Neuro physiology

Sensory receptorsSomatic sensation 1 and 2References : Guyton and Hall chapters 46, 47, 48 Done by : Dr. Reham Al-Dwairi

• Types of Sensory Receptors and the Sensory stimuli they detect

• there are five basic types of sensory receptors:• (1) mechanoreceptors: which detect mechanical compression or

stretching of the receptor or of tissues adjacent to the receptor.• 2) thermoreceptors: detect change in temperature.• (3) Nociceptors pain receptors : detect physical and chemical damage to

the tissues.• ( 4) electromagnetic receptors : detect light in the retina.• (5) chemoreceptor's: detect taste, smell, oxygen level in the arterial

blood ,…ets.

• Differential sensitivity of receptors: mean that is each type of receptor is highly sensitive to one type of stimulus for which it is designed.• What ever the type of stimulus that excites the receptor its

immediate effect is to change the membrane electrical potential we call this( receptor potential).• Mechanisms of receptor potential: all receptor acts By opening of ion channels either by stretching the

receptor membrane, or by chemicals , temperature , light.Maximum amplitude of most sensory receptor potentials is about 100

millivolts.•

• The frequency of repetitive action potentials transmitted from sensory receptors increase approximately in proportion to the increase in receptor potential.• This allows the receptor to have an extreme range of response , from

very weak to very intense.• Adaptation of receptors: All receptor responds at high impulse rate at

first and then at a progressively slower rate until finally the rate of action potentials decreased to very few or often to none at all.• All mechanoreceptors adapt completely within seconds or more.

Baroreceptors need hours or days to do so ( adaptation time for many carotid and aortic Baroreceptors is about 2 days) called nonadapting.• Chemoreceptor's and pain receptors never adapt completely

• Slowly adapting receptors (tonic receptors): detect continuous stimulus strength can continue to transmit information for many hours.

• Rapidly adapting receptors (rate, movement, or phasic receptors) detect change of stimulus strength , they react rapidly while the change is actually taking place.

• Nerve fibers come in all size between 0,5-20 micrometers in diameter. • The larger the diameter the greater the conducting velocity.• The range of conducting velocities is between 0,5 -120m/sec.• Types of nerve fibers:• Type A large myelinated fibers subdivided into α, β, δ and γ Conduct

impulses at fast velocities.• Type C fiber small unmyelinated conduct impulses at low velocities

• Transmission of signals of different intensity in nerve tracts in two ways: 1. Spatial summation: by increasing signal strength using greater

numbers of fibers

2. Temporal summation: increasing the strength by increasing the frequency of nerve impulses in each fiber.

• The central nervous system is composed of 100s-1000s or even millions of neuronal pools, each pool has its own special characteristics of organization that cause it to process signals in its own special way.

• We should know that discharge of a single exitatory presynaptic terminal almost never causes an action potential in a postsynaptic neuron.

• We need a large number of input terminals must discharge on the same neuron either simultaneously or in rapid succession to cause excitation.• When the stimulus is enough to

cause excitation of a neuron called excitatory stimulus or suprathreshold.• When the stimulus is not

enough to cause excitation in a neuron called subthreshold stimulus and the neuron facilitated neuron.

• Discharge zone /Excitatory zone / liminal zone.• Facilitated zone/ subliminal zone / subthreshold zone

• Divergance of signals passing through neuronal pools:• Two major type • Divergence in same tract ( ex.

corticospinal pathway in its control to the skeletal muscles).• Divergence into multiple tracts

( information transmitted in the dorsal columns of the spinal cord takes two course in the lower part of the brain

a) into the cerebellum. b)through the lower regions of the brain

in to the thalamus and cerebral cortex.

• Convergence of signals:• Signals from multiple inputs

uniting to excite a single neuron this provide enough spatial summation.• Ex: 1. peripheral nerve fiber

entering the cord.• 2. corticospinal fibers from the

cerebral cortex.• 3. Propriospinal fibers passing

from one segment of the cord to another.

reciprocal inhibition circuit :• Some times an incoming signal

to a neuronal pool causes an output excitatory signal going in one direction and at the same time an inhibitory signal going elsewhere.• This type of circuits is for

controlling the antagonistic pairs of muscles (help in preventing over activity in many parts of the brain).

