Questions -  Answers 

Very Short Answer Type Questions
1. Name the cranial meninges covering the brain of man.
A: a. Outer duramater
     b. Middle arachnoid mater
     c. Inner piamater
 

2. What is corpus callosum?
A: a. The two cerebral hemispheres are internally connected by a transversely arranged bundle of myelenated nerve fibres (beneath the cortex), called corpus callosum.
     b. It brings coordination of left and right cerebral hemispheres.
 

3. What do you know about arbor vitae?
A: The white mater of cerebellum is like a branching tree and hence, it is called arbor vitae (tree of life).
 

4. Why sympathetic division is called thoracolumbar division?
A: As the preganglionic neurons of sympathetic division arise from thoracic and lumbar regions of the spinal cord, it is called thoraco lumbar division.
 

5. Why parasympathetic division is called craniosacral division?
A: As the cell bodies of preganglionic neurons of parasympathetic division lie in brain and sacral region of spinal cord, it is called cranio sacral division.
 

6. Distinguish between absolute and relative refractory periods.
A: a. During absolute refractory period, even a strong stimulus can not initiate a second action potential (coincides with depolarisation and repolarisation.)
     b. The relative refractory period is the time during which a second action potential can be initiated by a longer than normal stimulus. (coincides hyper polarisation.)
 

7. What is all or none principle?
A: If the level of depolarisation is less than the threshold potential, the membrane potential will normally drop back to resting levels without further consequences. When depolarisation is just equal to or above the threshold potential, the action potential of equal amplitude is initiated. It means the nerve impulse is either conducted totally or not conducted at all. It is called all or none principle.
 

Short Answer Type Questions
1. Draw a labelled diagram of T S of spinal cord of man. 

A:
 
 

2. Distinguish between sympathetic and parasympathetic neural systems.  

A:
 

3. Give an account of synaptic transmission.

A: The place at which nerve impulse is relayed from the axon terminal of a neuron to the dendrite of adjacent neuron is known as synapse. It is formed by pre synaptic membrane (of axon terminal) and post synaptic membrane (of dendrite). Pre and post synaptic membranes are separated by a narrow gap known as synaptic cleft synapses are of two types, viz., electrical synapses and chemical synapses.
A. In electrical synapse, the pre and post synaptic membranes are in close proximity. They have electrically conductive links called gap junctions. Impulse transmission across the electrical synapse is much faster when compared to that of chemical synapse.
B. In chemical synapse, the pre and post synaptic membranes are separated by a synaptic cleft. In these synapses, neurotransmitters (chemicals) are involved in impulse transmission. The axon terminals contain synaptic vesicles (filled with neuro transmitters) and mitochondria besides some other cell organells. 
 i. When the action potential reaches the axon terminal, it depolarises the presynaptic membrane. Thus voltage gated channels of calcium open. Ca++ ions stimulate the synaptic vesicles towards the presynaptic membrane where they fuse with it. Thus the neurotransmitter is released into the synaptic cleft.
These neuro transmitter bind to the specific receptors of post synaptic membrane.
 ii. The most common neurotransmitter is the acetyl choline. Epinephrine, norepinephrine, serotonin, dopamine etc., are either excitatory or inhibitory neurotransmitters. Glycine, Gamma Amino Butyric Acid (GABA) are inhibitory neurotransmitters.
 iii. The post synaptic membrane has ligand gated channel (They are ion channels and respond to chemical signals - ligand). The entry of ions can generate a new action potential in the post synaptic membrane - which may be excitatory or inhibitory.
 iv. Excitatory post synaptic potentials (EPSP) cause depolarisation and inhibitory post synaptic potentials (IPSP) cause hyperpolarisation in post synaptic membrane.
 v. Then the neurotransmitter (e.g.: acetyl choline) is degraded by an enzyme (e.g.: acetyl choline esterase).
 vi. The post synaptic potentials are graded potentials and summation of these potentials occur at the axon hillocks.
 

