CNS – 3

~ Hypothalamus: (LAQ) {connections, functions, and disorders}

Introduction:

Hypothalamus controls the vegetative/visceral functions of the body. It is an integrating center where several neural and endocrine influences originating throughout the brain converge. After due processing, the neural and endocrine outputs of the hypothalamus diverge to all parts of the body.

Hypothalamus consists of a large number of nuclei and also fiber tracts.

~ Connections and functions:

- The main afferent connections of the hypothalamus are with the limbic system and the midbrain tegmentum.

- The main efferents from the hypothalamus are projected to the limbic system, midbrain, thalamus, pituitary, and the medulla.

(1)Regulation of activity of the anterior pituitary gland – The hypothalamus regulates the activity of the anterior

pituitary; it releases the “releasing factors” and “inhibiting factors” for the hormones of the anterior pituitary gland. {For example: the growth hormone (GH) of the anterior

pituitary is controlled by GH.RH and GH.IH secreted by the hypothalamus.}

(2)Regulation of the posterior pituitary hormone secretion – Hormones of the posterior pituitary gland (ADH & oxytocin)

are synthesized in the hypothalamic nuclei: the supra-optic and paraventricular nuclei of the hypothalamus.

Then, via the axons of those nerve cells (of nuclei), the hormones are transported into the posterior pituitary. The hormones are then released by the posterior pituitary into the circulation.

(3)Control of the circadian rhythm – - The body has internal “biological clock”; it follows the 24-

hour cycle. This cycle is circadian rhythm.- Variations in the body functions during daytime and

nighttime are called diurnal variations.- For example: cortisol levels are highest at 6 A.M. (morning),

and lowest at 6 P.M. (evening). Hypothalamus is the link between external environmental

changes and the body’s internal biological rhythm. Visual impulses (about day and night) are transmitted from

the retina into the optic tract (II cranial nerve). The optic nerve gives out collateral fibers that reach the hypothalamus. These fibers synapse in the suprachiasmatic nucleus (SCN) of hypothalamus.

From the SCN, the signals about light and darkness are forwarded to the pineal gland. Pineal gland secretes the hormone melatonin. This secretion increases in the

darkness (or nighttime). Thus, it forms a signal mechanism for body functions to be altered as the darkness sets in.

(4)Regulation of body temperature – In the anterior hypothalamus, there is “preoptic area”.

Neurons in this area are ‘temperature-sensitive’; that is, they can sense the temperature of the blood.

when the body temperature increases;

blood temperature is increased

as the blood circulates and reaches the brain

neurons in the preoptic area of hypothalamus

sense this increased blood temperature

they increase the signals to the cardiovascular center in medulla

blood flow to skin will increase; heat will be lost from skin;

body temperature returns to normal

(5)Regulation of body water and osmolality of body fluids - Osmolality of the plasma (and body fluids) is linked to its water content.Neurons in the hypothalamus can act as “osmoreceptors”; they can detect the osmolality of plasma.If the plasma water is decreased, the plasma will become hyperosmolar. Hypothalamus then regulates it in two ways:

- When the plasma water is decreased, plasma becomes hypertonic. Blood circulates, and reaches the brain.

- Hypothalamic neurons sense this increased osmolality of the plasma. Now, (1) the thirst center in the hypothalamus gets activated; and (2) neurons in the supraoptic nucleus of hypothalamus, projecting onto posterior pituitary, release ADH hormone from the posterior pituitary.

thirst center in hypothalamus release of ADH from

activated posterior pituitary

person drinks water/fluids; ADH acts on kidney tubules

water absorbed from G.I.T. water retention from kidneys

Water absorbed into the blood

;

Water is absorbed into the blood (from GIT and from kidneys); thus water content and osmolality of the body fluids/plasma returns to normal.

(6)Regulation of hunger and feeding – There are two hypothalamic centers concerned with hunger

and feeding:a. Ventromedial nucleus of hypothalamus: It acts as satiety

center. When the neurons of this center fire, it limits the food intake (it gives the feeling of “satiety” at the end of food intake).

b. Lateral hypothalamic area: It acts as hunger center.

Mechanism:

The neurons in the “satiety center” are said to be “glucostats” or “glucoreceptors”; they can sense the blood glucose level.

