Sensory, Motor, and Integrative Systems

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Sensory, Motor, and Integrative Systems

• Dr. Michael P. Gillespie

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Sensation

• Sensation is the conscious or subconscious
  awareness of changes in the internal or external
  environment.
• Destination of sensory nerve impulses-
• Spinal cord reflexes.
• Lower brain stem heart rate, breathing rate.
• Cerebral cortex we become aware of sensory
  stimuli.
• Perception is the conscious awareness and
  interpretation of sensations (primarily occurs in
  the cerebral cortex).

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Sensory Modalities

• Each unique type of sensation is called a sensory
  modality.
• Touch, pain, vision, hearing, etc.
• A given sensory neuron carries information for
  only one sensory modality.
• Two classes of sensory modalities
• General senses.
• Special senses.

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General Senses

• General senses refer to both somatic and visceral
  senses.
• Somatic senses include tactile sensations (i.e.
  touch, pressure, vibration, itch, tickle),
  thermal sensations (warm and cold), pain
  sensations, and proprioceptive sensations.
  Proprioceptive sensations monitor static
  positions and movements.
• Visceral senses provide information about the
  organs.
• Special senses include the sensory modalities of
  smell, taste, vision, hearing, and equilibrium or
  balance.

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Process of Sensation

• The process of sensation begins in a sensory
  receptor, which can be either a specialized cell
  or the dendrites of a sensory neuron.
• Each sensory receptor responds to a different
  stimulus.
• The receptor exhibits selectivity.

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Sensory Receptor Types
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Four Events in Sensation

• 1. Stimulation of the sensory receptor.
• Stimulation must occur within the receptive
  field.
• 2. Transduction of the stimulus.
• The receptor transduces (converts) energy in a
  stimulus into a graded potential.

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Four Events in Sensation

• 3. Generation of nerve impulses.
• When a graded potential reaches threshold, it
  triggers one or more impulses.
• Sensory neurons that conduct from PNS to CNS are
  referred to as first order neurons.
• 4. Integration of sensory input.
• Part of the CNS receives and integrates the
  sensory nerve impulses.

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Types of Sensory Receptors

• Sensory receptors can be classified according to
  several structural and functional
  characteristics.
• 1. Microscopic appearance.
• Type of potential produced
• Generator potentials and receptor potentials.
• 2. Location of receptors and the origin of the
  stimuli that activate them.
• 3. According to the type of stimulus they detect.

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Microscopic Structural Characteristics

• Free nerve endings of first-order sensory
  neurons.
• Bare dendrites.
• Pain, thermal, tickle, itch, and some touch
  sensations.
• Encapsulated nerve-endings of first-order sensory
  neurons.
• Dendrites are enclosed in a connective tissue
  capsule.
• Somatic and visceral sensations such as pressure,
  vibrations, and some touch sensations.
• i.e. pacinian corpuscles.

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Microscopic Structural Characteristics

• Separate cells that synapse with first-order
  sensory neurons.
• i.e. hair cells for hearing and equilibrium,
  gustatory receptor cells in taste buds,
  photoreceptors in the retina of the eye, etc.

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Types of Graded Potentials

• Sensory receptors produce two kinds of graded
  potentials in response to a stimulus.
• Generator potentials
• Occur in dendrites of free nerve endings,
  encapsulated nerve endings, and the receptive
  part of olfactory receptors.
• When a generator potential is large enough to
  reach threshold, it generates an action potential
  in a first-order neuron.
• Receptor potentials
• Occur in sensory receptors that are separate
  cells.
• Receptor potentials trigger release of a
  neurotransmitter through exocytosis of synaptic
  vesicles.

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Location of Receptors / Origin of Stimuli

• Exteroreceptors
• Located at or near the external surface of the
  body.
• Sensitive to stimuli outside the body.
• Monitor the external environment.
• Hearing, vision, smell, taste, touch, pressure,
  vibration, temperature, and pain.
• Interoreceptors
• Located in blood vessels, visceral organs,
  muscles, and the nervous system.
• Monitor the internal environment.
• Usually not consciously perceived however,
  strong stimuli may be felt as pain and pressure.