Long Answer Type Questions
1. Give an account of structure and functions of brain of man.
A: In man, brain is lodged and protected in a bony box in the skull called cranium. Brain is covered by three membranes called meninges, namely, outer duramater (double layered), middle arachnoid layer and inner piamater (thin and attached to outer surface of brain). Duramater and arachnoid layer are separated by sub dural space and arachnoid layer and piamater are separated by sub arachnoid space. Sub dural space is filled with cerebro spinal fluid (alkaline and colour less). It acts as a shock absorbing medium. Brain is divided into three parts, namely, fore brain (Prosencephalon), mid brain (Mesencephalon) and hind brain (Rhombencephalon).
A) Prosencephalon: It is the anterior and the largest part of the brain and is formed by olfactory bulbs, cerebrum and diencephalon.
a. Olfactory bulbs: In the anterior part of brain is a pair of olfactory bulbs. They receive sensory impulses of smell from olfactory epithelium. 
b. Cerebrum: It is the largest part of the brain and is divided into two cerebral hemispheres (left and right) by a deep longitudinal fissure. Beneath the cortex, left and right cerebral hemispheres are connected by a transeversly arranged bundle of myelenated nerve fibres called corpus callosum/ colossal commissure. It co - ordinates the left and right cerebral hemispheres. The outer part of cerebrum is formed by grey matter called cortex and inner part by white matter called medulla. Cortex contains nerve cell bodies medulla contains myelinated axons. Cerebral cortex contains many folds (gyri) among which fissures (deep grooves) and sulci (shallow grooves) are present. Cortex has three functional areas.
I. Sensory areas - Receive and interpret sensory impulses.
II. Motor areas - Control voluntery muscular movements.
III. Association areas - Deal with integrative functions (such as memory and communications)
      Each cerebral hemisphere is divided into four lobes- Frontal, parietal, temporal and occipital lobes.
c. Diencephalon/ Thalamencephalon: It has an epithalamus, thalamus and a hypothalamus.
i. Epithalamus is non nervous. It is fused with piamater and form the anterior choroid plexus. Behind the anterior choroid plexus, is a pineal stalk, which bears a pineal body at its tip. 
ii. Thalamus acts as a coordinating centre for sensory and motor signaling.
iii. Hypothalamus ventrally has a hallow out growth called infundibulum. It bears the pituitary gland.
       Infundibulum also contains several neurosecretory cells. Hypothalamus acts as an osmoregulatory, thermoregulatory, hunger, thirst and satiety centre.
B) Mesencephalon: It lies between the thalamus and pons varoli. Ventrally, it contains a pair of longitudinal bands of nervous tissue called crura cerebri. They connect the fore 
brain and hind brain. Dorsally, the mid brain has two pairs of spherical lobes called corpora quadrigemina (four optic lobes). The anterior pair is called superior colliculi (concerned with vision) and the posterior pair is known as interior colliculi (concerned with hearing).

C) Rhombencephalon: It is formed by cerebellum, pons and medulla oblongata.
a. Cerebellum is the second largest part of the brain and is composed of a median vermis and two lateral cerebellar lobes. Each cerebellar lobe consists of an anterior, a posterior and a floccular lobe. The inner white matter of cerebellum spreads like a branching tree. It is called arbor vitae. Cerebellum (the little brain/ gyroscope of the body) control and coordinate locomotor movements and maintaining equilibrium (Damage to cerebellum results in ataxia).
b. Pons varoli: Beneath the mid brain, just in front of cerebellum and above the medulla oblongata, pons varoli is present. It is in the form of transervese bridge of nerve fibres connecting the two cerebellar lobes. It acts as a relay centre between cerebellum, spinal cord and the remaining part of brain. It contains a pneumotoxic centre (controls respiratory muscles).
c. Medulla oblongata: It is the posterior part of the brain and is continued as spinal cord. Its roof is thin and vascular and is called posterior choroid plexus. Medulla oblongata consists of cardio vascular and respiratory centres and the centres of swallowing, cough, vomiting, sneezing, hiccuping.
Cavities of brain: Cavities of brain are known as ventricles. The cavities of right and left ventricles are known as 1st and 2nd ventricles respectively. They are also called paracoels or lateral ventricles. Diencephalon contains the third ventricle or diacoel and medulla oblongata contains the fourth ventricle or myelocoel. The two lateral ventricles open into the diacoel through separate openings called foramina of Manro. Diacoel and myelocoel are connected by a narrow tube called iter or ductus sylvius. 4th ventricle is continuous with central canal of spinal cord. All the ventricles are filled with cerebrospinal fluid. It was flushed four times in a day to remove metabolities and toxins.

