Nervous System

• The basic purpose of nervous system is to co-ordinate all activities of the body.
• It respond & adapts to change.
• The nervous system acts like a two way communicating system. The impulse are carried from the tissue to the nerve centres & from the nerve centers the message are carried to desired tissue.

Nervous System Structure

• Central nervous system (CNS)
  • Brain
    • Forebrain
    • Midbrain
    • Hindbrain
  • Spinal Cord
• Peripheral nervous system (PNS)
  • Cranial nerve (12 pairs)
  • Spinal nerve (31 pairs)
  • Somatic N.S
  • Autonomic N.S
    • Sympathetic (*Under Stress condition)
    • Parasympathetic (*Under normal condition)

Central nervous System:

• It consist of brain + spinal cord having billions of interacting neurons which acts like a "Central telephone exchange".
• It coordinate, integrate & control various nervous activities of the body.
• It is covered with three membranous layer called meninges. It is protective layers.

So, meninges consist of:

[Diagram showing: Skull bone -> Dura mater -> Subdural space (with CSF) -> Arachnoid mater -> Subarachnoid space (with CSF) -> Pia mater -> Brain surface]

1. Duramatter: (dura = hard)
   • It makes the outermost covering of the brain located just beneath cranial cavity. It is very hard tough & thickest membranous layer.
   • It is made up of fibrous connective tissue.
2. Arachnoid matter: (archnoid = web-like)
   • It is located in betⁿ duramater & piamater. It is a thin, delicate, transparent almost fine web-like structure.
3. Piamatter: (Pia = tender)
   • It is the innermost layer. It is a thin, tender very delicate & highly vascular. This layer remains closely attached to the surface of the brain.

• The space beneath the duramater & above the arachnoid is known as subdural space.
• The space between arachnoid & piamater is called subarachnoid space.
• All the spaces are filled with cerebrospinal fluid (CSF).

Cerebrospinal fluid (CSF):

It is a clear, watery fluid containing glucose, enzyme & salt which surrounds the brain as well as spinal cord. It is also present in the ventricles & the central canal of spinal cord. It is about 90-150 ml in adult.

Functions of CSF:

1. It protects the brain & spinal cord.
2. It formed spongy cushion in between the cranium & the brain. Thus it acts as shock absorbant.
3. It keeps the brain & spinal cord moistened.
4. It provide essential nutrient & oxygen to the brain tissue.
5. It removes all the unwanted excretory waste product from the brain.
6. It maintains a constant pressure inside the cranium in spite of fluctuation in the volume & pressure of blood in the cranial vessels.

The CNS has two distinct regions: white matter & grey matter.

1. white matter:
   • It looks white in colour due to the presence of fatty myelin sheath around the nerve fibre.
   • In the spinal cord, white matter forms the outer layer while the grey matter forms the central core.
2. Grey matter:
   • It looks grey in colour. It consist of cell bodies, dendrites & synapses of neurons.
   • In brain, grey matter is situated on the surface while the white is located deeper.

Brain:

• Brain is the master organ of the body which controls & co-ordinates almost all the system of the body.
• It is the anterior upper part of the CNS & is made up of soft nervous tissue called neurons (100 billion).
• The brain is located & protected inside the head in the bony skull cavity is known as cranium or cranial cavity or brain box.
• Brain weight about 1200 to 1400 gm in average adult.
• The brain remains covered by meninges.

Structure of brain:

The human brain is divisible into three main parts:

1. Forebrain or Prosencephalon: It is further divided into olfactory lobes, cerebrum & diencephalon.
2. Midbrain or Mesencephalon: It consist of optic quadrigemina or optic lobes & cerebral peduncle (crura cerebri).
3. Hind brain or Rhombencephalon: It consist of Cerebellum, Pons Varolii, Medulla oblongata.

I) Forebrain: It consist of:

1. Olfactory lobes: There is a pair of olfactory lobes. Each lobe is solid bulb like structure. There is no ventricles inside it. Each lobe is connected with stalk called olfactory tract to the olfactory region in the temporal lobe. This is present just above the optic chiasma. It helps in detection of smell.
2. Cerebrum: (cerebral hemisphere)

   [Diagram of brain showing central fissure, cortex, medulla, corpus callosum (made up of myelinated axon) & connections]

   [Lobes diagram: Frontal Lobe (Anterior), Parietal, Occipital, Temporal lobe, Central fissure, Lateral or Silvian fissure, Parieto-occipital fissure]

   [Lateral view diagram: Fore frontal lobe, Temporal lobe, Parietal lobe, Occipital lobe, Cerebellum, Pons, Medulla, Brain stem]