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Location of Receptors / Origin of Stimuli

• Mechanoreceptors
• Located in muscles, tendons, joints, and the
  inner ear.
• Provide information about body position, muscle
  length and tension, and the position and movement
  of your joints.
• There really is no such thing as a proprioceptor.
  Receptors such as mechanoreceptors participate
  in proprioceptive pathways. The term
  proprioceptor is vague and not appropriate
  however, its use is ubiquitous in the literature.

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Type of Stimulus Detected

• Most stimuli are in the following forms
• Mechanical energy i.e. sound waves or pressure
  changes.
• Electromagnetic energy i.e. light or heat.
• Chemical energy i.e. a molecule of glucose.

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Type of Stimulus Detected

• Mechanoreceptors
• Sensitive to mechanical stimuli such as the
  deformation, stretching, or bending of cells.
• Provide sensations of touch, pressure, vibration,
  proprioception, hearing, and equilibrium.
• Thermoreceptors
• Respond to changes in temperature.
• Nociceptors
• Respond to painful stimuli from physical or
  chemical tissue damage.

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Type of Stimulus Detected

• Photoreceptors
• Detect light that strikes the retina of the eye.
• Chemoreceptors
• Detect chemicals in the mouth (taste), nose
  (smell), and body fluids.
• Osmoreceptors
• Detect the osmotic pressure of body fluids.

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Somatic Sensations

• Somatic sensations arise from stimuli of sensory
  receptors in the skin or subcutaneous layer in
  mucous membranes of the mouth, vagina, and anus
  in muscles, tendons, and joints and in the inner
  ear.
• Somatic sensory receptors are distributed
  unevenly.
• Highest density tip of the tongue, lips,
  fingertips.
• Cutaneous sensations are those arising from
  stimulating the surface of the skin.

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Four Modalities of Somatic Sensation

• Tactile
• Thermal
• Pain
• Proprioceptive

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Tactile Sensations

• The tactile sensations include touch, pressure,
  vibration, itch, and tickle.
• Tactile receptors in the skin or subcutaneous
  layer include Meissner corpuscles, hair root
  plexuses, Merkel discs, Ruffini corpuscles,
  pacinian corpuscles, and free nerve endings.

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Structure and Location of Sensory Receptors
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Touch

• Sensations of touch arise from stimulation of
  receptors in the skin and subcutaneous layer.
• Rapidly adapting touch receptors
• Meissner corpuscles
• Corpuscles of touch.
• Located in the dermal papillae of hairless skin.
• Egg shaped mass of dendrites enclosed by a
  capsule.
• Hair root plexuses
• Free nerve endings wrapped around hair follicles.

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Touch

• Slowly adapting touch receptors
• Merkel discs (tactile discs or type I cutaneous
  mechanoreceptors.
• Saucer shaped, flattened free nerve endings that
  make contact with Merkel cells.
• Plentiful in the fingertips, hands, lips, and
  external genitalia
• Ruffini corpuscles (type II cutaneous
  mechanoreceptors).
• Elongated, encapsulated receptors located deep in
  the dermis, and in ligaments and tendons.
• Present in the hands and soles.
• Sensitive to stretching of digits and limbs.

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Pressure

• Pressure is a sustained sensation that is felt
  over a larger area than touch.
• It occurs with deformation of deeper tissues.
• Meissner corpuscles, Merkel discs, and pacinian
  corpuscles contribute to pressure sensation.
• Pacinian corpuscles (lamellated corpuscles) are
  large oval structures composed of a multi-layered
  connective tissue capsule enclosing a dendrite.
• Located in the dermis and subcutaneous layer in
  submucosal tissues around joints, tendons, and
  muscles in the periosteum and in the mammary
  glands, external genitalia, and certain viscera,
  such as the pancreas and urinary bladder.

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Vibration

• Vibration sensation results from rapidly
  repetitive sensory signals from tactile
  receptors.
• Meissner corpuscles and pacinian corpuscles
  detect vibration.
• Meissner lower-frequency vibrations.
• Pacinian higher-frequency vibrations.

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Itch

• Itch results from stimulation of free nerve
  endings by certain chemicals, such as bradykinin,
  often due to a local inflammatory response.

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Tickle

• Free nerve endings are thought to mediate the
  tickle sensation.

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Thermal Sensations

• Thermoreceptors are free nerve endings.
• The thermal sensations of coldness and warmth are
  detected by different receptors.
• Temperatures below 10 and above 48C primary
  stimulate pain receptors.

