C' Basic Anatomical Structures

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C. Basic Anatomical Structures
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1. The Spinal Cord

•  The spinal cord is divided into right and left
  sides by two grooves.
• The anterior median fissure is a deep wide groove
  on the ventral surface.
• The posterior median sulcus is a shallower,
  narrower groove on the dorsal surface.
•  

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1. The Spinal Cord

• The spinal cord consists of both grey and white
  matter.
• The grey matter forms an H-shaped area within the
  white matter and consists primarily of nerve
  cells bodies, dendrites, and unmyelinated axons
  of both motor neurons and interneurons.

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1. The Spinal Cord

• The white matter consists of bundles of
  myelinated axons of motor and sensory neurons
  that constitute the sensory and motor tracts of
  the spinal cord.
• The cross-bar of the H is formed by the grey
  commissure.

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1. The Spinal Cord

• In its center, is a small space called the
  central canal.
• The canal runs the length of the spinal cord and
  is continuous with the 4th ventricle of the
  medulla.
• It contains cerebrospinal fluid.

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1. The Spinal Cord

• Anterior to the grey commissure is the anterior
  white commissure, which connects the white matter
  of the right and left sides of the spinal cord.
• The upright portions of the H are further
  subdivided into regions.

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1. The Spinal Cord

• Those portions closer to the front of the cord
  are called the ventral (anterior) grey horns.
• The regions closer to the back of the cord are
  called the dorsal (posterior) grey horns.

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1. The Spinal Cord

• The regions between the ventral and dorsal grey
  horns are the intermediate lateral grey horns.
• The lateral grey horns are only present in the
  thoracic, upper lumbar, and sacral segments of
  the cord.

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1. The Spinal Cord

• The white matter, like the grey matter, is also
  organized into regions.
• The anterior white columns, the posterior white
  columns, and the lateral white columns are
  regions that result from the division of the cord
  by the grey matter.   

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1. The Spinal Cord

• Each column consists of bundles of myelinated
  fibers that run within the cord.
• These ascending bundle tracts are called
  fasciculi.
• They consist of sensory axons that conduct
  impulses that enter the spinal cord upward to the
  brain.

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1. The Spinal Cord

• The long descending tracts consists of motor
  axons that conduct impulses from the brain
  downward into the spinal cord where they synapse
  with other neurons whose axons pass out of the
  spinal cord to a muscle or gland.

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Spinal Nerves

• The 31 pairs of spinal nerves are the paths of
  communication between the periphery and the
  ascending sensory fasciculi as well as between
  the descending motor tracts and the periphery.

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Spinal Nerves

• The dorsal (posterior) root contains sensory
  nerve fibers only and conducts nerve impulses
  into the system from the periphery.
• These fibers extend into the dorsal grey horn.

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Spinal Nerves

• Each dorsal root has a swelling called the dorsal
  root ganglion.
• The dorsal root ganglion contains the cell bodies
  of the sensory neurons from the periphery.

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Spinal Nerves

• The ventral root contains motor axons only and
  conducts impulses away from the spinal cord
  toward the periphery.
• The cell bodies of the motor neurons are located
  in the grey matter.

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Spinal Nerves

• If the motor neuron supplies a skeletal muscle,
  then its cell body is located in the ventral grey
  horn.
• If the motor neuron supplies a smooth muscle, a
  cardiac muscle, or a gland through the autonomic
  nervous system, then its cells bodies are located
  in the lateral grey horns.

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Spinal Cord Functions

• The major function of the spinal tracts is to
  convey sensory impulses from the periphery to the
  brain and to conduct motor impulses from the
  brain to the periphery.
• A secondary function is to provide reflex
  integration.
• Reflexes are fast responses to certain stimuli in
  the internal or external environment that allow
  the body to maintain homeostasis.

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Spinal Cord Functions

• Reflexes that are carried out by the spinal cord
  alone are called spinal reflexes.
• Reflexes that involve brain centers and cranial
  nerves are called cranial reflexes.
• Reflexes that result in the contraction of
  skeletal muscles are called somatic reflexes.
• Reflexes that result in the contraction of smooth
  or cardiac muscle, or the secretion by glands are
  called visceral reflexes.

