1. SENSORY NERVOUS SYSTEM
Dr. Ayisha Qureshi
Assistant Professor
MBBS, Mphil

2. Imagine swimming in an indoor tank of salt water: there is no light, no sound and no breeze. The air and the water are the same temperature as your body. You are in a sensory deprivation chamber, and the only sensations you are aware of come from your own body. Your limbs are weightless, your breath moves in and out effortlessly, and you feel your heart beating. There is complete absence of the external stimuli. A few decades past, flotation tanks for sensory deprivation were a popular way to counter the stress of a busy world. These facilities are hard to find now, but they illustrate the role of the afferent division of the nervous system: to provide us with information about the environment outside and inside our bodies.

3. Stimulus & Modalities A stimulus is a change detectable by the body.
Stimuli exist in a variety of energy forms, or modalities, such as heat, light, sound, pressure, and chemical changes.
Sometimes we perceive sensory signals when they reach a level of consciousness, but other times they are processed completely at the subconscious level.
All the information regarding all these senses is send to the CNS via AFFERENT NEURONS.
With the sensory system, we will be covering all the somatic senses EXCEPT proprioception (Proprioception is when you close your eyes and the arms are raised above the head, you are aware of the position of the arms. This is called PROPRIOCEPTION). The special senses have already been covered.

4. Because the only way afferent
neurons can transmit information to the CNS about stimuli
is via action potential propagation, these forms of energy must be converted into electrical signals.
The conversion of stimulus energy into a graded potential is called Sensory transduction and is done by sensory receptors.

6. SENSORY NERVOUS SYSTEM
SOMATIC SENSES
Mechano-receptive
Tactile(Touch, tickle , Pressure, Vibration)
Position
Thermo-receptive
Hot
Cold
Pain
SPECIAL SENSES

7. Receptors are sensory afferent nerve endings that terminate in periphery as either part of a neuron or in the form of specialized capsulated structures. They act as biological transducers and convert various forms of energy acting on them into action potentials.
SENSORY Receptors

8. Classification of Receptors
INTEROCEPTORS
Receptors which give response to stimuli arising from WITHIN the body.
EXTEROCEPTORS
Receptors which give response to stimuli arising from OUTSIDE the body.

9. INTEROCEPTORS VISCEROCEPTORS PROPRIOCEPTORS Stretch Receptors
(Heart)
Baroreceptors
(pressure changes in b.v)
Chemoreceptors
(chemical changes in blood)
Osmoreceptors
(Osmotic Pressure changes Urinary Tract & Brain )
PROPRIOCEPTORS
Muscle Spindle
Golgi Tendon Organs
Pacinian Corpuscle
Free Nerve endings

10. MEISSNER’S CORPUSCLE & MERKEL’S DISC
EXTERCEPTORS
CUTANEOUS RECEPTORS
TOUCH
MEISSNER’S CORPUSCLE & MERKEL’S DISC
PRESSURE
PACINIAN CORPUSCLE
COLD
THERMORECEPTOR
WARMTH
PAIN
FREE NERVE ENDING’S
SPECIAL SENSES
CHEMORECEPTORS
(Taste & Smell)
TELERECEPTORS
(Vision & Hearing)

14. Receptors are present in the skin, the mucous membranes, fascia and deeper parts of the body. They are responsible for 4 different sensations:
Touch-pressure
Cold
Warmth and
Pain
The receptors are:
Encapsulated receptors: consist of multilayered capsules of connective tissue which surround a core of cells in which axons end after losing their myelin sheath.
- Meissner’s corpuscle: sensitive to light touch & are rapidly adapting. Are present just below the epidermis in the palmer surface of fingers, lips, margins of the eyelids.
- Pacinian Corpuscle: respond to vibration & deep pressure & is rapidly adapting. Present in deeper tissues and also in pleura, peritoneum, external genitalia and walls of many viscera. Also present in periostium, ligaments and joint capsules.
- Krause’s end bulbs: occur in conjunctivae, papillae of lips and tongue.
Expanded tips on sensory nerve endings:
- Merkel’s discs: which detects light, sustained touch and texture, and is slowly adapting. Present in hairless skin e.g. fingertips.
- Riffini’s end organs: in deeper layer of skin and deeper tissues, e.g. periostium, ligaments and joint capsules. They respond to deep, sustained pressure and stretch of the skin, such as during a massage, and are slowly adapting.
Naked or free nerve endings: are the most widely distributed receptors in the body and can be excited by touch, cold, warmth and pain.