When the blood glucose level falls, satiety center lifts the inhibition of the hunger center. The person will feel the hunger. When food intake occurs, and glucose level in blood rises, the satiety center becomes active and it inhibits the hunger center.

(7)Cardiovascular regulation –The hypothalamus acts as the relay station; there are corticohypothalamic descending pathways which discharge by emotions. Emotional effects on the CVS are executed by these fibers.

(8)Control of the ANS –

The hypothalamus is called the “head ganglion of the autonomic nervous system (ANS)”. Areas in hypothalamus are said to control sympathetic and parasympathetic nerve responses.

(9)Control of emotional responses –Stimulation of the periventricular nuclei in hypothalamus leads to fear and punishment reactions.

(10) Sexual drive -

Most anterior and most posterior portions of the hypothalamus control the sexual drive.

~ Effects of hypothalamic lesions:

- Lesion of the lateral hypothalamus (which controls hunger): It leads to decreased drinking and eating. There may be lethal starvation. Also, the animal becomes extremely passive.

- Lesion of the ventromedial areas of hypothalamus (which control satiety): It will produce opposite effects. There will be excessive drinking and eating. And, there will be hyperactivity.

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The cerebral cortex (LAQ)

~ Physiologic anatomy:

There are two cerebral hemispheres; they are connected with each other by a bundle of nerve fibers called the corpus callosum.

The most superficial part of each cerebral hemisphere is the “cerebral cortex” (or cerebral grey matter). It is 2 to 4 mm thick. Its total surface area is about 2200 cm2.

Beneath the cerebral cortex lies the subcortical white matter in which embedded are masses of the grey matter. These subcortical masses of grey matter or masses of nuclei mainly include the thalamus, hypothalamus, and the basal ganglia.

The entire cerebral hemisphere is marked by ridges called gyri, and the fissures called sulci.

Because of the major sulci, the entire cerebral hemisphere is divided into 4 lobes:1. Frontal lobe – It lies in front of the central sulcus. Motor

cortex is located here. It is concerned with initiation and control of the voluntary movements of the body.

2. Parietal lobe – It lies between the central sulcus and parieto-occipital sulcus. Somatosensory cortex is located here. It is concerned with perception of general sensations (such as touch, pressure, etc) from all over the body.

3. Temporal lobe – It lies below the lateral sulcus. Auditory cortex is located here. Sound perception occurs here.

4. Occipital lobe - It lies behind the parieto-occipital sulcus. Visual cortex is located here. Visual sense starting from the eyes is perceived here.

~ The typical cortex has 6 layers, numbered I to VI from outside to inside.

In the so-called neocortex (or isocortex), all 6 layers are present. 90% of the cerebral cortex has all 6 layers of cells.

10% of the cortex is “allocortex”. It includes parts of the limbic system. (It does not have all 6 layers.)

In all the lobes, the cortex has two functional divisions: (a) primary cortex, and (b) secondary cortex or association areas.

{Primary cortex in each lobe performs the primary function of that particular cortex; secondary or association areas perform the “analytical” function. For instance, primary sensory areas detect specific sensations – visual, auditory, or somatic – transmitted from periphery. Secondary sensory areas analyze the meanings of the specific sensory signals. Primary motor cortex has direct connections with muscles so as to cause specific muscle contractions. Association or supplementary motor areas provide “patterns” of motor activity.}

(1)Frontal lobe : (mainly the motor areas)- Located in front of the central sulcus; it is mainly concerned

with the motor functions.- It is subdivided into two main areas: (a) precentral cortex,

and (b) prefrontal cortex.(a)Precentral cortex: (immediately in front of the central

sulcus)

Area 4: Primary motor cortex. It is the area for initiation of the voluntary motor activity.

Area 6: Premotor areas. In front of the primary motor cortex; thought/idea of the voluntary movement begins here. It provides the “pattern” for the motor activity.

Area 8: frontal eye field. Situated in front of the area 6. It controls the conjugate movements of the eyeballs to the opposite side.

Area 44: Broca’s area for speech. Situated in the dominant hemisphere; it is the motor area for speech.