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Thermal Sensations

• Cold receptors
• Located in the stratum basale of the dermis.
• Attached to medium-diameter type A myelinated
  fibers.
• Temperatures between 10 and 40C activate them.
• Warm receptors
• Located in the dermis.
• Not as abundant as cold receptors.
• Attached to small-diamtere unmyelinated C fibers.
• Temperatures between 32 and 48C activate them.

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Phantom Limb Sensation

• Patients who have had a limb amputated may still
  experience sensations such as itching, tingling,
  or pain as if the limb were still there.
• This is called phantom limb sensation.
• Possible causes
• Impulses from the proximal portions of sensory
  neurons that previously carried impulses from the
  limb.
• Neurons in the brain that previously received
  input from the missing limb are still active,
  giving false sensory perceptions.

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Phantom Limb Sensation

• Treatments such as acupuncture, electrical nerve
  stimulation, and biofeedback can be helpful in
  treating phantom limb pain.

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Pain Sensations

• Pain serves a protective function by signaling
  the presence of noxious, tissue-damaging
  conditions.
• The subjective description and indication of the
  location of pain may help identify the underlying
  disease.
• The receptors for pain are called nociceptors
  (noci harmful).
• They are free nerve endings found in every tissue
  of the body except the brain.

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Pain Sensations

• Intense thermal, mechanical, or chemical stimuli
  can activate nociceptors.
• Tissue irritation or injury releases chemicals
  such as prostaglandins, kinins, and potassium
  ions that stimulate nociceptors.

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Pain Sensations

• Pain can persist long after the pain-producing
  stimulus is removed because the pain mediating
  chemicals linger.
• Conditions that elicit pain include excessive
  distention (stretching) of a structure, prolonged
  muscular contractions, muscle spasms, or
  ischemia.

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Types of Pain

• Types of pain based upon speed of impulses
• Fast pain
• Medium-diameter, myelinated A fibers.
• Occurs within 0.1 seconds after a stimulus is
  applied.
• Referred to as acute, sharp, or pricking pain.
• Needle puncture or knife cut to the skin.
• Not felt in deeper tissues.

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Types of Pain

• Slow pain
• Small-diameter, unmyelinated C fibers.
• Begins a second or more after the stimulus is
  applied.
• Increases in intensity over several seconds or
  minutes.
• Referred to as chronic, burning, or throbbing
  pain.
• Can occur in skin, deeper tissues, or internal
  organs.

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Types of Pain

• Types of pain based upon location of pain
  receptors
• Superficial somatic pain stimulation of
  receptors in the skin.
• Deep somatic pain - stimulation of receptors in
  skeletal muscles, joints, tendons, and fascia.
• Visceral pain stimulation of receptors in
  visceral organs.

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Localization of Pain

• Fast pain
• Very precisely localized to the stimulated area.
• i.e. pin prick
• Somatic slow pain
• Well localized, but more diffuse

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Localization of Pain

• Visceral slow pain
• Some is localized to the area of pain
• Much is referred to the skin that overlies the
  organ or to a surface area far from the
  stimulated organ.
• Know as referred pain.
• In general, the visceral organ and the area to
  which the pain is referred are served by the same
  segment of the spinal cord.

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Distribution of Referred Pain
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Analgesia

• Analgesia (an without, algesia pain) is pain
  relief.
• Types of analgesia
• Analgesic drugs such as aspirin and ibuprofen
  block the formation of prostaglandins, which
  stimulate nociceptors.
• Local anesthetics such as novacaine block the
  conduction of nerve impulses along the axons of
  first-order pain neurons.
• Morphine and other opiate drugs alter the quality
  of pain perception in the brain.
• Pain is still sensed, but no longer experienced
  as so noxious.

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Proprioceptive Sensations

• Proprioceptive sensations allow us to know where
  our head and limbs are located and how they are
  moving even if we are not looking at them.
• Kinesthesia (kin motion, esthesia
  perception) is the perception of body movements.
• Proprioceptive sensations arise in receptors
  termed proprioceptors.

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Proprioceptive Sensations

• Proprioceptors are embedded in muscles and
  tendons. These tell us the degree to which the
  muscle is contracted, the amount of tension on
  tendons, and the position of joints.
• Hair receptors in the inner ear monitor the
  orientation of the head relative to the ground
  and the head position during movements.
• The provide information for maintaining balance
  and equilibrium.
• Proprioceptors also allow for weight
  discrimination.

