Transcript 03.Lect-2 Sensory system-PSL-132-Dr.Zahoor.ppt

Sensory Pathways
DR. ZAHOOR ALI SHAIKH
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Before we talk about sensory pathways we
will trace the course of sensory impulse
from receptors to the spinal cord.
• From receptors impulses are carried by
sensory nerves, the sensory nerve have
cell bodies in dorsal root ganglia.
• Primary afferent fibers which carry
cutaneous sensations are
– Large myelinated fiber Aα and Aβ –
propriception, touch, pressure.
– Small myelinated fiber Aδ – fast pain
– Small unmyelinated C fibers – slow pain
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• Sensory fiber go to dorsal horn in the spinal
cord.
• Dorsal horn represent a gate in which
impulses in the sensory nerve fiber are
translated into impulses in ascending tracts.
• Dorsal horns are divided into laminae I – VII.
(I – superficial, VII – deepest).
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Sensory pathways or tracts.
1. Dorsal column or posterior column or
leminscal system.
– It carries sensations of fine touch, position,
viberation, two point discrimination &
stereognosis.
2. Anterolateral system
 Ventral spinothalamic tract carries crude
touch and pressure,
 Lateral spinothalamic tract carries pain and
temperature.
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Spinal Tracts
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DORSAL COLUMN
PATHWAY
• CARRIES FINE TOUCH,
POSITION, PRESSURE,
VIBRATION, TWO
POINT DESRIMINATION
stereognosis
• AFFERENT SENSORY
FIBERS Aβ TYPE.
• VERY FAST VELOCITY
30 – 70 m/s
• 3 NEURON SYSTEM
(SEE THE DIAGRAM)
ANTEROLATERAL PATHWAY
• CARRIES PAIN &
TEMPRATURE (lat. Sp.Th)
• CRUDE TOUCH &
PRESSURE ( VENT, Sp. Th)
• AFFERENT SENSORY
FIBERS Aδ (MYELINATED)
FAST PAIN
• C FIBERS( UNMYELINATED)
SLOW PAIN
• RELATIVELY SLOW
VELOCITY Aδ – 6 – 30 m/s.
C – 0.5 – 2 m/s.
• 3 NEURON SYSTEM
(SEE THE DIAGRAM)
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Different Sensations
TOUCH
• Touch receptors are more numerous in the skin of
fingers, lips & less in the skin of trunk.
• Many receptors are around hair follicles, subcutaneous
tissues & hairless areas.
• Touch receptors transmits impulses via Aβ sensory
fibers to CNS.
• Their diameter is 5 – 12 μm, conduction velocity is 30 –
70 m/sec.
• Some touch goes via C fibers.
• Touch information is transmitted by Dorsal column (fine),
and ventral spinothalamic tract (crude)
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Different Sensations
PROPRIOCEPTION
• It is awareness of position of body in space.
Information is carried by dorsal column, input goes
to cerebellum & sensory cortex.
• Disease of dorsal column produces Ataxia( due to
interruption of proprioception input to cerebellum).
• Sense organ for proprioception – spray endings,
pacinian corpuscle in synovia & ligaments, touch
receptors in skin & other tissues, muscle spindle –
these sensation are synthesize in the cortex into
conscious picture of position of body in space.
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Different Sensations
VIBRATORY SENSE
• When vibrating tuning fork is applied to
bony part we feel thrill.
– Receptor – pacinian corpuscle
– Sensation is carried by dorsal column.
– Vibration sensation decreases in diabetes
mellitus, pernicious anemia due to
degeneration of dorsal column.
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Different Sensations
TWO POINT DISCRIMINATION
• The minimal distance by which two touch stimuli must be
separated to be perceived as separate is called two point
threshold.
• It depends on touch receptors and cortical component.
• Its magnitude varies from place to place on body.
• Distance is smaller when touch receptors are most
abundant e.g. fingers where two stimuli can be felt if they
are separated by 3mm.
• But at the back points must be separated by 65mm or
more before they can be identified as separate.
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Different Sensations
STEREOGNOSIS
• The ability to identify objects by hand
without looking at them is called as
stereognosis.
• Normal person can identify objects like
coin, key.
• It depends on touch & pressure sensation,
dorsal column & sensory cortex.
• Impaired stereognosis is an early sign of
damage to the cerebral cortex. (parietal
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lobe lesion)
Different Sensations
TEMPERATURE
• Skin has cold sensitive area & heat sensitive
area. Receptors are free nerve endings.
• Cold receptors are 4 – 10 times more than warm
sensitive spots.
• Cold receptor responds from 10 – 38 oC.
• Warm receptor responds from 30– 45 oC.
• Afferent from cold receptors – Aδ & C fibers.
• Afferent from warm receptors – C fibers.
• Temperature sensation is carried via lateral
spinothalamic tract.
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THERMORECPTOR
NOCICEPTORS
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Different Sensations
PAIN
• Pain receptors or nociceptors – free nerve endings.
• Pain sensation are carried by two types of nerve fibers.
• Aδ (myelinated) -2 – 5 μm.in diameter, conduction
velocity – 12 – 30 m/sec.
– For fast pain (sharp localized)
• C fibers (unmyelinated) 0.4 – 1.2μm. in diameter,
conduction velocity – 0.5 – 2 m/sec.
– For slow pain (dull, diffuse)
– Both Aδ & C fibers terminate in dorsal horn.
Pain is carried via lateral spinothalamic tract.
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