(b) Prefrontal cortex/prefrontal lobe: (remainder of the frontal lobe, situated most anteriorly, in front of areas 4,6, & 8)

It is also called orbito-frontal cortex. It has to and fro connections with the thalamus,

hypothalamus, and many other regions of the cerebral cortex.

Area 24: of the cingulate gyrus; is connected with hippocampus. It forms a part of “Papez circuit”; involved in genesis of emotions

Areas 9, 10, 11, & 12: “Seat of intelligence”. Functions of the prefrontal cortex :

Control of some of the higher intellectual activities

Control of personality Control of behavior and social consciousness

(2)Parietal lobe : (somatosensory cortex)- Areas 3, 1, & 2: primary sensory area [S1]. Situated just

behind the central sulcus (in the post central gyrus). It is concerned with appreciation of general sensations

such as touch, pain, temperature, etc. Body representation is upside down. That is,

sensations from the face reach the lower aspect.- Areas 5 & 7: sensory association areas [S2].

It analyzes the sensations perceived by primary sensory areas.

Recognition of the objects placed in the hand, without looking at them – “stereognosis”. This is by analyzing the touch, pressure, texture of the object.

Tactile localization, two-point discrimination, and recognition of spatial relationship are the functions assigned to this cortex.

- Posterior parietal cortex: This area provides continuous analysis of the spatial coordinates of all parts of the body as well as of the surroundings of the body.

(3)Temporal lobe : (auditory cortex)- Areas 41 & 42: in the Heschl’s gyrus. Auditosensory area I &

II. These areas are concerned with perception of sound.- Areas 20, 21, & 22: auditopsychic areas. They are auditory

association areas.- Area planum temporale: large in musicians; recognition of

pitch of the sound- Wernicke’s area: area for language comprehension; behind

the primary auditory cortex.

It is the area where somatic, visual, and auditory association areas meet.

It is also called general interpretive area.(4)Occipital lobe : (visual cortex)

- Area 17: the primary visual cortex; areas 18 & 19: secondary or visual association areas.

- Primary visual cortex perceives an image; visual association areas (visuopsychic areas) interpret the exact meaning of a visual image.

- Area for recognition of faces; area for naming objects. These areas are located in the occipital lobe.

~ Angular gyrus:

Lies in the posterior parietal lobe; it fuses with the visual areas in the occipital lobe.

It is concerned with interpretation of visual information.

Lesion of this area: “dyslexia” or word blindness – The person may be able to see words and even know that they are words, but not be able to interpret their meanings.

~ Limbic lobe: The limbic system –

{Limbus means a ring; the term limbic system is applied to the parts of the cortical and subcortical structures that form a ring around the brain stem.}

- The limbic system consists of the limbic lobe or limbic cortex and the related subcortical nuclei.

- The limbic cortex includes: cingulated gyrus, isthmus, hippocampal gyrus, and uncus.

- The related subcortical nuclei include: (i) amygdala (the group of nuclei on the tip of the temporal lobe), (ii) septal nuclei, (iii) hypothalamus, and (iv) anterior thalamic nuclei.

~ This area was formerly called as ‘rhinencephalon’ because of its relation to olfaction (smell); however, only a small part of it is actually concerned with smell.

~ The limbic cortex is phylogenetically the oldest part of the cerebral cortex. Histologically, it is made up of a primitive type of cortical tissue, called “allocortex”.

~ Functions:

(1)The limbic system represents the primary area for the control of autonomic functions (heart rate, BP, G.I. movements, etc).

(2)It is known to regulate the emotional behavior of an individual; the emotions of rage and fear are elicited on stimulation of the limbic system.

(3)It plays an important role in the motivational drive of the individual.

(4)It is also concerned with olfaction.(5)It plays a role in memory, particularly the long-term memory.

~ Role of the hypothalamus:

- The non-hypothalamic parts of the limbic system receive information from cortical association areas, particularly

those in the frontal lobe. This information is sent directly to the hypothalamus. The emotional meaning of the external stimuli, information gathered from memory and understanding, is thus passed on to the hypothalamus.

- The hypothalamus then integrates the endocrine, autonomic, and some other motor activities that form appropriate emotional behavior.

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Higher functions of the cerebrum

~ The concept of the dominant hemisphere:

As known already, there are two cerebral hemispheres: the left and the right.