   • It is longest part of brain. It consist of two cerebral hemisphere (right & left) joined together by a broad curved thick band of nerves fibres called Corpus callosum.
   • Each cerebral hemisphere is divided into four lobes: frontal, parietal, temporal & occipital lobe by three deep & wide fissures.
   • The surface area of each hemisphere is increased by numerous folds called gyri. The gyri are separated by depression called sulci.
   • The cerebral hemisphere have an outer region of densely packed nerve cells called cerebral cortex. It forms the grey matter of the brain & inner medulla is white matter.
   • The Central sulcus separates the frontal lobe from the parietal lobe. The lateral sulcus divide the frontal lobe from the temporal lobe.
   • The parieto-occipital sulcus separates parietal lobe from the occipital lobe.
   • The cerebral hemisphere are hollow cavity & is known as ventricles of cerebral.
   code Code

   Functional areas of Cerebrum:

   1. Frontal lobe: It has motor area to control the voluntary movement of skeletal muscles & seat of learning.
   2. Parietal lobe: It has sensory area for the perception of stimuli of heat, cold, light, touch, pain & also for reading, writing & mathematical calculation.
   3. Temporal lobe: Primary auditory cortex receives stimulus of hearing. The olfactory cortex for smell & the hippocampal cortex for long-term memory.
   4. Occipital lobe: The visual cortex receive images detected by eye & interpret it (visualization).
3. Diencephalon:
   • It is the part of the forebrain that is located betⁿ cerebrum & the midbrain, just below the corpus callosum. Its main part are epithalamus, thalamus & hypothalamus.
   • i) Epithalamus: It consist of a narrow band of nerve tissue in the roof of third ventricle. Remaining attached with it is highly vascular area, the anterior choroid plexus which secretes CSF.
   • ii) Thalamus: It is a region present at the centre of the forebrain. It is composed of mass of grey matter. It lies in cerebral medulla & acts as relay center (collect stimuli & pass to cerebral hemisphere). It receives the impulse of pain, temp^v, touch etc.
   • iii) Hypothalamus: It is located below the thalamus in the floor of 3rd ventricle (Diocoel). The pituitary is attached to the hypothalamus by stalk, the infundibulum. A pair of small rounded structure lie behind the infundibulum called mammilary gland.
     • So, hypothalamus acts as connection between nervous & endocrine system.
     • It controls thrust, hunger, temp^v regulation, sleep, salivation, sweating, BP etc.
     • It secrets neurohormones also called releasing hormones that stimuli pituitary gland.

[Internal Diagram showing: Corpus callosum, Anterior choroid plexus, CSF, Pituitary, Medulla oblongata, Pons, Infundibulum, Hypothalamus, Thalamus, Cerebellum, Mamillary body]

Diencephalon Summary:

1. Epithalamus - Roof
2. Thalamus - They acts as relay center (Fore brain, Hind brain, Cerebellum)
3. Hypothalamus - Pons, Medulla oblongata

Midbrain: Optic lobes, Cerebral Peduncle

II. Midbrain (Mesencephalon):

• It is located in betⁿ the diencephalon & hind brain.
• It is made up of two parts:

1. Optic lobe or optic quadrigemina: It is made up of four little lobes which are in the form of round swellings. All these four lobes are divisible into two pairs: the upper pair - the Superior colliculi & the lower pair - the inferior colliculi.
   • Functions: They are the center for Vision.
2. Cerebral peduncle or Crura cerebri: These are two peduncle like bundle of nerve fibres lying on the ventral surface of mid brain.
   • Function: They help in coordination betⁿ forebrain & hind brain.

III. Hind brain:

It is located in between the brain & Spinal cord. It is the posterior part of the brain. It consist of cerebellum on the dorsal side & brainstem (pons varolii + medulla oblongata) on the ventral side.

1. Cerebellum: It is the second largest part of brain. It has two cerebral hemisphere just like Cerebrum. It also has grey matter on outer surface called cerebellar cortex & white matter in the centre the cerebellar medulla.

   Function:

   1. Maintain posture & equilibrium.
   2. It controls rapid muscular action such as running, talking, typing etc.
2. Pons Varolii: It lies just above the medulla & acts as a bridge carrying ascending & descending tract between brain & spinal cord. It is oval & paired structure.
   • It also send impulse to cerebral cortex & cerebellum.
   • It has many nuclei. It help in controlling involuntary function as it has breathing centre.
3. Medulla Oblongata: It is the posterior most part of brain & continues into the Spinal Cord. The roof of medulla is thin & highly vascular which forms posterior choroid plexus that secretes CSF.

   Functions:

   1. It controls involuntary function of the body through a number of centers like Cardiac centre (heartbeat), respiratory centre (respiration), vasomotor center (contraction of blood vessels), salivary center etc.
   2. It also control coughing, sneezing, vomiting, urinating, BP, peristalsis, swallowing of food etc.

