Nervous system or nervous co-ordination

Nervous system or nervous co-ordination

The system which co-ordinate, regulate and controlled all other systems is called nervous system. Functional Unit of nervous system is neuron or nerve cell. Nervous system of higher animal performs three functions these are Receiving, Processing or Modulating and Responding.

Function of Nervous system.

1. The stimuli or impulses are carried to CNS from muscles and vice versa.
2. Co-ordinates various organs system.
3. It stimulates and inhibits the activities of muscles, glands and viscera.
4. It helps to maintain the homeostatic condition.

Parts of nervous system

1. Central nervous system (CNS)-It includes brain and spinal cord.
2. Peripheral nervous system (PNS)-It includes the nerves arising from CNS.
3. Autonomic nervous system (ANS)-It includes nerves and ganglia extending up to visceral organs. It works involuntarily.

1. Central nervous system

CNS consist of two parts-

1. Brain

Brain is highly specialized delicate organ located in skull or cranium. It is about 1.4 kg. Brain is covered by three layers known as meninges.

                          I.      Outer layer is duramater-It is just located below the skull. It is highly vascular tough white fibrous tissue. It supports the brain spinal cord.

                       II.      Middle layer is arachnoid-It is the middle, delicate and thin fibrous covering below the subdural cavity.

                     III.      Inner layer piamater-It is thin delicate and highly vascular innermost layer. It is separated from arachnoid membrane by a subarachnoid space. The space is filled with spongy connective tissue and cerebrospinal fluid (CSF).

Cerebrospinal fluid (CSF)

It is secreted from anterior and posterior choroids plexus. It performs following functions.

• Act as cushion shock absorber.
• Act as medium for exchange of different materials through the blood vessels.
• Keep the brain and spinal cord moist.
• It makes the brain weight less.

Structure of human brain

Human brain is divided into three parts;

1.      Fore brain(prosencephalon)

2.      Mid brain(Mesocephalon)

3.      Hind-brain(Rhombencephalon)

Fig: mid-sagittal section of human brain

Forebrain is distinguished in to 3 parts-

1. Olfactory lobes
2. Cerebrum
3. Diencephalon

1. Olfactory lobes have anterior olfactory bulb and posterior olfactory tract. These parts are fully covered by cerebral hemisphere and ventral view can be seen. Cavity of lobes is called 1^st ventricle. Olfactory lobes are poorly developed in human. Function of olfactory is to receive the sense of smell.

Fig: Lobes of cerebrum

fig: Olfactory lobes

2. Cerebrum is more complex and large part of brain. It is divided in to left and right hemisphere. Both hemispheres are connected by nerve fibers called corpus callosum. Outer layer of cerebrum is known as cerebral cortex formed of convolutions. Raised parts are called gyri and depressions are called sulci. Each hemispheres can be divided in to- frontal parietal, temporal and occipital lobes by sulci.

There are different areas on the cerebrum these are-Motor area for movement, sensory area for heat, cold, light, pressure, touch etc auditory area for hearing, visual area for seeing olfactory area for taste and smell, taste area and smell area. Each hemisphere receives information from opposite side of the body. Cavity of each cerebral hemis- sphere is called lateral ventricles.

Functions-

a)      It controls all mental and conscious activities (such as intelligence, memory, reason, will, feelings and emotions).

b)      It is the site of originator of voluntary acts and interpreter of sensations.

c)      It is a control center of reflex actions.

3. Diencephalon lies just below the corpus callosum and the mid brain. It has three parts-epithalamus, thalamus and hypothalamus. Epithalamus is anterior part and folded to form anterior choroids plexus which secrete CSF. There is a pineal body behind the choroids plexus. It secrets melatonin hormone.

Thalamus is the part which consists of optic chaisma (crossing of optic nerves). It is a sensory relay station.

Hypothalamus consist of a hypophysis (pituitary gland) attached by stalk called infundibulum. Pituitary gland   is a master gland. The cavity behind the lateral ventricle is called third ventricle. It is connected with ventricle of cerebrum by foramen of monro.

Functions:

a)      It serves as relay center between spinal cord and brain stem.

b)      It regulates emotions and perceptions.

Mid-brain

It is the brain which connects forebrain with hind brain. It has two parts;

Corpora quadrigeminia; These are two pairs of round lobes.1^stpair is called superior colliculi which concern with vision.2^nd pair is called inferior colliculi which concern with hearing.

Crura cerebri; These are two bundles of fibers which lie on the lower surface of the mid brain. It relay the impulses back and forth between the cerebrum, cerebellum, pons and medulla oblongata.

fig: Mid brain

Hind brain

It consists of cerebellum, pons varolli and medulla oblongata.

Cerebellum; It is the second largest part of brain located behind posterior part of brain. It has three lobes. Cavity of cerebellum is called fourth ventricle communicating with third ventricle through aqueduct of sylvius.

Functions

a)      Co-ordinate muscular body movement.

b)      It controls reflex action of skeletal muscle activities.