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The Reflex Arc

• The reflex arc is the shortest route that can be
  taken by an impulse passing from a receptor to
  and effector.

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The Reflex Arc

• The basic components of the reflex arc are a
  receptor, a sensory neuron, an interneurons, a
  motor neuron, and an effectorskeletal muscle,
  smooth muscle, or gland.

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2. The Brainstem and Cranial Nerves

• The rhombencephalon is that portion of the brain
  that lies above the spinal cord and from which a
  number of cranial nerves emerge.
• The longitudinal organization of the neural tube
  is still recognizable in the brainstem, although
  its altered by the enlargement of the central
  canal into the fourth ventricle.

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2. The Brainstem and Cranial Nerves

• In keeping with its embryonic organization, the
  ventral portion of the brainstem is motor in
  nature (red), while the dorsal portion is sensory
  in nature (blue).

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2. The Brainstem and Cranial Nerves

• Intermediate regions between these two areas are
  viscero-motor (white) and viscero-sensory
  (yellow).

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2. The Brainstem and Cranial Nerves

• The secondary divisions of the rhombencephalon
  include the myelencephalon from which the medulla
  oblongata and 4th ventricle arise and the
  metencephalon from which the pons, cerebellum,
  and cerebral aqueduct differentiated.
• As we ascend in a rostral direction, the first
  structure upon which we will focus is the medulla
  oblongata.

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Medulla

• The medulla oblongata is the most inferior
  portion of the brainstem.
• The inferior portion is closed and continuous
  with the spinal cord at roughly the level of the
  foramen magnum.
• The superior portion of the medulla is open and
  contains the 4th ventricle.

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Ventral Medulla

• Just below the pons, on the ventral (anterior)
  side of the medulla, are two roughly triangular
  structures called the pyramids.

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Ventral Medulla

• The pyramids are composed of the largest motor
  tracts coming from the cerebral cortex, through
  the cerebral peduncles, on their way to the
  spinal cord.

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Pyramidal Decussation

• In the area just above the transition from
  brainstem to spinal cord, most of the fibers
  contained in the pyramids decussatecrossto the
  contralateral side.
• The motor fibers which undergo this decussation
  in the pyramids belong to the lateral
  corticospinal tracts.

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Lateral Corticospinal Tract

• After they decussate, they descend in the lateral
  columns of the spinal cord, terminating in the
  ventral grey horns.
• Corticoorigin of the motor tracts
• Spinaltermination point of the motor tracts
• Lateralthe area of the spinal cord in which the
  fibers pass.

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Lateral Corticospinal Tract

• In the ventral grey horns, the motor fibers
  synapse with motor neurons that terminate in
  skeletal muscles.

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Lateral Corticospinal Tract

• As a result of the crossing, fibers that
  originate in the left hemisphere activate muscles
  on the right side of the body, and fibers that
  originate in the right hemisphere, activate
  muscles on the left side of the body.

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Doral Medulla

• The dorsal portion of the medulla contains two
  pairs of prominent nuclei the right and the
  left nucleus gracilis and nucleus cuneatus.
• These nuclei receive sensory fibers from
  ascending white matter tracts of the posterior
  column of the spinal cord.

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Posterior Column

• Lumbar and thoracic fibers from the lower limb
  area ascend in a bundle called the fasciculus
  gracilis (2) to end in the nucleus gracilis (4).
• Cervical fibers from the upper limb area ascend
  in another bundle called the fasciculus cuneatus
  (3) to end in the nucleus cuneatus (5).

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Posterior Column

• The second order fibers arising from these nuclei
  (6, 6, 6) cross the midline (7) forming the
  medial lemniscus (6), and then continue their
  ascent through the brainstem to the thalamus.
• Because of this crossing over, nearly all sensory
  impulses received on one side of the body are
  perceived in the opposite hemisphere of the
  cerebral cortex.

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Posterior Column

• In other words, we have contralateral sensory
  perception of upper and lower body stimuli
  leading to conscious appreciation of touch,
  pressure, vibration, and joint position/movement
  because of this decussation of the fasciculi
  gracilis and cuneatus.