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Mechanoreceptors

• Three types
• Muscle spindles
• Located within skeletal muscles
• Tendon organs
• Located within tendons
• Joint kinesthetic receptors
• Located within synovial joint capsules

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Muscle Spindles

• Muscle spindles are located in skeletal muscles.
• They consist of several slowly adapting sensory
  nerve endings that wrap around 3-10 specialized
  muscle fibers, called intrafusal muscle fibers.
• Muscle spindles monitor changes in the length of
  skeletal muscles.
• The main function of a muscle spindles is to
  measure muscle length (how much a muscle is being
  stretched).

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Muscle Spindles

• They participate in stretch reflexes.
• Activation of the muscle spindle causes
  contraction of a skeletal muscle, which relieves
  stretching.
• They help maintain the level of muscle tone (the
  small degree of muscle contraction present while
  the muscle is at rest).

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Tendon Organs

• Tendon organs are located at the junction of a
  tendon and a muscle.
• They consist of a thin capsule of connective
  tissue that encloses a few tendon fascicles.
• The participate in tendon reflexes to protect
  tendons and their associated muscles from damage
  due to excessive tension.
• Tendon reflexes decrease muscle tension by
  causing muscle relaxation.

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Muscle Spindles Tendon Organs
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Joint Kinesthetic Receptors

• Several types of joint receptors are present
  within or around the articular capsule of
  synovial joints.
• Free nerve endings and Ruffini corpuscles respond
  to pressure.
• Pacinian corpuscles respond to acceleration and
  deceleration of the joint.
• Articular ligaments contain receptors similar
  tendon organs that adjust reflex inhibition of
  adjacent muscles.

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Somatic Sensory Pathways

• Somatic sensory pathways relay information from
  the somatic sensory receptors to the primary
  somatosensory area in the cerebral cortex and to
  the cerebellum.
• Three sets of neurons
• First-order neurons
• Second-order neurons
• Third-order neurons

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First-order Neurons

• Conduct impulses from somatic receptors into the
  brain stem or spinal cord.
• Impulses from the face, mouth, teeth, and eyes
  travel along the cranial nerves.
• Impulses from the neck, trunk, limbs, and
  posterior aspect of the head travel along spinal
  nerves.

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Second-order Neurons

• Conduct impulses from the brain stem or spinal
  cord to the thalamus.
• The axons decussate in the brain stem or spinal
  cord before ascending.
• Consequently, all somatic sensory information
  from one side of the body reaches the thalamus on
  the opposite side.

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Third-order Neurons

• Conduct impulses from the thalamus to the primary
  somatosensory cortex on the same side.

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Relay Stations

• Regions within the CNS where neurons synapse with
  other neurons that are part of a particular
  sensory or motor pathway are known as relay
  stations.
• The Thalamus serves as a major relay station.
• Neural signals are being relayed from one region
  of the CNS to another.

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Direct Motor Pathways
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Somatic Sensory Pathways

• Somatic sensory impulses ascend to the cerebral
  cortex via three general pathways.
• Posterior column-medial lemniscus pathway.
• Anterolateral (spinothalamic) pathways.
• Trigeminothalamic pathway.

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Somatic Sensory Pathways
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Posterior Column-Medial Lemniscus Pathway

• This pathways conveys information for touch,
  pressure, vibration, and conscious proprioception
  from the limbs, trunk, neck, and posterior head.
• Posterior column in spinal cord.
• Medial lemniscus in brain stem.

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Posterior Column-Medial Lemniscus Pathway

• First order neurons from the upper limbs, upper
  trunk, neck, and posterior head travel in the
  cuneate fasciculus.
• First order neurons from the lower limbs and
  lower trunk travel along the gracile fasciculus.
• The axons synapse with second order neurons in
  the cuneate and gracile nuclei respectively.
• The axons of the second-order neurons decussate
  in the brain stem and enter the medial lemniscus.

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Posterior Column-Medial Lemniscus Pathway

• The second-order neurons traveling in the medial
  lemniscus synapse with third-order neurons in the
  thalamus.
• Axons from the third order neurons project into
  the primary somatosensory area of the cortex.