In general, higher functions of the nervous system are developed on one side of the brain or one cerebral hemisphere than the other.

Dominant hemisphere or categorical hemisphere:

Higher functions, such as language (speech), are developed in one hemisphere. This hemisphere is called the dominant or categorical hemisphere.

Non-dominant hemisphere or representational hemisphere:

This hemisphere is specialized in “spatio-temporal coordinates”; that is, orientation/relation of body parts with each other, and orientation of the body with respect to the surrounding.

~ About 90% or more of the human population is right-handed. For them, the left cerebral hemisphere is the dominant hemisphere.

The individuals who are left-handed: in about 30% left handed individuals – right hemisphere is dominant; in the remaining 70% - left hemisphere is dominant.

~ Speech: (The language function)

Definition:

Speech is the production of articulate sounds which bear a definite meaning.

The speech requires formation of the proper words and expression of those words verbally.

Speech: It has two components –

1. Proper word formation in the brain. The centers in the brain that cause this are collectively called “central speech apparatus”.

2. Production of the words verbally. This would require respiratory system, vocal cords, and organs of the oral cavity. This is called “peripheral speech apparatus”.

Brain mechanism for speech:

For the proper speech to be executed, following neural mechanisms/centers will be necessary:

a. We must be able to hear sounds. This requires an intact auditory pathway, from the ears to the auditory cortex (area 41).

b. We must be able to understand the sounds/spoken words. This requires the audio-psychic area (or auditory association area; areas 20, 21).

c. We must be able to express the thoughts in spoken words. This process involves the activity of the Wernicke’s area.

Wernicke’s area is located in the superior temporal gyrus; in the dominant (categorical) hemisphere. It is concerned with interpretation and understanding of auditory and visual informations. It is called the “sensory area of speech”. Wernicke’s area integrates the auditory, visual, and other sensory information necessary to form the speech.It then sends the information to another area, the “Broca’s area”.

Broca’s area: It is located in the inferior frontal gyrus, in the dominant (categorical) hemisphere. It processes the information received from Wernick’e area, to form a detailed and coordinated pattern for vocalization (speech). This pattern is then projected to the motor cortex which initiates the appropriate movements of the lips, tongue, and larynx to produce.Thus, Broca’s area is the “motor area of speech”.

~ Peripheral speech apparatus:

The signals initiated by the Broca’s area are sent to the peripheral speech apparatus for execution (that is, production of spoken words).

It involves two processes: (a) phonation – means production of sound, and (b) articulation – means conversion of that sound into the specific words.

Phonation: As the expired air is coming out of lungs, it causes vibrations of the vocal cords. This will produce a sound.

Articulation: The organs of the mouth (lips, tongue, palate) articulate in a specific manner while the sound is coming out; so that the sound is converted into a specific word.

~ Applied physiology:

Aphasia:Defect of speech that results from lesions of the central speech apparatus (speech centers of the brain).Types of aphasias – a. Wernicke’s aphasia :

Spoken or written word can be understood, but inability to interpret the thought that is expressed. It results from a lesion of the Wernicke’s area (superior temporal gyrus).

b. Motor aphasia :The person may be capable of deciding what he or she wants to say but cannot make the vocal system emit the decided words.

~ Neurophysiology of learning & memory: (short note)

Definition:

“Learning” is acquisition of new information.

“Memory” is the retention and storage of that information.

Learning:

Learning is due to “synaptic plasiticty”. That is, there are changes in the synaptic function; the modification of the synaptic transmission is the basis for learning.Learning is mainly of two types – (1) non-associative learning: in this form of learning, the animal learns about a single stimulus, and (2) associative learning: in this form of learning, the animal learns about the relation of one stimulus to another.(1)Non-associative learning :

In this form of learning, the animal learns about a single stimulus. Examples of this type of learning are habituation & sensitization.a. Habituation:

It is a simple form of learning in which a neural stimulus is repeated many times. Initially, it evokes a reaction. However, as the stimulus is repeated, it evokes less and less response.

Eventually, the subject becomes habituated to the stimulus, and learns to ignore it. E.g. ticking of a clock may not allow a person to fall asleep initially. But, later on, the person’s nervous system learns to ignore it so that he/she can sleep.

