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: Pacinian corpuscle (pressure)

C: Meissner’s corpuscle (touch)

D: Muscle spindle (stretch)

A

B C

D

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 spin

al cord to thalamus– 3rd : projects to somatosensory cortex

sensation

Sensory pathway Spinothalamic pathway(anterolateral syste

m)

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 co

rd through dorsal root 2nd order neuron crosses over in

spinal cord; ascends to thalamus

3rd order neuron projects from thalamus to somatosensory cortex

spinothalamicpathway

Pain , temperature, crude touch and pressure signals

Spinothalamic Pathway

Small sensory fibres:

Pain, temperature, some touch

Primary somatosensory cortex (S1)

Thalamus

Medulla

Spinal cord

Spinothalamic tract

spinothalamic pathwayspinal cord injury

Loss of sense of (superficial sensations):•crude Touch•Pain•Warmth/coldin right leg

Spinothalamic damage

Dorsal column pathway• Function

– Carries fine touch, vibration and conscious proprioception signals

(deep sensations)

• Three-order neuron– 1st order neuron enters spinal co

rd 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 cloumnpathway

fine touch, vibration and conscious proprioception signals

Dorsal column pathwayDorsal column pathway

Large sensory nerves:

Fine Touch, vibration, two-point discrimination, proprioception

Primary somatosensory cortex (S1) in parietal lobe

Thalamus

Medulla

Mediallemniscus

Spinal cord

Dorsal column

Dorsal columnnuclei

Dorsal column pathway• Function

– Sensory nuclei of the trigenimal nerve which subserve the same sensory functions for the head

Dorsal column damage

dorsal column pathway

spinal cord injury

Loss of sense of (deep sensations):•Fine touch•proprioception•vibrationin 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 tend

on 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

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

Ipsilateral Caudal Aspect of the body and legs

Dorsal column damage

• Sensory ataxia (loss of coordination)

• Patient staggers; cannot perceive position or movement of legs

• partially compensated by visual surveillance

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 simila

r effects

anterolateral view

"inner chamber” in Greek

Compositions of thalamus

Lateral-lower part of bodyMedial –upper part

thalamus

2 Associated nuclei

Receive their driving inputs fromOther cortical areas

ventral-lateral nucleus

cerebullum ,globus pallidus—motor Pulvinar nucleusMedial ,lateral geniculate--sensors

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

Projection system of thalamus

Projection system of thalamus

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

Somatosensory cortex

Located in the post-central gyrus of the human cerebral cortex Ⅰ

Sagittal section coronal 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).

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.

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.

Visual cortex Primary visual cortex Secondary visual area

s

bitamporalnasal side

Pain

: Easily localized, occur first

: occur second

nociceptornociceptor

Aδ nerve C nerve

spinothalamicpathway

to reticularformation

Impulses transmitted to spinal cord by– Myelinated Aδ nerves: fast pain (80 m/s)– Unmyelinated C nerves: slow pain (0.4 m/s)

Pain

reticular formation

spinothalamicpathway

thalamus

somato-sensorycortex

Impulses ascend to somatosensory cortex via:– Spinothalamic pathway (fast pain)– Reticular formation (slow pain)---cingulate

gyrus

Viseral sensation: Pain ?

without proprioceptor few thermoceptor

Visceral pain Poorly localized; may be “referred” Slow pain Mostly caused by distension of hollow organ

s 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

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 c

onverged 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