Reverberatory (oscillatory) circuits as a cause of signal prolongation: caused by positive feedback to re-excite the input of the same circuit.• all part of the brain is connected either directly or in directly mean that

an excitation in one part will re-excite the other and like so…this will lead to continues cycle of re-excitation the nervous system prevent this from happening by two ways:

1. inhibitory circuits. 2. fatigue of synapses. (mean simply that synaptic transmition becomes

progressively weaker the more prolonged and more intense the period of excitation.

Means of stabilizing the nervous system• Automatic short-term adjustment of pathway sensitivity by the fatigue

mechanism.• Long term changes in synaptic sensitivity caused by automatic

downgrading or upgrading of synaptic receptors.

Somatic sensation 1:general organization ,the tactile and position senses

• Somatic senses are the nervous mechanisms that collect sensory information from the body associated with special senses (vision, smell, hearing, taste and equilibrium)• Classified into three physiologic types: 1.Mechanoreceptive somatic senses respond to( tactile and position 2.thermoreceptive sense respond to ( cold and heat). 3. pain sense respond to( tissue damage).

• Other classification of somatic sensation: 1. exteroreceptive sensation on the (surface of the body). 2. proprioceptive sensations to detect (physical state of the body). 3. visceral sensation in ( internal organs). 4. deep sensation in ( fasciae, muscle and bone)..

Tactile receptors are six type: 1. free nerve endings, 2. meissners corpuscle . 3. meissners corpuscle with expanded tip tactile receptors. 4. hair end-organ. 5.ruffinis end-organs. 6. pacinian corpuscles

• Transmission of tactile sensation:All the specialized sensory receptor transmits their signal in type A β

nerve fiber with velocity from 30-70 m/sec. Free nerve ending tactile receptors transmitted by small type A δ

myelinated fiber velocity of 5-30 m/sec. Some tactile free nerve ending transmitted by C unmyelinated fibers

velocity from a fraction of meter up to 2 m/sec.All the different tactile receptor involve in detection of vibration (rapidly

repetitive sensory signals.• Tickling and itch:Transmitted by very small type C unmyelinated fiber (similar to those

transmits aching pain, slow type of pain).

Sensory path ways for transmitting somatic signals into the central nervous system:

Anterolateral system Dorsal column-medial lemniscal system

Small myelinated fiber few meter per second up to 40 m/sec

Large myelinated fiber velocity 30-110 m/sec

Signals interring the spinal cord from the dorsal spinal root synapse in the dorsal horn of gray matter then cross to the opposite side throw the lateral white column then terminate at all level of the brain stem and in

the thalamus

Carries signals in the dorsal columns of the cord then signals synapse cross to the opposite side in the medulla then pass up word throw the brain stem to the thalamus

• Somatosensory cortex:Cerebral cortex divided into 50 areas called brodmanns areas based on

histological structural differences.The large central fissure (central sulcus) extend horizontally across the

brain, signals from all modalities of sensation terminate in the cerebral cortex posterior to the central fissure and anterior half of the parietal lobe is concerned with reception and interpretation of somatosensory signals.

EX.. Visual signals terminate in the occipital lobe. Auditory signals in the temporal lobe.

• Somatosensory area I has a high degree of localization of the different area of the body, while its poorly localized in somatosensory area II.• Each side of the cortex receive sensory information exclusively from

the opposite side of the body (with the exception of a very small amount of sensory information from the same side of the face).

• Bilateral excision of somatosensory area I lead to loss of following types of sensory judgment:

1. The person is unable to localize discretely the different sensation in the different parts of the body.

2. The person is unable to judge critical degree of pressure against the body.

3. The person is unable to judge the weight of objects.4. The person is unable to judge shapes or forms of objects

(astereognosis).5. The persons is unable to judge texture of materials.

• Somatosensory association area : Brodmanns area 5 and 7 of the cerebral cortex, play important role in

deciphering the sensory information that enter the somatosensory area.