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Lateral Medulla

• On each lateral surface of the medulla, an oval
  shaped projection called the olive contains the
  inferior olivary nucleus.
• Its fibers project to the cerebellum ensuring the
  efficiency of voluntary movements, especially
  precision movements.

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Lateral Medulla

• Also contained within each olive is the accessory
  olivary nucleus which has fibers that project to
  the cerebellum to monitor maintenance of
  equilibrium, postural changes, and locomotion.

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Medullary Cranial Nerves

• The nuclei of origin for cranial nerves VIII, IX,
  X, XI, and XII attach at various locations along
  the length of the medulla.
• Within the substance of the medulla is an area
  through which the cranial nerves pass and in
  which certain nuclear centers are found.

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Reticular Formation

• This substance is known as the reticular
  formation (3, 3, 3).
• It occupies the anterior and lateral districts of
  the medulla oblongata and is situated behind the
  pyramid and olive.

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Reticular Formation

• The reticular formation is caused by the
  intersection of bundles of fibers running at
  right angles to each other, some being
  longitudinal, others more or less transverse in
  direction.

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Reticular Formation and Cranial Nerves

• In the substance of the reticular formation are
  the nuclei of origin or termination of several
  cranial nerves.
• Specifically, the nucleus of origin of the
  Hypoglossal (XII) nerve, the nucleus of
  termination of the Vagus (X) nerve, and the
  nuclei of origin of motor fibers of the
  Glossopharyngeal (IX), vagus (X), and accessory
  (XI) nerves.

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Medullary Reflex Centers

• Also contained within the medullary reticular
  formation are three vital reflex centers the
  cardiac center, which regulates the rate of
  heartbeat and the force of contraction the
  medullary rhythmcity center, which
  regulates/adjusts the basic rhythm of breathing
  and the vasomotor center, which regulates the
  diameter of the blood vessels.
• Other centers in the medulla coordinate
  swallowing, vomiting, coughing, sneezing, and
  hiccoughing (hiccupping).

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Pons

• The metencephalon is the other secondary division
  of the rhombencephalon.
• It further differentiates into the pons and the
  cerebellum.
• If we move rostrally, the pons is found superior
  to the medulla.
• The word pons means bridge, and it is the
  portion of the brainstem located immediately
  inferior to the midbrain and anterior to the
  fourth ventricle.

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Pons

• It measures about 1 in length, and like the
  medulla, its basic anatomy can be organized into
  anterior (motor) and posterior (sensory)
  divisions.

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Ventral Pons

• The anterior portion of the pons is called the
  pontine bulb.
• It contains large numbers of descending motor
  fibers from the corticospinal tracts.

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Dorsal Pons

• In the dorsal pons, we have the medial lemniscus
  (blue), the site of ascending sensory information
  tracts.
• The reticular formation of the pons contains a
  collection of pigmented cells containing
  neuromelanin and the neurotransmitter
  norepinephrine.

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Pontine Reticular Formation

• This collection of pigmented cells is termed the
  locus coeruleus, with projections to the
  hypothalamus, cerebellum, cerebral cortex, and
  the spinal cord.
• This blue spot has been implicated in
  maintaining arousal, dreaming, and the regulation
  of mood.

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Pontine Reticular Formation

• Other important nuclei in the reticular formation
  of the pons comprise the pontine respiratory
  center.
• Specifically, the pneumotaxic area is found in
  the upper pons the apneustic area is found in
  the lower pons.
• Together they work with the medullary rhythmicity
  area in the medulla to inhibit (PA) or facilitate
  (AA) respiration.

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Reticular Formation Cranial Nerves

• The cranial nerve nuclei and tracts that are
  located in the reticular formation are the
  Trigeminal (Vth), and the Abducens (VIth) nerves.
• The Facial (VIIth) and the Vestibulococchlear
  (VIIIth) are located at the junction of the pons
  and medulla.

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Cerebellum

• The second largest portion of the brain is the
  cerebellum.
• It lies behind the pons and medulla oblongata.
• Between its central portion and these structures
  is the cavity of the fourth ventricle.

