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Classification of thalamic nuclei

  1. Non-specific nuclei - These are the midline and intralaminar nuclei . these project diffusely and non-specifically to the whole of the neocortex. These nuclei receive input from the reticular formation (the ARAS). Impulses from the nuclei are responsible for the diffuse secondary response ( EEG) . The alerting effects of reticular activation, are relayed through them.
  2. Specific nuclei
  • Sensory relay nuclei
  • Medial geniculate body (concerned with hearing)
  • Lateral geniculate body (concerned with vision)
  • Ventro posterior lateral and ventro-posterior medial :
  • These nuclei are the sites of termination of the ascending somatic afferent tracts the medial lemniscus (carrying sensation from all parts except the face) ends in the ventroposterior lateral ; the trigeminal leminscus (carrying sensations from face and taste sensation ) ends in the ventroposterior medial nucleus
  • Nuclei concerned with control of posture and movement
  • Ventrolateral nucleus -This is the chief motor nucleus of the thalamus. It receives fibres from the cerebellum (the dentato – thalamic fibres cf cerebellum). Fibres from the ventrolateral nucleus project to the primary motor cortex (area 4) and pre- motor cortex (area 6)
  • Ventro anterior nucleus -It receives fibres from the cerebellum and basal ganglia. It projects to the premotor cortex.


Nuclei concerned with visceral efferent control mechanism


a) Fornix

Cingulate gyrus (area 24)

The above circuit is called the papez circuit


b) Lateral dorsal nucleus -Its connections are similar to anterior thalamic nuclei.


c) Dorsal – medial nucleus


Thalamic nuclei serving integrative and perceptual function



1.  Pulvinar and lateral posterior nucleus:

(Which is intercalated between the somatic, visual and auditory cortex)

Reticular nucleus

Functions of thalamus

These can be predicted from the connections mentioned above.

  • It is a great sensory relay station it is an important relay station for all sensory systems except smell
  • Motor function it is also a relay station for the motor fibres from the basal ganglia and cerebellum on their way to the cerebral cortex
  • It also relays a part of the ARAS


Hans Berger made the first systematic analysis of EEG:

1.  Waves in the EEG


Delta, theta, alpha, beta (from delta to beta , the frequency increases and the amplitude decreases)

  1. Salient features of the different waves

Alpha rhythm -this is the wave seen at rest , with the eyes closed and the mind wandering .Its frequency is 8-12 /second and the amplitude is 50-100 v . it is most marked in the parieto – occipital lobe. The frequency of the alpha rhythm is decreased by:

  1. low blood glucose
  2. low body temperature
  3. low level of adrenal glucocorticoid hormones
  4. high Paco2

Alpha Block

(also called arousal response / alerting response desynchronisation)

  • The alpha rhythm can be made to disappear by focused attention, by mental concentration and sensory stimulation
  • (the ascending reticular formation activity is responsible for the EEG alerting response ; stimulation of ARAS causes the EEG rhythm to change from slow to high frequency small waves )
  • Beta rhythm -it has a frequency of 18-30 / second. It is seen over the frontal region. It is the wave seen when the eyes are opened; It is the wave seen in the alerting response
  • Theta rhythm-frequency is 4-7/second. It has large amplitude. It occurs in children
  • Delta rhythm -frequency is < 4/second

Sources of EEG

  • It is due to the constantly shifting / fluctuating dipole between the dendrites of the cortical cells and the cell bodies


Evoked cortical potential

This is an EEG change produced by some form of stimulation

Evoked cortical potential consist of:


  1. Primary evoked potential: This consist of positive and a small negative wave. It has a latency of 5-12 ms. It is highly specific in its location (over the primary receiving area of the cortex for the particular sensory pathway that has been stimulated to evoke). It is produced by conduction of sensory signal through specific sensory pathway.
  2. Diffuse secondary response

This consists of a larger, more prolonged positive deflection. It has a latency of 20-80 ms and may last 30 seconds. Its activity can be recorded form most of the cerebral cortex. It is produced due to spread of impulses through the ARAS to cerebral cortex.



NREM sleep (slow-wave sleep) - 4 stages

REM sleep



Stage 1 of NREM

Low amplitude, high frequency EEG activity

Stage 2 of NREM

Sleep spindles (alpha – like 10-14 /seconds, 50 v

amplitude), K complexes

Stage 3 of NREM

Low frequency, increased amplitude

Stage 4 of NREM

Maximum slowing, (least frequency ), large waves

(rhythmic slow waves, synchronized )


Rapid, low voltage EEG


All the stages of sleep are reversible except the stage from REM to awake state     

Duration / percentage of various stages:

  1. Percentage of REM out of total sleeping time
  2. Premature infants =80%
  3. Full term neonates =50%
  4. 20-65 years = 25%
  5. After 65 years, it decrease (in elderly = 15%)

Genesis of sleep

  1. REM
The mechanism that triggers REM sleep is located in the pontine reticular formation. PGO spikes originate in the lateral pontine tegmentum. The spikes are due to discharge of cholinergic neurons.
  1. NREM
    NREM (non-rapid eye movement) sleep is dreamless sleep. During NREM, the brain waves on the electroencephalographic (EEG) recording are typically slow and of high voltage, the breathing and heart rate are slow and regular, the blood pressure is low, and the sleeper is relatively still.