2. Explain the transmission of nerve impulse through a nerve fibre with the help of suitable diagrams.
A: The signal that travels along a nerve fibre and hence in the release of neuro transmitters is called a nerve impulse. Neurons conduct nerve impulses (action potentionals) because membrane potential is established across the neuronal membrane. Ions move in and out through axolemma of a neuron through channels - such as:
i. Leakage Channels - (Na⁺, K⁺): K⁺ leakage channels are more than Na⁺ leakage channels.
ii. Ligand - Gagted Channels - Located in post synaptic membrane and open or close in response to chemical stimuli.
iii. Voltage Gated Channels - Open in response to a change in membrane potential. They are Na⁺ and K⁺ voltage gated channels. Na⁺ channels are of 2 types (Activation channel, Inactivation channel).

Resting Membrane Potential
The membrane potential of a neuron that is not transmitting signals is called the resting membrane potential. It depends on the ionic gradients that exit across the axolemma (membrane) and the differential permeability of axolemma. At the resting potential voltage gated channels of Na⁺ or in resting stage. Voltage gated channels of K⁺ are closed. As the axolemma has more K⁺ leakage channels than
Na⁺, the cell is roughly hundred times more permeable to K⁺ than Na⁺. Consequently, the resting membrane potential is about - 70 mV on the inner side of axolemma. At this phase, the axolemma (membrane) is said to be polarised. If the inner side becomes less negative, it is said to be depolarised. If the inner side becomes more negative, it is said to be hyperpolaraised.
   
Action potential
The momentary change (about 1 - 2 milli seconds) in electrical potential on the membrane (axolemma) of a neuron that occurs when it is stimulated, resulting in the transmission of an electrical impulse, is termed as action potential.

   
A stimulus depolarises the axolemma when it is depolarised, the voltage gated channels of K⁺ open more slowly than those of Na⁺. As a result, action potential begins to increase in membrane permeability to Na⁺, which depolarises the membrane, followed by an increased permeability to K⁺, which repolarises the membrane. 
 

i. Depolarising (resting) phase: When a nerve fibre is stimulated when the depolarisation reaches the threshold level (-55 mV), voltage gated channels of Na⁺ (both activation and inactivation channels) and voltage gated channels of K⁺ close. Due to rapid influx of Na⁺, the membrane potential shoots rapidly upto + 45 mV (spike potential).
        If the level of depolarisation is less than the threshold potential, the membrane potential will normally drop back to resting levels without further consequences.
When depolarisation is just equal to or above threshold potential, the action potential of equal amplitude is initiated. It means the nerve impulse is either conducted totally or not conducted at all. This is called all or none principle. 

ii. Repolarising (falling) phase: Once the membrane potential has risen to peak, the voltage gated channels of Na⁺ are inactivated and voltage gated channels of K⁺ open. Efflux of K⁺ repolarises the membrane (axolemma).

iii. Hyperpolarization or undershoot: During this phase, membrane potential briefly becomes even more negative (-90 mV) that it normally is at rest. This is called hyperpolarization. This occurs because of increased permeability of K⁺ that exit while voltage gated K⁺ channels remain open. Na⁺ channels come to resting state. When the membrane potential returns to normal ( -70 mV), the K⁺ channels will close.
 

Refractory periods
Few milli seconds after initiation of an action potential, the neuron can not generate another action potential to a normal threshold stimulus. This brief period is called refractory period.
i. Absolute refractory period - During this period, even a very strong stimulus can not initiate a second action potential. This period is equal to periods of depolarization and repolarization.
ii. Relative refractory period - The time during which a second action potential can be initiated by a larger than normal stimulus. It is equal to the period of hyper polarization.

Speed of conduction
    The speed of conduction is directly proportional to the diameter of the axon. In myelinated neurons, the voltage gated channels of Na⁺ and K⁺ are concentrated at nodes of Ranvier. As a result the impulse jumps from one node to another. Hence it is called jumping or saltatory transmission. In non myelenated nerve fibres the conduction is continuous.

Sodium - Potassium pump:

The Na+ and K⁺ ions diffuse in and out across the axolemma (membrane). If this diffusion is unchecked, the resting membrane potential may be disturbed. These flows of ions are effect by sodium - potassium pumps located in the walls of axons. These pumps expel three Na⁺ ions for every two K⁺ ions imported.