response
• Following parts represent the typical reflex arc
• Receptor The end of the dendrite or some
  specialized receptor cell as in a special sense
  organ(skin) that detects the stimulus

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Reflex Arc

• Sensory neuron (or afferent neuron) A cell that
  transmits impulse from receptor towards CNS.
•  Central neuron( interneuron or associated
  neuron) A cell or cells in the CNS carry
  impulses to and from the brain within the brain
  or to different regions of the spinal cord. 

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Reflex Arc

• Motor neuron (or efferent neuron) A cell that
  carries impulses away from CNS
• Effector A muscle or a gland outside the CNS
  that carries out a response

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Reflex Arc

• Reflexes are involuntary actions involving
• a. skeletal muscle
• b. spinal cord and
• c. brain stem
• e.g.
• withdrawal reflex (withdrawal from painful
  stimulus) and
• knee jerk reflex(patellar REFLEX), results in
  quadriceps to contract and leg to extend)

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Simple Reflex Arc
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Organization of the Nervous System
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Central Nervous System (CNS)

• CNS develops from the embryonic neural tube
• The neural tube becomes the brain and spinal cord
• The opening of the neural tube becomes the
  ventricles
• Four chambers within the brain
• Filled with cerebrospinal fluid

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Nervous tissue is organized into brain and spinal
cord as

• Gray matter collection of neuron cell bodies ,
  dendrites and axon terminals or bundles of
  unmyelinated axons and ganglia.
• White matter Bundles of axons of several
  neurons, which are wrapped in myelin sheath.

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CENTRAL NERVOUS SYSTEM

• Site for initiating responses and integrating
  mental process (emotions and intelligence)
• Constitutes spinal cord and brain.
• Protected by three connective tissue covering
  called meninges.

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• Outer layer is the duramater .
• It is made of dense white fibrous connective
  tissue and blood vessels
• It continues along the spinal cord and ends in a
  called epidural space.
• This space is not seen in brain
• In spinal cord it is also made of adipose
  connective tissue to protect spinal cord

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• Middle layer is arachnoid mater
• It is made of connective tissue rich in collagen
  fibers.
• The space between arachnoid and third layer pia
  mater is known as subarachnoid space.
• It is filled with cerebrospinal fluid.
• It is not supplied with nerves or blood vessels

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• c. Inner layer is Pia mater
• It is made of thin connective tissue layer
• It is in direct contact with the brain
• It is supplied with blood vessels and nerves

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Cerebrospinal Fluid (CSF)

• Similar to blood plasma composition
• Forms a watery cushion to protect the brain
• Circulated in arachnoid space, ventricles, and
  central canal of the spinal cord

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Central Nervous system

• Brain
• Spinal Cord

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Brain

• Brain is the largest organ of nervous system.
• 12 pairs of cranial nerves arise from brain
• It is enclosed in skull.
• Brain is subdivided into
• Cerebrum (cerebral hemispheres)
• Cerebellum
• Diencephalon
• Brain stem

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Cerebrum or cerebral hemispheres

• has two large cerebral hemispheres connected by
  nerves fibers called
• Corpus callosum.
• Surface of cerebrum has ridges or gyri
• Gyri are separated by grooves called sulcus

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Deep grooves separate cerebrum into 4 distinct
lobes.

• Frontal lobe motor function of muscles, speech
  and intellectual processes.
• Parietal lobe sensations and speech.
• Occipital lobe vision
• Temporal lobe sensory areas of smell, auditory,
  memory, language comprehension

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• Basal nuclei grey matter surrounded by white
  matter in the cerebral cortex.
• Its function is not understood
• May have a role in voluntary functions

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Limbic system below the cerebral cortex

• Parts of cerebrum, thalamus and hypothalamus
• Relates conscious and subconscious aspects of
  brain
• Results in relating an action and sensory
  stimulus to pain, pleasure, anger etc
• Thus helping in survival
• Gives raise to feelings of emotions such as
  pleasure, pain, anxiety

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cerebellum

• Posterior to brain stem
• Separated from brain stem by forth ventricle
• Divided into two hemispheres connected medially
• Each hemispere is made of grey matter
  superficially and white matter interiorly
• Important for Muscle coordination received from
  cerebral cortex in
• Skeletal muscle contraction
• muscle tone and posture
• Damage to cerebellum causes tremors and problem
  with equilibrium, posture