Effect of removing the somatosensory associations area (Amorphosynthesis):

The person is loses the ability to recognize complex object and complex form by the process of felling them on the opposite side of the body. The person is mainly oblivious to the opposite side of the body-that is forget that its there.

• Position sense (Proprioceptive sense)1. Static position sense.2. Dynamic proprioceptionPosition sensory receptors depend on knowing the degree of angulations

of all joints in all planes and their rate of changes, both skin tactile receptor and deep receptors near the joint are used.

For determining joint angulations in midranges of motion, the most important receptor are the muscle spindles.

Processing of position sense information in the dorsal column-medial lemniscal pathway.

• Transmission of less critical sensory signal in the Anterolateral pathway.

1. the velocity of transmission are only 1/3 – 1/2 those in the dorsal column-medial lemniscal system .

2. The degree of spatial localization of signals is poor.3. The gradation of intensities are less accurate.4. The ability to transmute rapidly changing or rapidly repetitive

signals is poor.

• Function of the thalamus in somatic sensation:When the somatosensory cortex of human being is destroyed, that

person losses most critical tactile sensibilities.Loss of somatosensory cortex has little effect on ones reception of pain

sensation and only moderate effect on the perception of temperature (brain stem and thalamus and other associated basal regions of the brain play dominant roles in discrimination of these sensibilities.

• Each spinal nerve innervate segmental field of the skin called a dermatome .

Somatic sensation2 : pain , headache, and thermal sensationThe pain is mainly protective mechanism for the body, it causes the

individual to react to remove the pain stimulus:Classified into fast and slow pain:

slow (burning, aching, throbbing, nauseous and chronic) pain

Fast (sharp, priking, acute and electrical) pain

Associated with tissue destruction lead to prolonged unbearable suffering (skin and any deep tissue or organ)

Ex: needle stuck, cut with a knife and electric shock

Transmitted by C fiber velocity 0.5-2 m/sec Transmitted in small type A δ fibers velocity 6-30 m/sec

Poorly localized Much more exactly to localized the pain

• Pain receptors are all free nerve endings excited by mechanical, thermal and chemical stimuli.

Mechanical and thermal stimuli excite fast pain. while chemicals like bradykinin, serotonin, histamine, potassium ions,

acids, acetylcholine and substance p excite slow pain receptors.The pain receptor adapt very little and sometimes not at all

(nonadapting nature).Tissue ischemia as a cause of pain related to increase metabolism in a

tissue with blocked blood supply. muscle spasm as a cause of pain due to stimulating mechanosensitive

pain receptors.

• Dual pain pathway in the cord and brain stem:1. Neospinothalamic tract for fast pain.2. Paleospinothalamic pathway for slow chronic pain.Pain impulses entering the brain stem reticular formation, the thalamus

and other lower center can cause conscious perception of pain.Cortex plays an important roles in interpreting the quality of pain.

• Pain suppression (analgesia) system in the brain and spinal cord:Injection of minute quantities of morphine either into preiventricular

nucleus around the third ventricle or into the periaqueductal gray area of the brain stem causes extreme degree of analgesia.

Morphine like agent (opiates acts at many other point in analgesia system including the dorsal horns of the spinal cord.

More important of the opiate substances are β-endorphine, met-enkephaline, leu-enkephaline, and dynorphine.

• Inhibition of pain transmission by tactile sensory signals, for example:Relief pain by acupuncture and rubbing the skin near painful area are

often effective in relieving pain.Treatment of pain by electrical stimulation. stimulating electrodes are placed on selected areas of the skin so

patient can personally control the degree of stimulation

• Referred pain:Normally person feels pain in a part of his body that is considerably

remote from the tissue causing the pain.Referred pain is the pain usually initiated in one of the visceral organs

and referred to an area on the body surface.Visceral pain:All the true visceral pain that originates in the thoracic and abdominal

cavities is transmitted by C type nerve fiber transmitted only the chronic-aching-suffering type of pain.