Mechanism : With repeated stimulation, there will be decreased release of neurotransmitter from the presynaptic terminal.

b. Sensitization: (opposite of habituation) A repeated stimulus produces a greater response if it

is coupled with an unpleasant or a pleasant stimulus.

E.g. the mother who sleeps through many kinds of noise but wakes promptly when her baby cries.

Mechanism : augmented postsynaptic responses.(2)Associative learning :

This type of learning is based on observing repeatedly the association between two events. For example, observing repeatedly that dark clouds are followed by rain makes us learn that dark clouds lead to rain (a ‘cause-effect’ relationship). As a result, the next time we observe dark clouds while leaving home, we carry an umbrella (change in behavior).

This type of learning occurs by the “conditioning” of the animal to paired stimuli.

a. Classical conditioning: (a reflex or passive process)- This type of learning was studied by Pavlov in dogs. {It is also

called Pavlov’s conditioned reflex.}- If a dog is presented with food, there is reflex salivation in

the dog. This is innate reflex, and food is the “unconditioned stimulus” (US).

- Ringing of a bell does not produce salivary secretion normally. In the experiment, a bell is rung and then immediately food is presented to the dog. This is done repeatedly. The dog ‘learns’ to expect food after the ringing of bell. Then, only ringing of bell will cause reflex salivation in the dog. Sound of the bell is called the “conditioned stimulus” (CS). The reflex response to the CS is called “conditioned reflex”.

- Thus, pairing of two stimuli (CS & US) will cause the animal to learn. CS should be immediately followed by the US. If the

CS is presented repeatedly without the US, the conditioned reflex eventually dies out. This is called “extinction” or “internal inhibition”. If there is external stimulus causing disturbance, between CS and US, the conditioned response may not occur. This is called “external inhibition”.

b. Operant conditioning: (animal operates “actively” on the environment)

- It is a form of conditioning in which the animal is taught to perform some task (“operate on the environment”), in order to obtain a reward or avoid punishment.

- The US is the pleasant or unpleasant event. For example, an animal is taught that by pressing a bar it can prevent an electric shock to the feet.

- This type of learning is an “active” form. (compare with classical conditioning which is a reflex or passive process)

Memory :- Memory is retention and storage of the learned information.- It is the ability to recall past events at the conscious or

unconscious level.It is also due to the synaptic plasiticity or modulation of the synaptic transmission.

Declarative or explicit memory –

It involves conscious recall of events.

Non-declarative or implicit memory – (reflexive memory)

It includes classical conditioning, skills, habits; it is generally unconscious.

Types of declarative memory:

Depending on how long a conscious memory lasts, it is divided into following types – (1) short-term memory, (2) intermediate-term memory, and (3) long-term memory.

(1)Short-term or recent memory:- It involves mechanisms that can cause immediate recall of

events that occurred some time ago (seconds/minutes/hours).

- E.g. remembering and recalling a phone number for some time by repeatedly going through the digits.

- Mechanism: POST TETANIC POTENTIATIONIf a particular synapse in the brain is stimulated tetanically (repeated quick successive stimuli for short duration), the transmission at that synapse is enhanced for some time thereafter. This is “post tetanic potentiation”.Repeated quick successive stimuli will cause Ca++ to accumulate in the presynaptic neuron. Neurotransmitter release by this neuron will be greater as long as the Ca++ content in it is high. Hence, the response at this synapse is also potentiated.

(2)Intermediate-term memory:- It may last for several minutes or even upto weeks.- Eventually it will be lost unless converted into long-term

memory.

- Mechanism: (cAMP mediated activation of protein kinase in the neurons)

There is facilitation at the synapses of brain. The postulated neurotransmitter at these synapses is serotonin.

Serotonin acts on its receptors on the presynaptic terminal. This leads to activation of adenylyl cyclase enzyme, and formation of cAMP in the neuron. cAMP activates protein kinase enzyme. This leads to prolonged facilitation of synaptic transmission.

(3)Long-term memory:In this type, the events and information can be recalled even after years.

- Long-term memory is believed to result from actual structural changes at the synapses. Structural and functional changes lead to facilitation of synaptic transmission on a long-term basis.