Brain Stem:

It includes Diencephalon, midbrain, pons & medulla.

Ventricles of human brain:

[Diagram of ventricles: Left ventricle (I), Right ventricle (II), Cerebrum, Foramen of monro, Third ventricle (Diocoel), Midbrain, Iter of sylvius or aqueduct of sylvius, Fourth ventricle (IV), Cerebellum, Pons varolii, Medulla oblongata, Central canal of Spinal chord]

• The cavities of different part of brain is called ventricles. There are four irregular shaped cavities or ventricle within the brain containing Cerebro Spinal fluid.

B) Spinal cord:

• It is a tubular or cylindrical structure in the neural canal of vertebral column. It extends from the lower end of medulla oblongata ie 1st cervical vertebrae to 1st lumbar vertebrae.
• It is about 45cm long & about 35 gm in weight.
• The spinal is protected by meninges & bone of vertebral column.
• Spinal Cord give rise to 31 pairs of spinal nerves.

T.S of Spinal cord:

[Diagram showing: Dorsal septum, Dorsal commissure, Central canal (lined by Ciliated Columnar cells also called Ependyma), Dorsal funiculus, grey matter, white matter, Ventral commissure, Ventral horn, Lateral funiculus, Ventral funiculus, Ventral fissure, Ventral root (efferent or motor)]

• Histologically, the spinal cord is made up of two types of nervous tissue grey matter & white matter. Just reverse of what is present in brain (grey on surface, white in inner). Here, white matter is on periphery the grey matter is internal forming H shaped (butterfly shaped) central area with a small central canal.
• This canal is continuous with the 4th ventricle of brain & is filled with cerebrospinal fluid.

function:

1. It controls reflex action of the body.
2. It conduct sensory & motor impulse to and from the brain.

Peripheral Nervous System (PNS):

The spinal cord is the nervous tissue connects betⁿ the brain & the rest of the body. PNS consist of three components:

1. Spinal nerve - 31 pairs
2. Cranial nerve - 12 pairs
3. The autonomic part of the nervous system

Each nerve is made up of bundle of numerous nerve fibre. It may be motor, sensory or mixed nerve depending on type of fibre it contains. The nerve are protected by 3 layers called epineurium, perineurium & endoneurium.

1. Spinal nerves: The spinal nerves 31 pair arise from both side of the spinal cord. The motor & sensory nerve root unite to form spinal nerve.

   There are five group of spinal nerves named according to the vertebrae they are associated with:

   1. Cervical Spinal nerves (C1 to C8) -> 8 Upper limb
   2. Thoracic " (T1 to T12) -> 12 Trunk
   3. Lumbar " (L1 to L5) -> 5 lower limb
   4. Sacral " (S1 to S5) -> 5 pelvis
   5. Coccygeal " (Co1) -> 1

   The lumbar, sacral & coccygeal nerves leaves the cord near its terminal & gives the appearance of horse tail (cauda equina).

2. Cranial nerves: (Mnemonic: CO³ T² AF AGUSH)
   • The cranial nerves arises from the ventral side of brain.
   • These are 12 pair of cranial nerves in mammals.

Autonomic Nervous System (ANS):

• The autonomic nervous system is autofunctioning "self governed".
• This system controls & coordinates the involuntary activities of various organ such as heart beat etc.
• It consist of two neurons connected through a ganglion. The preganglion neuron arises before the ganglion.

Functionally ANS has two parts:

1. Sympathetic
2. Parasympathetic

1. Sympathetic Nervous System:
   • It mobilizes the body for fight of flight. It prepares the body to deal with excitement & stressful situation. Its function is similar to adrenaline & non-adrenalin hormone of supra-adrenal gland.
   • It originates in the thoracic & the lumbar region of spinal cord. Preganglionic fibres are short. Postganglionic fibres are long. Nor-adrenaline is produced at terminal end of postganglionic fibre. Also called adrenergic effect. It controls the body in emergencies.
2. Parasympathetic Nervous System:
   • It originates in the cranial region of brain & sacral region of spinal cord. Preganglionic fibres are long whereas star postganglionic fibres are short. Acetylcholine is produced at the terminal end of postganglionic fibres & also called cholinergic fibres.
   • The effect of parasympathetic is of a peace maker & tries to slow down ie bring back to normal condition (inhibitory effect).

Nerve Impulse & its Conduction:

Stimulus:

• It is a sudden change in the environment (external or internal) which is strong enough to excite the nerve or muscle.
• The weak electrochemical change is the nerve impulse.