Stimulation of certain sleep zones can produce sleep

  1. Diencephalic sleep zone
    (In the posterior hypothalamus and the near by intralaminar and anterior thalamic nuclei) for producing sleep it requires low frequency stimulation
  2. Medullary synchronizing zone
    (In the reticular formation of the medulla oblongata at the level of the nucleus of the tractus solitarius)
    This also requires low frequency stimulation for producing sleep
  3. Basal forebrain sleep zone (includes preoptic area and the diagonal band of Broca)
    This can produce sleep by both low as well as high frequency stimulation.

Control of posture and movement

Posture control

Stretch reflex is fundamental to posture control. The major factor in posture control is by a change in the threshold of the stretch reflex. This can be achieved:

1. Directly by a change in the excitability of the motor neuron

Indirectly by change in the rate of discharge of gamma efferent nerve to muscle spindle

  • There are 6 supraspinal influences on the stretch reflex viz:
  1. Cortex =inhibits gamma motor neuron
  2. Basal ganglia=inhibits gamma motor neuron
  3. Cerebellum= inhibits gamma motor neuron
  4. Reticular formation (facilitatory area tonic ally active)= stimulates gamma motor neuron
  5. Reticular formation (inhibitory area)= inhibits gamma motor neuron
  6. Vestibular nuclei = stimulates alpha motor neuron
  7. Stretch reflex can be made hyperactive either by increasing gamma stimulation or by increasing alpha stimulation

1.  Transection at various levels helps to find out the role of each structure

  1. Spinal Components-(only the spinal cord intact , transection above the spinal cord) there is initially a period of spinal shock during which all spinal reflexes are profound depressed
  2. Features after the spinal shock is over

Hyperactive stretch reflexes

Presence of positive supporting reaction (magnet reaction)and negative supporting reaction

When suitably stimulated, spinal animals can even produce walking movements indicating the presence of Loco- motor pattern generators . However the spinal locomotor pattern generator needs to be turned on by the mesencephalic locomotor region.

Automatic reflexes

i. Reflex contraction of the urinary bladder and rectum can occur

ii. BP is generally normal at rest but there are wide swings

iii. Bouts of sweating and blanching of the skin occur

Sexual responses
Erection and ejaculation is possible by local stimulation

Mass reflex -Irradiation from one reflex center to another can occur. Minor stimulation of skin can cause evacuation of the urinary bladder rectum, sweating, pallor, BP swings, in addition to withdrawal response


Medullary Components

Transection at the superior border of the pons causes decerbration. It leaves the following components intact, the spinal cord, medulla , pons and cerebellum

1.  Features

  1. No stage of spinal shock decerebrate rigidity immediately occurs. The reason for this is increased general excitability of the motor neuron pool
  2. Increased discharge of gamma motor neurons (because the inhibitory influence of the cerebral cortex and basal ganglia on the gamma motor neuron is removed)
  3. (Although the cerebellum also has an inhibitory influence on gamma motor neuron,  removal of the cerebellum in humans causes hypotonia )
  4. The decerebrate rigidity in animals is most marked in the antigravity muscle. However, in humans the pattern of decerebrate rigidity is extensor in all the 4 limbs
  5. (note that in decorticate rigidity, there is extensor rigidity in the legs and moderate flexion in arms )
  6. tonic labyrinthine and tonic neck reflexes are present: these reflexes are responsible for the change in the pattern of rigidity with change in the position. They are not righting reflexes
  7. righting reflexes are absent


Mid Brain Components

  •  Mid Brain Components Here, the transection is made at the superior border of the mid brain

1.  Features

  1. extensor rigidity is seen only when the animal lies quietly on its back
  2. the animal can rise to standing position walk , and right itself
  3. righting reflexes are present. All the righting reflexes (except the optical righting reflex, which is cortical) are
  4. integrated at the mid brain (the rightly reflexes operate to maintain the normal standing position and keep an
  5. animals head upright)
  6. grasp reflex present
  7. pupillary light reflex present (if the optic nerves or intact)
  8. nystagmus present
  9. vestibular placing reaction present


Cortical Components

Decortication in many species of animals causes little motor deficiency.In primates , the deficit is more severe but movement in still possible


1.  Features

  1. because the hypothalamus is present , temperature regulation and other visceral and homeostatic functions are present
  2. inability to react in terms of past experience
  3. Decorticate rigidity: this is because of loss of cortical inhibition of gamma motor neuron
  4. As in mid brain , the rigidity is present only when the animals is at rest
  5. Hopping and placing reaction absent

2.  Postural reflexes


Integrated in

Antigravity reflexes, attitudinal reflexes, (ie. Tonic labyrinthine & tonic neck reflexes)



Med brain, thalamus

Righting reflexes (except optical righting reflex)

Mid brain

Optical righting reflex


Visuo spinal reflex

Mid brain

Conditioned reflex



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