• Causes of true visceral pain: 1. Ischemia: due to formation of acidic metabolic end products or tissue-

degenerative products. 2. chemical stimuli: gastric juice leak from rupture gastric and duodenal ulcer

cause usually sever pain. 3. Spasm of a hollow viscus. 4. Overdistention of a hollow viscus. extreme overfilling lead to pain due to

stretching.Insensitive viscera: parenchyma of liver and the alveoli of the lungs.While liver capsule is extremely sensitive to both direct trauma and stretch and

the bile ducts also sensitive to pain.In the lung both bronchi and the parietal pleura are very sensitive to pain.

• Pain from the parietal wall overlying a viscus is sharp.• Any pain that originate internally can be localized only generally.• Visceral pain transmitted via sensory fibers in the autonomic nerves

and the sensation are referred to surface area of the body• Parietal sensation are conducted directly into the local spinal nerves

from the parietal peritoneum, pleura, or pericardium usually localized directly over the painful area.• Person generally localized the pain to the dermatomal segment from

which the visceral organ originated in the embryo .• Heart C-3 and T-5 so pain from the heart is referred to the side of the

neck over the shoulder , pectoral muscle, down the arm and into the substernal area of the upper chest

• Stomach originate from T-7 to T-9 pain from it referred to the anterior epigastrium above the umbilicus.

• Some clinical abnormalities of pain and other somatic sensation: 1. Hyperalgesia: pain pathway sometimes become excessively excitable. caused by: a. excessive sensitivity of the pain receptor themselves called

( primary Hyperalgesia). b. facilitation of sensory transmissions called ( secondary

Hyperalgesia ). 2. herpes zoster (shingles): herpes virus cause infection in the dorsal

root ganglia lead to pain in the dermatomal segment normally sub served by the ganglion lead to segmental type of pain.

3. tic douloureux: occurs in some people over one side of the face in the sensory distribution area of 5th and 9th cranial nerve( trigeminal or glossophryngeal neuralgia).

Pain felt as sudden electrical shock for few seconds or some time as a continues pain.

the pain is usually blocked by surgically cutting the peripheral nerve from the hypersensitive area this operation leave the side of the face anesthetic which is annoying, some times the operation is unsuccessful indicating that the lesion is in the sensory nucleus in the brain stem itself.

headache headache are type of pain referred to the surface of the head from

deep head structures.Intracranial headache:The brain itself is totally insensitive to pain.Tugging on the venous sinuses around the brain, damaging the

tentorium or stretching the dura at the base of the brain can lead to headache.

stimulation of pain receptors in the cerebral vault above the tentorium lead to pain in the front half of the head in the surface areas supplied by the 5th cranial nerve.

Nerve impulses beneath that tentorium inter the central nervous system through the glosspharyngeal, vagal and 2nd cervical nerves lead to pain in the posterior part of the head (occipital headache).

Types of intracranial headache:1. Headache of meningitis.2. Headache caused by low CSF. 3. Migraine headache (2 theories) explain its causes.

a. abnormal vascular phenomena.b. excess local potassium in the cerebral extracellular fluid.

Prodromal sensation proceed headache such as nausea, loss of vision, visual aura and sensory hallucinations, started 30 mins - 1 hs. before headache.

4. Alcoholic headache5 headache caused by constipation .

Extracranial types of headache:

1. Headache resulting from muscles spasm (neck muscle & scalp)2. Headache caused by irritation of the nasal and accessory nasal

structures.3. Headache caused by eye disorders.

Thermal sensation

• The human being can perceive different gradations of cold and heat from freezing cold to cold to cool to indifferent to worm to hot to burning hot. • Three types of receptors : cold heat and pain receptors.• Cold receptors are small type A sigma myelinated nerve ending with

velocities about 20 m/sec.• Warmth signal transmitted mainly over c fibers with velocities of 0,4-2

m/sec.• Freezing cold and burning hot sensation feel almost alike.

• Mechanism of stimulation of the thermal receptors:• Receptors stimulated by changes in metabolic rate, temperature lead

to alter the rate of intracellular chemical reactions more than two folds, for each 10 cc changes.• Thermal sensation transmitted in pathways parallel to those for pain

signals. • It is known that removal of the post central gyros in the human being

reduces but does not abolish the ability to distinguish gradation of temperature.

The end Thank you