- Structural changes : Increase in the number of presynaptic terminals Changes in the dendritic geometry Increase in the number of transmitter vesicles

- There is evidence which suggests that activation of genes and new protein synthesis is involved in the processes responsible for memory.

- Molecular or biochemical basis of long-term memory : (long-term potentiation)

The synaptic facilitation that occurs on a long-term basis is postulated to be due to the phenomenon called “long-term potentiation” (LTP) at the synapses.

Role of hippocampus in long-term memory:LTP is known to occur at the synapses in the hippocampus.The neurons projecting from other parts of the brain to the hippocampus release glutamate as the transmitter.Glutamate acts on its receptors, the NMDA receptors. This leads to increased entry of Ca++ in the post- synaptic neurons. There is long-term potentiation of the hippocampal neurons.

- Consolidation and storage of memory:Hippocampus promotes storage of memories.

~ Applied physiology: (AMNESIA)

- Loss of memory is called amnesia.- Bilateral lesion of hippocampus leads to anterograde

amnesia. That is, person is not able to establish new long-term memories after hippocampal lesion because hippocampus is the storage site for memory.

- Retrograde amnesia: Inability to recall memories from the past.

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Electroencephalogram (EEG) and sleep

~ Electroencephalogram (EEG):

The background electrical activity of the brain can be recorded from the scalp. Record of these brain potentials is called electroencephalogram (EEG).

- These potentials are recorded by placing electrodes on the scalp.

- These potentials are NOT ACTION POTENTIALS of the neurons; they are post-synaptic potentials (EPSPs & IPSPs) of the synapses in the brain.

- German psychiatrist Hans Berger introduced the term EEG; hence the waves of the EEG are also called “Berger rhythm”.

Waves of EEG – (1)Alpha waves: (waves of quiet wakefulness)

When an adult human is awake but mind is at rest eyes closed and mind is wandering, his EEG record will mainly show these () waves.

Frequency of these waves: 8 – 12 Hz. These waves are recorded mainly from the parieto-

occipital area.(2)Beta waves: (waves of alert wakefulness)

When the person is awake and mentally alert, with eyes open, his EEG record will mainly show these () waves.

Frequency: 18 – 30 Hz; low amplitude

These waves are generally recorded over the frontal region of the brain.

Paradoxically, these waves are also seen in REM type of sleep.

(3)Theta waves: (large amplitude, slower frequency) These waves are generally recorded in children. Frequency: 4 – 7 Hz; large amplitude They are also known to be generated in the

hippocampus. These waves are seen in the EEG when the person

falls asleep; in stage 2 & 3 of the sleep.(4)Delta waves: (large amplitude, slowest frequency)

These waves have the slowest frequency: less than 4 Hz.

They are recorded in very deep sleep, and in organic brain disease.

These waves do not require activity of the lower brain regions.

~ Gamma oscillations:

- Very high frequency: 30 – 80 Hz- These rapid oscillations in EEG are seen when the individual

is aroused and focuses attention on something.

Changes in EEG pattern at different states of wakefulness and sleep: (note that: when the person is awake, the waves are high frequency and low amplitude. As the person falls asleep, and goes

into deeper sleep, progressively the waves become low frequency and high amplitude.)

Alert wakefulness (beta waves)

Quiet wakefulness (alpha waves)

Stage 1 sleep (low voltage waves)

Stage 2 & 3 sleep (theta waves)

Stage 4 slow wave sleep (delta waves)

REM sleep (beta waves)

~ Sleep:

Definition:

It is the unconsciousness from which the person can be aroused by sensory or other stimuli.

(Compare with coma; the unconsciousness from which the person cannot be aroused.)

Two types of sleep:During each night, a person goes through stages of two types of sleep: (1) slow-wave sleep (the brain waves are very large but very slow), and (2) rapid eye movement sleep (“REM sleep”) (there are rapid eye movements during this type of sleep)o When a person falls asleep, about 90 minutes of slow-wave

sleep (non-REM sleep) and then a bout of REM sleep for 5 to 30 minutes. This pattern will occur throughout the sleep duration.

(1)Slow-wave sleep: (or nREM sleep)- As the person falls asleep, it begins with the nREM sleep.

And, except for the bouts of REM sleep intermittently, the major proportion of the sleep is slow-wave sleep.