15. SENSORY RECEPTORS
Pacinian Corpuscle
Free nerve ending’s

16. General properties of receptors

17. The following are the properties of the Sensory Receptors:
Receptor Potential.
Specificity of stimulus & the Adequate stimulus.
Effect of strength of stimulus.
Adaptation (also Desensitization).
Muller’s doctrine of specific nerve energies
Law of projection.
Threshold.
Sensory unit
Receptive field.

18. 1. RECEPTOR POTENTIAL
The changes in sensory receptor membrane potential is a graded potential called the receptor potential.

19. (SENSORY TRANSDUCTION)
Transduction is the conversion of stimulus energy into information that can be processed by the nervous system, which is an action potential.
Stimulus ↓
Receptor
(SENSORY TRANSDUCTION)
Graded Potential
(RECEPTOR POTENTIAL)
Afferent Neuron
Action Potential
A transducer is a device that converts a signal from one form of energy into a different form. For example, the transducer in a radio converts radio waves into sound waves.

20. How is a physical or a chemical stimulus converted into a change in membrane potential?
(chemical/ mechanical/ thermal) ↓
Receptor which is either:
Specialized ending of the afferent neuron, OR
A separate receptor cell associated with a peripheral nerve ending.
Membrane permeability altered
(usually by opening of ligand-gated or stimulus sensitive cation channels)
A graded potential is generated. It is called RECEPTOR POTENTIAL.
There is summation, and if the stimulus is strong, it leads to a greater permeability change in the receptor which leads to a large Receptor potential.
If the Receptor Potential is large enough
An Action Potential is generated
(by opening of the voltage-gated Na channels in the afferent neuron next to the receptor)
Usually, it opens the ligand-gated sodium channels in most of the receptors which leads to depolarization and a graded potential. Only photoreceptors are hyperpolarized on stimulation by closing of the cation channels.

21. In some cells, the receptor potential initiates an action potential that travels along the sensory fiber to the CNS. In other cells, receptor potentials influence neurotransmitter secretion by the receptor cell, which in turn alters electrical activity in an associated sensory neuron.

22. THE INITIATION OF THE ACTION POTENTIAL
The initiation site of action potentials in an afferent neuron differs from the site in an efferent neuron or interneuron.
In the other two types of neurons (interneuron & the efferent neuron), action potentials are initiated at the axon hillock located at the start of the axon next to the cell body.
However, in the afferent neuron, action potentials are initiated at the peripheral end of fiber next to the receptor, a long distance from the cell body.

23. 2. SPECIFICITY OF STIMULUS & ADEQUATE STIMULUS

24. (1) its nature, or modality and (2) its location (3) Intensity
If all stimuli are converted to action potentials in sensory neurons and all action potentials are identical, how can the central nervous system tell the difference between heat and pressure, or between a pinprick to the toe and one to the hand?
All stimuli once received by the receptor are converted into action potentials and all of them are carried by the afferent neurons. This means that the CNS must distinguish four properties of a stimulus to be able to specify a stimulus:
(1) its nature, or modality and
(2) its location
(3) Intensity
(4) Duration

25. Adequate Stimulus
Each sensory receptor has an adequate stimulus, a particular form of energy to which it is most responsive. For example, thermoreceptors are more sensitive to temperature changes than to pressure, and mechanoreceptors respond preferentially to stimuli that deform the cell membrane, receptors in the eye are sensitive to light, receptors in the ear to sound waves, and warmth receptors in the skin to heat energy. Because of this differential sensitivity of receptors, we cannot “see” with our ears or “hear” with our eyes.
Some receptors can respond weakly to stimuli other than their adequate stimulus, but even when activated by a different stimulus, a receptor still gives rise to the sensation usually detected by that receptor type. They respond to most other forms of energy if the intensity is high enough. Photoreceptors of the eye respond most readily to light, for instance, but a blow to the eye may cause us to “see stars”, an example of mechanical energy of sufficient force to stimulate the photoreceptors.
Sensory receptors can be incredibly sensitive to its preferred stimulus.

