1 Sensory Pathways DR. ZAHOOR ALI SHAIKH

Before we talk about sensory pathways we will trace the course of sensory impulse from receptors to the spinal cord.

3 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 A α and A β – propriception, touch, pressure. –Small myelinated fiber Aδ – fast pain –Small unmyelinated C fibers – slow pain

4 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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6 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.

Spinal Tracts

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14 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)

15 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)

16 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. Different Sensations PROPRIOCEPTION

17 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. Different Sensations VIBRATORY SENSE

18 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. Different Sensations TWO POINT DISCRIMINATION

19 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 lobe lesion) Different Sensations STEREOGNOSIS

20 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 o C. Warm receptor responds from 30– 45 o C. Afferent from cold receptors – Aδ & C fibers. Afferent from warm receptors – C fibers. Temperature sensation is carried via lateral spinothalamic tract. Different Sensations TEMPERATURE

21  THERMORECPTOR  NOCICEPTORS

22 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. Different Sensations PAIN