Characteristics of nerve fibres:

1. Excitability: It is an inherent property of the nerve fibres which arises in receptors as a response to stimuli. This is transmitted along the nerve fibres as a nerve impulse. So nerve cells are excitable because their membrane are polarised.
2. Conductivity: The transmission of excitation along the nerve fibre is called conductivity. It is in the form of action potential.
3. Threshold stimulus: A nerve fibre also need a minimum strength of stimulus called threshold stimulus.
4. All or none law: There is always a threshold value necessary to stimulate nerve fibre. This is called all or none law. So when it respond it gives a maximum action potential.
5. Saltatory conduction: When an impulse travel along a myelinated neuron depolarization occurs only at the nodes. So, impulse jump over myelin sheath from one node to other node of Ranvier. This process is called saltatory (saltere = to leap) conduction.

Mechanism of Conduction of Nerve Impulse:

It is the most accepted mechanism of nerve impulse conduction which was proposed by Hodgkin & Huxley (Ionic theory). This theory state that nerve impulse is an electrochemical event caused by differencial permeability of neurilemma to Na⁺ & K⁺ ion which is in turn is regulated by the electric field across the membrane.

According to this theory the process of nerve impulse conduction is divisible into 3 main phases:

1. Polarization: (Resting Potential):
   • It is the potential that exist in an axon at rest - ie when it is not stimulated.
   • Resting potential refers to the potential differences that exist between the inside & outside of the axon membrane. It is about -70mV.
   • At rest, ie when an axon is not stimulated the inside of the membrane is negatively charged & the outside is positively charged.
   • The extracellular fluid has high concentration of Na⁺ whereas axoplasm - ie intracellular fluid has high concentration of K⁺.
   • Neurons has special ion channel called voltage gated ion channels. At polarized condition Na⁺ + K⁺ channels are closed.
   • [Diagram: Axon membrane showing +++ outside and --- inside in axoplasm ICF, resting membrane]
2. Depolarization (Action Potential):
   • When a stimulus is applied to the nerve a disturbance occurs at a point of stimulation to bring changes in the potential.
   • The permeability of the membrane changes & becomes more permeable to Na⁺ ions.
   • The gates of Na⁺ ion channel opens. Thus Na⁺ ions diffuse from ECF to inside & the number of positive ion inside the axon increases than outside.
   • The membrane potential changes from -70mV to +40mV in nerve fibre. This potential difference betⁿ ECF and ICF is called charge potential.
   • This change in potential is called as action potential & the membrane is called depolarized.
3. Repolarization:
   • At the peak of action potential, the gates of Na⁺ ion channel close & so the membrane becomes less permeable to Na⁺ & more permeable to K⁺.
   • The K⁺ ion channel gates open & K⁺ ion diffuses out of axon.
   • As a result sodium-potassium pump operates in the membrane which transport sodium ion out & potassium ion against the concentration gradient by the help of carrier molecule. It involves energy (ATP) & called active transport.
   • Thus 3 Na⁺ ion are expelled out & 2 K⁺ ion comes inside the axoplasm. Such pump are called electrogenic (sodium-potassium pump).
   • The nerve fibre regains the original polarity ie outside electropositive due to Na⁺. Thus membrane is said to be polarized & has resting potential of -70mV.
   • The whole process of depolarization & repolarization is very fast. It takes just 1-5 milliseconds.

Synapse:

• It is a junction betⁿ the axon terminal of one neuron & the dendrite of next neuron. It is separated by a small gap known as synaptic cleft.
• There are 2 types of synapses:
  1. Electrical synapses
  2. Chemical synapses

1. Electrical Synapses transmission: At the electrical synapses the membrane of pre & post synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across three synapses. So transmission of an impulse is very similar to impulse conduction along a single axon.
2. Chemical synaptic transmission:
   • The membrane of the pre-post synaptic neurons are separated by a fluid filled space called synaptic cleft.
   • Neurotransmitter are involved in the transmission of impulses. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane & release their neurotransmitter in synaptic cleft by exocytosis.
   • The released neurotransmitter bind to their specific receptors present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in post synaptic neurons.
   • Once the neurotransmitter has acted upon the target cell the rest is immediately broken down by an enzyme found in post-synaptic membrane (acetylcholine).

[Diagram of Synapse: Myelin sheath, Axon, Synaptic vesicles, pre-synaptic membrane, Neurotransmitter, Post synaptic membrane, post-synaptic cell, Receptor molecules, Voltage-gated Ca⁺⁺ channel]

Q) What do you call the conduction of impulse along a myelinated nerve fibre?

1. Along a myelinated nerve fibre, the conduction of impulse is called saltatory conduction.
2. This is so because the ionic changes & consequent depolarization takes place only at the nodes of Ranvier free from myelin sheath leading to the jumping of action potential from one node to the next.

[Diagram: Myelinated fibre showing Active node with +/- exchange and Inactive node under myelin sheath]