26. Modality/ Nature of the stimulus
The 1:1 association of a receptor with a sensation is called labeled line coding. Stimulation of a cold receptor is always perceived as cold, whether the actual stimulus was cold or an artificial depolarization of the receptor. The blow to the eye that causes us to see a flash of light is another example of labeled line coding. A blow to the eye is seen as “white light” as the photoreceptors were stimulated.
The afferent neuron with its peripheral receptor that first detects the stimulus is known as a first-order sensory neuron. It synapses on a second-order sensory neuron, either in the spinal cord or the medulla, depending on which sensory pathway is involved. This neuron then synapses on a third-order sensory neuron in the thalamus, and so on. With each step, the input is processed further. A particular sensory modality detected by a specialized receptor type is sent over a specific afferent and ascending pathway (a neural pathway committed to that modality) to excite a defined area in the somatosensory cortex. That is, a particular sensory input is projected to a specific region of the cortex. Thus, different types of incoming information are kept separated within specific labeled lines between the periphery and the cortex. In this way, even though all information is propagated to the CNS via the same type of signal (action potentials) the brain can decode the type and the location of the stimulus.

27. The table summarizes how the CNS is informed of the type (what
The table summarizes how the CNS is informed of the type (what?), location (where?), and intensity (how much?)
of a stimulus.

28. Location of the stimulus
The location of a stimulus is also coded according to which receptive fields are activated. The sensory region supplied by a single sensory neuron is called a receptive field.
For example, touch receptors in the hand project to a specific area of the cerebral cortex. Experimental stimulation of that area of the cortex during brain surgery is interpreted as a touch to the hand, even though there is no contact.
Also, lateral inhibition of the less activated regions leads to release of inhibitory NT that inhibits the region around the stimulated area. The contrast leads to a better localization of the stimulated area. (Tactile localization)

29. Receptive fields and convergence

30. 3. Effect of Strength & Duration of Stimulus:

31. For individual sensory neurons, intensity discrimination begins at the receptor. If a stimulus is below threshold, the primary sensory neuron does not respond. Once stimulus intensity exceeds threshold, the primary sensory neuron begins to produce action potentials.
As stimulus intensity increases, the receptor potential amplitude (strength) increases in proportion, and the frequency of action potentials in the primary sensory neuron increases, up to a maximum rate.
Similarly, the duration of a stimulus is coded by the duration of action potentials in the sensory neuron. In general, a longer stimulus generates a longer series of action potentials in the primary sensory neuron

33. 4. ADAPTATION also called Desensitization.
It is the decrease in response of receptors on being continuously stimulated.

34. When a stimulus persists continuously, some receptors adapt, or cease to respond. Thus, the receptor “adapts” to the stimulus by no longer responding to it to the same degree.
Receptors fall into one of two classes, depending on how they adapt to continuous stimulation:
Tonic receptors
Phasic receptors

35. Types of receptors based on their adaptation
TONIC RECEPTORS
PHASIC RECEPTORS
Tonic Receptors are slowly adapting receptors that respond rapidly when first activated, then slow down and maintain their response.
Pressure sensitive baroreceptors, irritant receptors, and some tactile receptors and proprioceptors fall into this category. In general, the stimuli that activate tonic receptors are parameters that must be monitored continuously by the body.
It is important that these receptors do not adapt to a stimulus and continue to generate action potentials to relay this information to the CNS.
Phasic receptors are rapidly adapting receptors that respond when they first receive a stimulus but stop responding if the strength of the stimulus remains constant. Many of the tactile receptors in the skin belong to this class.
Some phasic receptors, most notably the Pacinian corpuscle, respond with a slight depolarization called the off response when the stimulus is removed. They are important in situations where it is important to signal a change in stimulus intensity rather than the status quo information.
When you put something on, you soon become accustomed to it, because of these receptors’ rapid adaptation. When you take the item off , you are aware of its removal because of the “off” response.
Because these receptors adapt rapidly, you are not continually conscious of wearing your watch, rings, and clothing. A good example is of the touch receptors. When we wear a ring in the morning, the tactile receptors are stimulated. However, because the stimulation is continuous, after a little while the receptors show adaptation. Now we are not continuously aware of the ring while we go about doing different things, otherwise, we would not be able to focus. When we take off the ring, then the receptor is switched off,i.e. it is stimulated again to make us aware of the fact that the ring is being taken off.

