The sensory function of brain Nervous system Ⅲ The sensory function of brain

Senses

Five basic types of sensory receptors Mechanoreceptors Thermoreceptors Nociceptors(pain receptors): physical or chemical damage Electromagnetic receptors:light on the retina of the eye chemoreceptors

Sensory receptor A: Free nerve endings (pain, temperature) B C D A: Free nerve endings (pain, temperature) B: Pacinian corpuscle (pressure) C: Meissner’s corpuscle (touch) D: Muscle spindle (stretch)

sensation

sensation Somatic senses Mechanoreceptive somatic senses Stimulated by mechanical compression or stretching of some tissue of the body Tactile sense Touch , pressure, vibration , tickle senses position sense Static position and rate of movement senses Thermoreceptive senses Detect heat and cold Pain senses

sensation Sensory pathway: 1st : enters spinal cord from periphery 2nd : crosses over (decussates), ascends in spinal cord to thalamus 3rd : projects to somatosensory cortex

oblongata

Sensory pathway Spinothalamic pathway(anterolateral system) Dorsal column pathway (dorsal column– medial lemniscal system) Spinocerebellar pathway

Spinothalamic pathway Function Carries pain, temperature, crude touch and pressure signals (superficial sensations) Three-order neuron 1st order neuron enters spinal cord through dorsal root 2nd order neuron crosses over in spinal cord; ascends to thalamus 3rd order neuron projects from thalamus to somatosensory cortex

spinothalamic pathway Pain , temperature, crude touch and pressure signals

Spinothalamic Pathway Primary somatosensory cortex (S1) Thalamus Medulla Small sensory fibres: Pain, temperature, some touch Spinothalamic tract Spinal cord

Spinothalamic damage spinothalamic pathway spinal cord injury Loss of sense of (superficial sensations): crude Touch Pain Warmth/cold in right leg

Dorsal column pathway Function Three-order neuron Carries fine touch, vibration and conscious proprioception signals (deep sensations) Three-order neuron 1st order neuron enters spinal cord through dorsal root; ascends to medulla (brain stem) 2nd order neuron crosses over in medulla; ascends to thalamus 3rd order neuron projects to somatosensory cortex

dorsal cloumn pathway fine touch, vibration and conscious proprioception signals

Dorsal column pathway Large sensory nerves: Primary somatosensory cortex (S1) in parietal lobe Dorsal column nuclei Thalamus Medulla Medial lemniscus Dorsal column Large sensory nerves: Fine Touch, vibration, two-point discrimination, proprioception Spinal cord

Dorsal column pathway Function Sensory nuclei of the trigenimal nerve which subserve the same sensory functions for the head

Dorsal column damage dorsal column spinal cord injury pathway Loss of sense of (deep sensations): Fine touch proprioception vibration in left leg

Central Pathways

Differences between the two system Velocities The dorsal column–medial lemniscal system: large, myelinated nerve fibers 30 to 110 m/sec, The anterolateral system :smaller myelinated fibers a few meters per second up to 40 m/sec.

Differences between the two system spatial orientation : in spinal cord the dorsal column–medial lemniscal system :high degree of with respect to their origin the anterolateral system : much less spatial orientation lower parts of the body---center of the cord higher parts---lateral layers

Spinocerebellar pathway Function : receives inputs from golgi tendon organs and muscle stretch receptors. proprioception signals Receptors : muscles & joints Three-order neuron 1st order neuron: enters spinal cord through dorsal root 2nd order neuron: ascends to cerebellum 3rd order neuron to cortex The ventral spinocerebellar tract conveys proprioceptive information from the body to the cerebellum. It is part of the somatosensory system and runs in parallel with the dorsal spinocerebellar tract. Both these tracts involve two neurons. The ventral spinocerebellar tract will cross to the opposite side of the body then cross again to end in the cerebellum (referred to as a "double cross"), as compared to the dorsal spinocerebellar tract, which does not decussate, or cross sides, at all through its path.

Spinocerebellar pathway Division Peripheral Process of First Order the Neuron Region of Innervation dorsal (posterior) spinocerebellar tract from muscle spindle (primarily) and golgi tendon organs Ipsilateral Caudal Aspect of the body and legs ventral (anterior) spinocerebellar tract from golgi tendon organs The ventral spinocerebellar tract conveys proprioceptive information from the body to the cerebellum. It is part of the somatosensory system and runs in parallel with the dorsal spinocerebellar tract. Both these tracts involve two neurons. The ventral spinocerebellar tract will cross to the opposite side of the body then cross again to end in the cerebellum (referred to as a "double cross"), as compared to the dorsal spinocerebellar tract, which does not decussate, or cross sides, at all through its path.