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Diencephalon has hypothalamus and thalamus
surrounding third ventricle and made of grey
matter

• Present below cerebrum and above midbrain
• It is divided into
• Hypothalamus
• Thalamus

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• Hypothalamus It is combination of nervous and
  endocrine organs
• Forms the floor of third ventricle
• Controls involuntary functions like
• maintenance of homeostasis,
• regulation of sleep,
• temperature,
• BP, etc

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• Thalamus above the midbrain
• Forms the lateral walls of the third ventricle
• central relay station from spinal cord,
  brainstem, and cerebellum to cerebral cortex.

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• Besides hypothalamus and thalamus,
• diencephalon harbors
• pineal body,
• pituitary gland

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d. Brain Stem Medulla, pons and midbrain.
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Mid brain

• superior to pons.
• coordinating the visual and auditory activities
• Tactile( general touch) response.
• It helps in the movement of head and neck to
• eye and hearing responses

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Pons

• Made of axons transporting from cerebellum and
  rest of the brain and spinal cord
• relays information from spinal cord and medulla
  to cerebral cortex
• regulates respiratory movements rate, visual and
  auditory functions

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Medulla oblongata

• Medulla Lies superior to spinal cord and
  inferior to pons.
• It is the vital center for regulation of
  autonomous activities such as
• Heartbeat ( cardiac center),
• breathing, ( respiratory center)
• vasoconstriction (vasomotor center) and others
  such as
• reflex centers for vomiting, coughing, sneezing
  hiccups, swallow.
• Relay center between cerebral cortex and spinal
  cord

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• 4 ventricles
• 2 Lateral cerebral hemispheres
• Third ventricle- diencephalon
• Fourth ventricle base of cerebellum connects to
  central canal of spinal cord

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Cerebrospinal fluid

• Surrounds brain and spinal cord and protects them
  from mechanical shock
• produced by ependymal cells of lateral
  ventricles,third and fourth ventricles.
• CSF exchanges its nutrients and excretory
  material with the blood vessels of brain called
  dural sinuses.

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Spinal Cord

• It is enclosed in the vertebral column
• Spinal cord continues from the medulla oblongota
  
• To the second lumbar vertebra
• Connecting link between organs and brain with the
  help of
• 31 pairs of spinal nerves arise from spinal cord
  to various organs.
• Made of
• gray matter surrounding central canal in the form
  of butter fly
• white matter surrounding the grey matter.

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Gray matter.made of dendrites, cellbodies and
unmylinated axons of interneurons

• Grey matter looks like butterfly. It is divided
  into
• Dorsal horn sensory nerve fibers from sensory
  organs end
• ventral horn from which motor fibers arise to
  skeletal muscles

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White matter surrounds grey matter or butterfly

• has axons wrapped in myelinated sheath.
• form columns in the tracts( nerves) in the
  spinal cord
• Ascending tracts carry information from spinal
  cord to brain
• Descending tracts carry information from brain
  to spinal cord

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Spinal Cord enclosed in vertebral column.

• It has two main functions.
• Communation center
• Reflex center

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• Spinal cord provides means of communications
• between brain and various organs with the help of
  spinal nerves
• conduction of sensory impulses upward
• through ascending tracts
• to the brain
• conduction of motor impulses from brain down
• through descending tracts to the efferent
  neurons
• that supply muscles or glands

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• It is the center for reflex actions
• automatic, involuntary responses to changes
• occurring inside or outside the body. E.g.
• Withdrawal reflex

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Blood Brain Barrier

• Includes the least permeable capillaries of the
  body
• Excludes many potentially harmful substances
• Useless against some substances
• Fats and fat soluble molecules
• Respiratory gases
• Alcohol
• Nicotine
• Anesthesia

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Traumatic Brain Injuries

• Concussion
• Slight brain injury
• No permanent brain damage
• Contusion
• Nervous tissue destruction occurs
• Nervous tissue does not regenerate
• Cerebral edema
• Swelling from the inflammatory response
• May compress and kill brain tissue

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Cerebrovascular Accident (CVA)

• Commonly called a stroke
• The result of a ruptured blood vessel supplying a
  region of the brain
• Brain tissue supplied with oxygen from that blood
  source dies
• Loss of some functions or death may result