36. 5. MULLER’S DOCTRINE OF SPECIFIC NERVE ENERGIES &
Muller’s doctrine states that the action potential produced by all receptors is the same in all nerve fibers provided the diameter of the nerve fiber is the same.
the nature of perception of a stimulus by the cns is defined by the pathway over which the sensory information is carried. Hence, the origin of the sensation is not important.

37. 6. LAW OF PROJECTION
E.g. stimulation of a cold receptor over the knee by cold or electrical stimulation of the fiber originating from this receptor before and after its entry into the brain or spinal cord will all produce a sensation of cold over the knee.
Stimulation of nerve fiber anywhere along its course produces the specific sensation in the area of the body from where it originated.

38. 6. THRESHOLD
All receptors need a minimum strength of stimulus to start showing activity; this strength is called the threshold.

39. 7. SENSORY UNIT
A sensory unit is just like a motor unit.
The sensory unit is a single primary afferent nerve including all its peripheral branches.

40. 8. RECEPTIVE FIELD.
THE AREA OF THE BODY WHOSE SENSORY NERVE SUPPLY COMES FROM a single SENSORY UNIT IS CALLED A RECEPTIVE FIELD.

41. Summary
1. Each receptor is most sensitive to a particular type of stimulus. 2. A stimulus above threshold initiates action potentials in a sensory neuron that projects to the CNS. 3. Stimulus intensity and duration are coded in the pattern of action potentials reaching the CNS. 4. Stimulus location and modality are coded according to which receptors are activated. 5. Each sensory pathway projects to a specific region of the cerebral cortex dedicated to a particular receptive field. The brain can then tell the origin of each incoming signal.

42. SENSORY CLASSIFICATION OF THE NERVE FIBERS
Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves.
Type C fibers are the small unmyelinated nerve fibers that conduct impulses at low velocities. The C fibers constitute more than one half of the sensory fibers in most peripheral nerves, as well as all the postganglionic autonomic fibers.
Note that a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, a distance in 1 second that is longer than a football field.
Conversely, the smallest fibers transmit impulses as slowly as 0.5 m/sec, requiring about 2 seconds to go from the big toe to the spinal cord.
Some signals need to be transmitted to or from the central nervous system extremely rapidly; otherwise, the information would be useless. An example of this is the sensory signals that apprise the brain of the momentary positions of the legs at each fraction of a second during running. At the other extreme, some types of sensory information, such as that depicting prolonged, aching pain, do not need to be transmitted rapidly, so slowly conducting fibers will suffice. Nerve fibers come in all sizes between 0.5 and 20 micrometers in diameter-the larger the diameter, the greater the conducting velocity. The range of conducting velocities is between 0.5 and 120 m/sec.

43. THE SENSE OF TOUCH: TACTILE SENSE

44. Sense of touch is also called the Tactile sense
Sense of touch is also called the Tactile sense. Sense of pressure is not separate from the sense of touch as it is only sustained touch.
Receptors:
Free nerve endings.
Pacinian corpuscles.
Meissner’s corpuscles.
Ruffini’s end organs
Merkel’s discs.
Hair end organs.
Location:
All cutaneous receptors (skin)
Dermal tissue
Within the mouth (tip of tongue esp.)
Tendons
Periostium
Nerve fibers carrying the tactile sensations:
A-beta nerve fibers
C nerve fibers

45. Tactile Localization
This is the capacity to localize the area where a touch stimulus is applied. The lips and the fingers have the best developed tactile localization and also possess a low touch threshold.