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Posterior Column-Medial Lemniscus Pathway
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Anterolateral Pathway to the Cortex
(Spinothalamic)

• This pathway conveys information for pain,
  temperature, itch, and tickle from the limbs,
  trunk, neck, and posterior head.
• First order neurons connect to a receptor of the
  limbs, trunk, neck, or posterior head.
• Cell bodies are located in the dorsal root
  ganglion.
• The first order neurons synapse with second order
  neurons in the spinal cord.
• Cell bodies are located in the posterior gray
  horn of the spinal cord.

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Anterolateral Pathway to the Cortex
(Spinothalamic)

• The axons of the second order neurons decussate
  and move to the brain stem via the spinothalamic
  tract.
• The axons of the second order neurons synapse
  with third order neurons in the thalamus.
• The third-order neurons project to the primary
  somatosensory area of the cortex on the same side
  as the thalamus.

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Anterolateral Pathway to the Cortex
(Spinothalamic)

• Figure 16.6

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Trigeminothalamic Pathway to the Cortex

• This pathway conveys information for most somatic
  sensations from the face, nasal cavity, oral
  cavity, and teeth.
• First-order neurons extend from somatic sensory
  receptors in the face, nasal cavity, oral cavity,
  and teeth into the pons via the trigeminal nerve.
• They synapse with second order neurons in the
  pons.

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Trigeminothalamic Pathway to the Cortex

• The second order neurons decussate and ascend the
  trigeminothalamic tract to the thalamus.
• They synapse with third-order neurons in the
  thalamus.

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Trigeminothalamic Pathway to the Cortex

• Figure 16.7

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Mapping the Primary Somatosensory Area
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Somato-Sensory and Somato-Motor Maps in Cerebral
Cortex
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Sensory Homunculus
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Somatic Sensory Pathways to the Cerebellum

• The posterior spinocerebellar and anterior
  spinocerebellar tracts convey nerve impulses from
  proprioceptors to the cerebellum.
• This informs the cerebellum of body movements and
  allows it to coordinate them for smooth,
  controlled movements.
• This helps us to maintain posture and balance.

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Somatic Motor Pathways

• Lower motor neurons
• Have cell bodies in the brain stem and spinal
  cord.
• Innervate skeletal muscles
• Referred to as the final common pathway because
  only LMNs provide output from the CNS directly to
  skeletal muscle fibers
• Upper motor neurons
• Carry signals form the cerebral cortex to LMNs.
• Execution of voluntary movements.
• Maintain balance and coordination.

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Direct Motor Pathways

• Lateral corticospinal tract
• Anterior cotricospinal tract
• Corticobulbar tract

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Indirect Motor Pathways

• Rubrospinal
• Tectospinal
• Vestibulospinal
• Medial and lateral reticulospinal

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Lateral Corticospinal Tract (Crossed Pyramidal
Tract)

• The lateral corticospinal tract provides fine
  motor control to the limbs and digits.
• The fibers decussate in the medulla.

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Anterior Corticospinal Tract (Direct Pyramidal
Tract)

• The anterior corticospinal tract conducts
  voluntary motor impulses from the precentral
  gyrus to the motor centers of the cord.

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Corticobulbar Tract

• Connects the cerebral cortex to the brain stem.
• bulbar refers to the brainstem.
• Controls the muscles of the face, head, and neck.
• Innervates the cranial motor nuclei.

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Rubrospinal

• Controls large muscle movement such as the arms
  and legs.
• Some fine motor control.
• Facilitates flexion and inhibits extension in the
  upper extremities.

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Tectospinal

• Coordinates head and eye movements.
• Mediates reflex postural movements in response to
  visual and auditory stimuli.

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Vestibulospinal

• The vetsibulospinal tract is a descending tract
  that originates from the vestibular nuclei of the
  medulla.
• The vestibulospinal tract facilitates extensor
  (antigravity) muscle tone.
• It assists in maintaining equilibrium.
• It participates with cranial nerves II, IV, and
  VI in controlling eye movements.
• It helps to control head and neck position.

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Reticulospinal

• The reticulospinal tract is an extrapyramidal
  tract which travels from the reticular formation.
• It has integrative functions that help to
  coordinate automatic movements of locomotion and
  posture.

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Spinal Tracts
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Referred Pain Distribution
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Stages of Sleep
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Reticular Activating System
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Input and Output to Cerebellum