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Alzheimers Disease

• Progressive degenerative brain disease
• Mostly seen in the elderly, but may begin in
  middle age
• Structural changes in the brain include abnormal
  protein deposits and twisted fibers within
  neurons
• Victims experience
• memory loss, irritability, confusion and
• ultimately, hallucinations and death

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Peripheral nervous system (PNS)

• Peripheral nervous system (PNS)
• Part of the nervous system outside the CNS
• Consists of cranial nerves and spinal nerves
• Carries messages to and from the spinal cord and
  brain

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Peripheral Nervous System (PNS)

• Two Functional Divisions
• Sensory (afferent) division
• Carry impulses from sensory receptors located in
  skin, skeletal muscles, joints, visceral organs
  to the brain
• Motor (efferent) division
• Transmits impulses from the CNS to effector
  organs, such as muscles, glands

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Motor Division

• Motor division has two main parts
• Somatic nervous system
• Conscious control of skeletal muscles
• Autonomic nervous system (ANS)
• Regulates smooth muscle, cardiac muscle, and
  glands
• Two divisions of ANS
• Sympathetic
• Parasympathetic

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Peripheral Nervous System (PNS)

• PNS consists of nerves (bundles of axons) extend
  from brain and spinal cord
• And ganglia (neuronal cell bodies) outside the
  CNS
• These nerves can be
• Sensory nerves carry impulses to the brain or
  spinal cord
• Motor nerves carry impulses to muscles or
  glands, away from CNS
• Mixed nerves combination of both sensory and
  motor nerves

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Transverse section of a nerve

• Nerves consists of axons (some myelinated some
  not) are surrounded by delicate connective tissue
  called endoneurium
• Groups of nerve fibers are bound into bundles or
  fascicles by heavier connective tissue layer
  called perineurium
• Finally, all the fascicles are bound together by
  third layer of connective tissue, epineurium

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Cranial nerves

• There are 12 pairs of nerves originating from the
  brain and serve head and neck
• Only the pair of vagus nerves extend to thoracic
  and abdominal cavities
• I and II arise from cerebral hemispheres
• All others arise from brain stem
• I, II and VIII ( sensory nerves)
• Rest are mixed motor nerves

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Cranial nerves from cerebrum

• I (olfactory) nerve
• Sensory for smell
• Convey information from
• nose to base of frontal lobe and temporal
  lobe of cerebral hemispheres
• II (optic) nerves
• Sensory for vision
• Convey information from eye to occipital lobes of
  cerebral hemispheres

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Cranial nerves arising from midbrain

• III (occulomotor) nerve
• Is a combination of somatic motor and autonomic
  motor nerves
• They regulate the amount of light entering eye
  and also focusing lens
• IV (trochlear) nerve
• controls the movement of eye muscle
• Smallest nerve

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Cranial nerves arising from pons

• V( trigeminal) nerve
• largest nerve
• Three branches
• Mandibulary
• ( motor), motor fibers to chewing muscles
• Maxillary
• (sensory) , conveys impulses from upper teeth,
  upper lip
• Ophthalmic
• Sensory
• Convey impulses from scalp, eye and nose

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Cranial nerves arising from Pons

• VI abducens
• Motor fiber to eye muscle
• Arising at the site of pons
• Important for eye movement
• VII (facial)
• Activates the muscles of facial expressions and
  carries sensory impulses from the taste bud
• VIII (vestibulocochlear)
• Purely sensory
• Transmit impulses for hearing and balance

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Cranial nerves arising from Medulla

• IX (glassopharyngeal)
• Is important for taste and
• swallowing
• Supplies motor fibers to tongue and pharynx
• X (vagus)
• Fibers carry sensory impulses from and motor
  impulses to larynx, pharynx, thoracic and
  abdominal viscera
• Regulate digestive and heart activity

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Cranial nerves arising from Medulla

• XI (accessory)
• Important for mastication,
• Supplies nerves to tongue, soft palate, pharynx
  etc and muscles of neck
• XII (hypoglossal)
• Is important for tongue function such as
  speaking, chewing, swallowing

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Cranial Nerves
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Spinal nerves

• There are 31 pairs of spinal nerves each numbered
  according to the level of the spinal cord from
  where it arises
• Each spinal nerve is a mixed nerve consisting of
  sensory and motor nerve      