46. TWO-POINT DISCRIMINATION
This is the capacity to distinguish two tactile stimuli when an area of skin is stimulated by two stimuli simultaneously at a certain distance from each other.
A high tactile discrimination is said to present when one can distinguish between the two points.
More detail in the practical copy (examination of the sensory system).

48. THE DORSAL COLUMN MEDIAL LEMNISCUS SYSTEM

49. DCML is a crossed system
DCML is a crossed system. It originates from mechano-receptors located in the body wall and projects to the contralateral cerebral hemisphere via a 3-neuron projection system.
Also called:
- Dorsal white column system
- The Lemniscal system
It is constituted of 2 tracts called:
- Fasciculus Gracilis
- Fasciculus Cuneatus
The dorsal column-medial lemniscal system is composed of large, myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec.
The dorsal column-medial lemniscal system, as its name implies, mainly in the dorsal columns of the cord.
FUNCTIONS: It carries the following sensations:
Fine tactile sensations
Tactile localization
Tactile discrimination
Sensation of vibration
Conscious kinesthetic sense (sensation or awareness of various muscular activities in different parts of the body).
Stereognosis (It is the ability to recognize the known objects by touch with closed eyes). `
Another difference between the two systems is that the dorsal column-medial lemniscal system has a high degree of spatial orientation of the nerve fibers with respect to their origin, while the anterolateral system has much less spatial orientation. These differences immediately characterize the types of sensory information that can be transmitted by the two systems. That is, sensory information that must be transmitted rapidly and with temporal and spatial fidelity is transmitted mainly in the dorsal column-medial lemniscal system; that which does not need to be transmitted rapidly or with great spatial fidelity is transmitted mainly in the anterolateral system

50. Touch receptor/proprioceptor
- Fasciculus Gracilis fibers from sacral, lumbar & lower thoracic ganglion
- Fasciculus Cuneatus contains fibers from Upper thoracic and cervical ganglion ↓
First order neuron
Posterior root ganglion (Cell bodies)
Spinal cord (post. Column. Same side)
Medulla Oblongata (the cuneate & gracile nuclei)
Second order neurons
Internal Arcuate Fibers arise tocross over to form the SENSORY DECUSSATION
Ascends as the MEDIAL LEMNISCUS through the pons & midbrain on the contralateral side
THALAMUS (Ventral Posterolateral nucleus of thalamus)
Third order neurons
Cerebral cortex
(Primary Somatosensory Cortex)

51. The cell body for the first order neuron lies in the Dorsal Root Ganglion. The distal axon innervates the mechanoreceptor while the proximal axon enters the dorsal column of the spinal cord to ascend ipsilaterally to terminate in the nucleus gracilis. For the second order neuron, the cell body lies in the nucleus gracilis and cuneatus of medulla. For the third order neuron, the cell body lies in the VPL of thalamus.

52. Lesion of the DCML at the level of T8 on the left side causes what kind of impairment?
It causes absence of light touch, vibration and position sensation in the left leg and lower left trunk. It is because only gracilis exists below T6 and the tract has not decussated/crossed over as yet. So the impairment is on the same side as the lesion.

53. Lesion of the DCML at the level of C3 on the left side produces what kind of impairment?
It causes absence of light touch, vibration and position sense in the lower and upper trunk and lower and upper limb below the level of the lesion on the left side. This is because both gracilis and cuneatus are present at this level but decussation has not occured.

54. Lesion of the right medial lemniscus produces what impairment?
It causes absence of light touch, vibration and position on the entire left side.

55. Lesion of the right somatosensory cortex or the right internal capsule produces what impairment?
Absence of all sensations including the face on the entire left side.

56. Effects produced by lesions of the posterior white columns leads to:
The lesion produces the following effects on the same side:
Loss of tactile localization and two-point discrimination.
Loss of vibratory sense.
Astereognosis (loss of appreciation of difference in weight and inability to identify objects placed in hand by feeling them)
Position and movement sense is lost leading to impairment in the performance of voluntary motor functions.