- The person goes through 4 stages of slow-wave sleep, which becomes deeper and deeper with these stages. Stage 1: low voltage waves in the EEG Stage 2: sleep spindles and large “K-complexes” are

seen in the EEG Stage 3: theta waves Stage 4: delta waves (slowest waves, deepest sleep)

- Sleepwalking (“somnambulism”), bed-wetting, and nightmares occur during slow-wave sleep.

- Although dreams may occur during slow-wave sleep, there is no consolidation of those dreams in the memory. Hence,

those dreams are not likely to be remembered when the person wakes up.

(2)REM sleep: (or paradoxical sleep)- In a night, bouts of REM sleep (5 to 30 minutes each) occur

every 90 minutes. There may be 5 to 6 episodes of REM sleep every night.

- In young adults, it may occupy 25% of the total sleep duration.

- There are ‘Rapid Eye Movements’ during this type of sleep, hence it is called REM sleep.

- During the episode of REM sleep, the brain is highly active. EEG record of this sleep shows predominance of beta () waves which are the waves of alert wakefulness. Hence, it is called “paradoxical sleep”.

- This type of sleep is associated with active dreaming. There may also be consolidation of dreams into the memory, so that the person can remember the dreams (which occurred during REM sleep). Along with this, there may be tooth-grinding (“bruxism”) in some individuals.

- The muscle tone is throughout the body is depressed.- Heart rate and respiratory rate become irregular.

Probable cause: (genesis of REM sleep)There are cholinergic neurons in the reticular formation of the pons. Discharge of these neurons is thought to initiate the REM sleep.

~ Applied physiology:

- Insomnia: It generally means inability to fall asleep normally. (Sleep latency will be more; there may be intermittent awakening during sleep, and total duration of sleep reduced.)

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Cerebrospinal fluid [CSF] (short note)

Introduction:

It is the fluid circulating around the brain and the spinal cord. It is found the ventricles of the brain, in the cisterns around the outside of the brain, and in the subarachnoid space around the brain and the spinal cord.

Composition of the CSF:

It is a clear, colorless alkaline fluid; specific gravity: 1005 – 1008 Volume: about 150 ml Daily secretion: about 500 ml It is almost cell-free and protein-free It contains less glucose than plasma; (glucose: about 50 mg% in

CSF)

~ Formation, flow, and absorption of CSF:

- About 2/3rd of CSF is formed as a secretion from the choroid plexuses in the ventricles, mainly the two lateral ventricles.

- Additional amounts are secreted by the ependymal surfaces of the ventricles. Some amount is also secreted by blood vessels of the brain and the spinal cord.

- After its formation, it passes from the lateral ventricle into the 3rd ventricle. Some fluid gets added here. Then, along the aqueduct of Sylvius it comes into the 4th ventricle. from here, it passes out via foramina of Luschka and foramen of Magendie, and via cisterna magna it comes into the subarachnoid space. CSF then circulates in the subarachnoid space around the brain and the spinal cord.

- It is mainly absorbed by the subarachnoid villi into the venous (dural) sinuses.

~ CSF pressure:

About 130 mm of water; in lateral lying position

130 mm of water = 13 cm of water

13 cm of water = 10 mm of Hg {1 mm Hg = 1.3 cm of water}

Functions of CSF:1. CSF serves as a fluid buffer that provides optimum environment

to neurons of the CNS.2. Protective function: CSF provides the cushioning effect to the

delicate structures of the cranial vault. 3. Regulates contents of the cranium: CSF acts as a reservoir and

regulates contents of the cranium. For example, if the blood volume of brain increases, then CSF drains away the excess amount of fluid.

4. It helps in transfer of metabolic waste products of brain into the blood.

5. It may serve as a medium for nutrient supply to the CNS.

~ Applied physiology:

- Lumbar puncture:It is the procedure by which CSF can be accessed through the lumbar segments of the spinal cord. A needle is inserted between the two lumbar vertebrae (L2-3), to reach the subarachnoid space. For diagnosis:CSF is collected by lumbar puncture; it is then analyzed for infections of the CNS, or malignancies.For therapeutic purpose:Drugs can be instilled into the CSF; for the purpose of anesthesia or antibiotics against CNS infections