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Spinal nerves

• Each nerve is attached to the spinal cord by two
  roots
• Dorsal root (sensory) arising from dorsal horn
  (posterior)  
•      

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Spinal nerves

• ventral root (motor) arising from ventral horn
  (anterior)
• It is mainly made of axons from motor neurons
• Their cell bodies are located in the gray matter
  of the spinal cord

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Spinal nerves

•  Each spinal nerve continues a short distance
  from the spinal cord and emerges from the
  intervertebral foramina
• branches out further to different organs

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Spinal nerves

• Each spinal nerve has two roots
• Dorsal root
• Ventral root
• Each root has several branches
• Each branch is called
• Ramus
• Each rami can be dorsal( sensory) or ventral(
  motor)

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Spinal nerves

• The spinal nerves arising from the respective
  region of the spinal cord
• a. 8 cervical (C1-C8),  b. 12 thoracic
  (T1-T12),   c. 5 lumbar (L1-L5),   d. 5
  sacral (S1-S5), and   e. 1coccygeal(Co).

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Spinal nerves

• Cauda equina  
• 5 lumbar (L1-L5),
• 5 sacral (S1-S5), and
• 1coccygeal(Co).
• together taper out forming Cauda equina

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Plexuses

• Except thoracic region,
• Spinal nerves form a network called plexuses.
• These networks distribute branches to the parts
  of the body
• There are 3 different plexuses  

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Plexuses

• Cervical plexus
• Formed by C1-C4. Serve to Muscles
• and skin of neck and shoulder
• Brachial plexus
• Formed by C5-T1
• They are musculocutaneous nerve
• Radial nerve
• Median nerve
• Ulnar nerve
• Supply arm, forearm and hand
• Phrenic nerve C3-C5
• Supplies diaphragm

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• Lumbosacral plexus formed by T12-S5
• Important nerves arising from them are
• Femoral nerve
• muscles and skin of thighs and legs
• Sciatic nerve
• muscles and skin of thigh legs and feet

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• T2-T11 intercoastal nerves
• Thoracic spinal nerves serves to muscles of
  respiration,
• and upper abdomen
• And receive information from thorax and abdomen

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Autonomous nervous system (ANS)

• Motor subdivision of the PNS
• Consists only of motor nerves
• Also known as the Involuntary nervous system
• Regulates activities of cardiac and smooth
  muscles and glands
• Two subdivisions
• Sympathetic division
• Parasympathetic division

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PNS Differences Between Somatic and Autonomic
Nervous Systems

• Nerves
• Somatic one motor neuron
• Autonomic preganglionic and postganglionic
  nerves
• Effector organs
• Somatic skeletal muscle
• Autonomic smooth muscle, cardiac muscle, and
  glands

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PNS Differences Between Somatic and Autonomic
Nervous Systems

• Neurotransmitters
• Somatic always use acetylcholine
• Autonomic use acetylcholine, epinephrine, or
  norepinephrine

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PNS Anatomy of the Sympathetic Division

• Originates from T1 through L2
• Ganglia are at the sympathetic trunk (near the
  spinal cord)
• Short pre-ganglionic neuron synapses with long
  post-ganglionic neuron and transmit impulse from
  CNS to the effector
• Norepinephrine and epinephrine are
  neurotransmitters to the effector organs

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PNS Anatomy of the Parasympathetic Division

• Originates from the brain stem and S1 through S4
• Always uses acetylcholine as a neurotransmitter

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PNS Autonomic Functioning

• Sympathetic fight or flight
• Response to unusual stimulus
• Takes over to increase activities
• Exercise, excitement, emergency, and
  embarrassment
• Sympathetic nervous system increases heart rate,
  blood pressure, blood glucose levels

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PNS Autonomic Functioning

• Parasympathetichousekeeping activites
• Conserves energy
• Maintains daily necessary body functions
• digestion, defecation, and diuresis (urination)

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Development Aspects of the Nervous System

• The nervous system is formed during the first
  month of embryonic development
• Any maternal infection can have extremely harmful
  effects
• The hypothalamus is one of the last areas of the
  brain to develop
• No more neurons are formed after birth, but
  growth and maturation continues for several years
• The brain reaches maximum weight in young adult