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Cerebellum

• The cerebellum also lies inferior to the
  occipital lobe of the cerebrum.
• It is separated from the cerebrum by the
  transverse fissure.

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Cerebellum

• The cerebellum is somewhat oval in form, but
  constricted medially and flattened.
• Its greatest diameter is from side.

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Cerebellum

• Shaped like a butterfly, the central constricted
  area is the vermis, which means worm-shaped.
• The lateral projections are referred to as
  hemispheres.

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Cerebellar Hemispheres

• Each hemisphere consists of lobes that are
  separated by deep and distinct fissures.
• The anterior lobe plays a prominent role in
  postural adjustment.

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Cerebellar Lobes

• The posterior lobe plays a major role in
  coordinating voluntary movement.

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Cerebellar Lobes

• The flocculonodular lobe is concerned with the
  sense of equilibrium.
• Eye movements and postural adjustments to gravity
  are also mediated in this lobe.

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Cerebellar Cortex

• The surface of the cerebellum is called the
  cortex, and it consists of grey matter in a
  series of slender parallel ridges called folia.
• They are less prominent than the convolutions
  (gyri) of the cerebral cortex.

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Cerebellar White Matter

• Beneath the grey matter are white matter tracts
  termed arbor vitae that resembles the branches of
  a tree.

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Cerebellar Nuclei

• Deep within the white matter are masses of grey
  matterthe cerebellar nuclei.
• These nuclei give rise to nerve fibers that
  convey information out of the cerebellum to other
  parts of the nervous system.

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Cerebellum

• In the adult the proportion between the
  cerebellum and cerebrum is about 1 to 8.
• In the infant, the proportion is about about 1 to
  20.

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Cerebellar Function

• Functionally, the cerebellum is a motor area of
  the brain concerned with coordinating
  subconscious movements of skeletal muscles.
• The cerebellar peduncles are the fiber tracts
  that permit information to pass into and out of
  the cerebellum.

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Cerebellar Peduncles

• The three major bundles of fibers are the
  superior, middle, and inferior cerebellar
  peduncles.

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Cerebellar Peduncles

• The superior cerebellar peduncles (4) connect
  the cerebellum with the midbrain.
• They contain mostly efferent fibers that bring
  information out of the cerebellum into the
  midbrain.

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Cerebellar Peduncles

• Fibers constituting the superior cerebellar
  peduncle arise from several deep cerebellar
  nuclei.
• These fibers pass rostrally for a while and then
  cross at the level of the midbrain to form the
  decussation of the superior cerebellar peduncle.
• These fibers then continue rostrally to terminate
  in the red nucleus and the motor nuclei of the
  thalamus.

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Cerebellar Peduncles

• The middle cerebellar peduncles (5) connect the
  cerebellum with the pons.
• These peduncles contain only afferent fibers and
  thus bring information into the cerebellum.

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Cerebellar Peduncles

• Pontine grey nuclei are involved in conveying
  information from the motor cortex to the
  cerebellum regarding intended movement(s) of
  contralateral arm and leg.
• Because there are lots of pontine grey neurons,
  the cortico-ponto-cerebellar circuit in very
  important in overall motor function.

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Cerebellar Peduncles

• Cortico-pontine fibers arise in the ipsilateral
  motor association cortex and provide the pontine
  grey neurons with information regarding intended
  movement(s) of the contralateral side of the body.

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Cerebellar Peduncles

• Specifically, neurons in the left pontine grey
  nuclei project to the right side of the
  cerebellum via the right middle cerebellar
  peduncle.
• Neurons in the right pontine grey nuclei project
  to the left side of the cerebellum via the left
  middle cerebellar peduncle.

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Cerebellar Peduncles

• The inferior cerebellar peduncles (7) connect
  the cerebellum with the medulla.
• These peduncles contain both efferent and
  afferent fibers bringing information into and out
  of the cerebellum.

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Cerebellar Peduncles

• Specifically, the inferior cerebellar peduncle
  contains fibers that arise from cells in the
  ipsilateral spinal cord.
• The largest component of the inferior cerebellar
  peduncle consists of fibers that arise from the
  contralateral inferior olive (medulla) and from
  the vestibular nuclei in the reticular formation
  on the border of the pons and medulla.