Dorsal column damage Sensory ataxia (loss of coordination) Patient staggers; cannot perceive position or movement of legs partially compensated by visual surveillance ataxia [英] [ə‘tæksiə] 共济失调

Spinocerebellar tract damage Cerebellar ataxia a failure of the fine coordination of muscle movements Clumsy movements Incoordination of the limbs (intention tremor) Wide-based, reeling gait (ataxia) Alcoholic intoxication produces similar effects reeling gait 步态蹒跚

"inner chamber” in Greek thalamus anterolateral view "inner chamber” in Greek

Compositions of thalamus Geniculate [英] [dʒiˈnikjulit] ventral posterior thalamus pulinar

Lateral-lower part of body Medial –upper part

thalamus 2 Associated nuclei Receive their driving inputs from Other cortical areas ventral-lateral nucleus cerebullum ,globus pallidus—motor Pulvinar nucleus Medial ,lateral geniculate--sensors ventral lateral pulvinar nucleus

thalamus 3 nonspecific projection nucleus “nonspecific nuclei”connect to association areas of cortex and/or limbic structure. Anterior nuclei Medial nuclei Intralaminar and reticular nuclei limbic structure intralaminar reticular nonspecific projection nuclei: Dispersion in projection

Projection system of thalamus Discrete fenlide

Projection system of thalamus Reticular formation

Projection system of thalamus Function Specific projection system of thalamus Specific senses: visual or auditory cortex (except olfaction) Non-specific projection system of thalamus Maintain and alter the excitatory situation of cortex

Sensory areas of cerebral cortex postcentral gyrus

Somatosensory cortex Located in the post-central gyrus of the human cerebral cortex Ⅰ gyrus [英] [ˈdʒaiərəs] coronal section Sagittal section From side

Somatotopic map of the somato-sensory cortex

Somatosensory cortex Cross projection Each side of the cortex receives sensory information exclusively from the opposite side of the body (the exception: the sensor information from face is bilateral projection). exclusively

Somatosensory cortex The finer the sense, the larger the somato-sensory cortex area The lips, face and thumb are represented by large areas in the somatic cortex, whereas the trunk and lower part of the body, relatively small area. exclusively

Somatosensory cortex Inversely projection “Up is down and down is up ” The head in the most lateral portion, and the lower body is presented medially. exclusively

Proprioception sensory area

Visual cortex Primary visual cortex Secondary visual areas

bitamporal nasal side chiasma [英] [kaiˈæzmə] bitamporal

Pain : Easily localized, occur first : occur second

Pain Impulses transmitted to spinal cord by Myelinated Aδ nerves: fast pain (80 m/s) Unmyelinated C nerves: slow pain (0.4 m/s) spinothalamic pathway to reticular formation Aδ nerve C nerve nociceptor nociceptor

Impulses ascend to somatosensory cortex via: thalamus spinothalamic pathway reticular formation Impulses ascend to somatosensory cortex via: Spinothalamic pathway (fast pain) Reticular formation (slow pain)---cingulate gyrus

Viseral sensation: Pain ? without proprioceptor few thermoceptor

Spasm of a hollow viscus

Visceral pain Poorly localized; may be “referred” Slow pain Mostly caused by distension of hollow organs or ischemia (localized mechanical trauma may be painless)

Visceral pain Agina: due to ischemia of the heart muscle, generally due to obstruction or spasm of the coronary arteries.

Refered pain is pain perceived at a location other than the site of the painful stimulus Site of pain may be distant from organ umbilicus

Visceral pain myocardial infarction :  (heart attack), where pain is often felt in the neck, shoulders, and back rather than in the chest, the site of the injury. 

Mechanism of refered pain Convergence theory The primary afferent axons of skin and viscera converged on the same interneurons in the pain pathways.

Mechanism of refered pain Convergence theory This theory explains why referred pain is believed to be segmented in much the same way as the spinal cord.  Fail to explain why there is a delay between the onset of referred pain after local pain stimulation.  Threshold for the local pain stimulation and the referred pain stimulation are different, but according to this model they should both be the same.

Mechanism of referred pain Facilitated theory The primary afferent axons of skin and viscera closed to each other

Mechanism of referred pain Facilitated theory to explain why there is a delay between the onset of referred pain after local pain stimulation.

Summary Spinothalamic pathway Dorsal column pathway Specific projection system Non-specific projection system Sensory area of cerebral cortex Referred pain