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Cerebellar Peduncles

• With information coming into the cerebellum from
  the spinal cord (bottom up) and cortex (top
  down), the cerebellum is constantly kept apprised
  of the status of the body in space.
• Specifically, it receives input signals from
  proprioceptors in muscles, tendons, and joints,
  and from receptors for equilibrium, and from
  visual receptors (e.g., the eyes).

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Cerebellar Functions

• In conjunction with other motor areas of the
  brain, such as the motor cortex of the cerebrum
  and the basal ganglia, the cerebellum receives a
  duplicate set of signals regarding the motor
  plan.
• The cerebellum compares input information
  regarding the actual status of the body with the
  intended movement determined by the motor areas
  of the brain.

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Cerebellar Functions

• If the intent of the motor plan is not being
  attained by the skeletal muscles, the cerebellum
  is going to detect the variation and send
  feedback signals to the motor areas to either
  stimulate or inhibit the activity of the skeletal
  muscles.
• The interaction of the cerebellum with the motor
  cortex and basal ganglia produces smooth,
  coordinated movements of the bodys skeletal
  muscles.

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The Mesencephalon

• The midbrain extends from the lower portion of
  the diencephalon to the pons.
• It is about 1 in length and is organized around
  the cerebral aqueduct, which passes down through
  the midbrain, connecting the third ventricle with
  the fourth ventricle.

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Anterior Midbrain

• The area anterior to the cerebral aqueduct is
  called the tegmentum.
• The tegmentum is continuous with the reticular
  formation of the pons, and consists of
  longitudinal and transverse fibers, together with
  a considerable amount of gray substance.

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Anterior Midbrain

• The principal gray masses of the tegmentum are
  the red nuclei.
• The red nuclei are found in the reticular
  formation and are the site of termination for
  fibers from the cerebellum and cerebral cortex.

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Ventral Midbrain

• The red nuclei are important relay centers for
  cortico-motor impulses that govern muscle tone,
  body posture, and walking.
• Damage to the red nucleus results in tremor at
  rest, alteration in muscle tone, and
  choreiform-athetoid movements.

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Ventral Midbrain

• The midbrain reticular formation also contains
  cranial nerve nuclei and tracts for the
  oculomotor nerve (III) and the trochlear nerve
  (IV).
• CN III mediates some movements of the eyeball and
  changes in pupil size and lens shape.
• CN IV is involved also in eyeball movement.

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Ventral Midbrain

• The substantia nigra runs the vertical length of
  the anterior midbrain between the cerebral
  peduncles.
• It is important for the control of involuntary
  coordinated movements and the rapid onset of
  movement.
• When damaged, tremor at rest, loss of coordinated
  movements, and a mask-like face result.

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Cerebral Peduncles

• The cerebral peduncles (d, f, g) are a pair of
  fiber bundles on the ventral midbrain.
• They consist mostly of descending motor tracts
  connecting the cortex and the pons, and the
  cortex and the spinal cord.

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Dorsal Midbrain

• The area of the midbrain found posterior to the
  cerebral aqueduct is termed the tectum (roof).
• On the midbrain tectum we fine an important
  landmark referred to as the quadrageminal plate.

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Dorsal Midbrain

• It is comprised of the paired superior colliculi
  (2) and the paired inferior colliculi (3).
• The superior colliculi serve as reflex centers
  for movements of the eyeballs and head and neck
  in response to visual and other stimuli.
• The inferior colliculi serve as reflex centers
  for movements of the head and trunk in response
  to auditory stimuli.

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Dorsal Midbrain Colliculi

• Visual information taken in by the eye is carried
  by the optic nerve and then forwarded to the
  superior colliculus.
• From the superior colliculus, this stimulus is
  relayed on to the lateral geniculate body in the
  thalamus.
• From the lateral geniculate body, it is relayed
  to the primary visual cortex in the occipital
  lobe for recognition and perception.

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Dorsal Midbrain Colliculi

• A similar relationship exits with the medial
  geniculate bodies of the thalamus and the
  inferior colliculi of the midbrain.
• They are relay centers for auditory information
  as it travels upstream to the primary auditory
  cortex.