Pons varollii (pons =bridge); it is situated in front of cerebellum and above the medulla oblongata and join medulla oblongata with the mid brain. Its fibre is of white matter.

Medulla oblongata; It lies between the pons varolii and the spinal cord. It consists of white matter. It is continuous with the spinal cord. Lower part of it consists of non vascular folded structure called posterior choroid plexus.

Functions;

a)      It is centre for cardiac, respiratory and vasomotor.

b)      It controls complex activities such heart action, respiration, sneezing, coughing etc.

Q. Write differences between cerebrum and cerebellum.

Brain Stem

Mid brain, pons and medulla oblongata collectively forms the brain stem. It connects the fore brain with spinal cord. It is continues with spinal cord. Death is declared clinically when there is cessation of brain stem function.

Ventricles of Human Brain

Ventricles are the cavities within the brain filled with cerebrospinal fluid (CSF).It secreted by anterior and posterior plexuses. There are four ventricles.

a)      Right and left lateral ventricles.

b)      Third ventricle (Dioecoel)

c)      Fourth ventricle (Metacoel)

Right and left lateral ventricles lie within the cerebral hemispheres below the corpus callosum. They communicate with third ventricles by foramen of monro.

Third ventricle situated below the lateral ventricles between two parts of the thalamus. It is communicated with third ventricle by narrow canal iter or cerebral aqueduct.

Fourth ventricle is a lozenge-shaped cavity situated below and behind the third ventricle, between the cerebrum and pons varolii. It continues below the central canal of spinal cord.

 fig: ventricles of brain

Cerebro-Spinal Fluid (CFS); It is lymph like intracellular fluid. It consists of salts, glucose, and small amount of albumin, globulin and few traces of urea and creatinine.

CSF is secreted by anterior and posterior choroid plexus in ventricles of brain, central canal or spinal cord and apace around the brain and spinal chord. Total volume of CSF is 150ml.the rate of formation is 20ml/hr.CFS can be obtained by lumber puncture.

Function of CSF

1. It protects brain and spinal cord.
2. It maintains uniform pressure around these delicate structures.
3. It acts as a cushion and shock absorber.
4. It keeps the brain and spinal cord moist and exchange of substances between CFS and nerve cells.
5. It excretes harmful metabolic wastes, drugs and other substances from the brain to the blood.
6. It nourishes the nerve cells.
7. It also acts as buffer.
8. It act as buoyancy and makes the brain low weight.

2. Spinal cord

            It is a posterior part of CNS which runs mid dorsally within the vertebral column. It is elongated, almost cylindrical part. It extends from the medulla oblongata within vertebral column to the level of second lumber vertebra. It measures about 42 to 45cm long and 2 cm thick. It is surrounded by the same three meninges found in the brain.

Internal structure

            Internally, spinal cord is divided in to left and right symmetrical halves. Posterior is median sulcus and anterior is median fissure. In the there is a central canal surrounded by a butterfly shaped area of grey matter. Around the grey matter there is white matter. Grey matter is an H-shaped with two dorsal and ventral horns. Roots of spinal nerve are originated from the horns. There are 31 pairs of spinal nerves arise from different segments of spinal cords. Each spinal nerve carries both sensory and motor impulses. Each spinal nerve connects with nerve roots.

         

Fig: T.S. of spinal cord

1. Dorsal nerve root: It originates from the dorsal horn of grey matter. It consists of only sensory fibers. It bears the dorsal root ganglion containing only sensory cells.
2. Ventral nerve root: It originates from the ventral horn of grey matter. It is made up of only motor fibers. It does not bear ganglion.

Functions of spinal cord

i.                     It is center of spinal reflex action.

ii.                   The stimuli are passed from and to the brain through the spinal cord.

Q. Write differences between ascending tract and descending tract.

                                                              

Q. Write differences between grey matter and white matter of CNS.

2. Peripheral Nervous System(PNS)

It consists of:

1. Cranial nerves: Those arising from the brain.
2. Spinal nerves: those arising from the spinal cord.

Types of Nerve Fibres

1.      Types on the basis of structures:

a.       Myelinated or medullated nerve fibres-

These fibre have myelin sheath. These are found in white matter of brain, spinal cord and nerves.

b.      Non-myelinated or non- modulated nerve fibre-These are without myelin sheath. They are found in grey matter of brain and spinal cord.

2.      Types on the basis of function of nerve impulse:

a.       Afferent nerve fibres (sensory): These conduct nerve impulses from effectors organs to the CNS. Such as optic nerve.

b.      Efferent nerve fibres (motor): These conduct nerve impulses from CNS to body organs. Such as occulomotor nerve (for eye movement).

c.       Mixed nerve fibres: These are both sensory and motor in function such as spinal nerves.            

3.      Types on the basis of number of their processes

a.       Unipolar: These neurons have only one axon with dendrites. They are found in dorsal root ganglia of spinal nerve.

b.      Bipolar: These neurons have two processes. In which one may be dendrite and other is axon. These are found in the retina of eye.

c.       Multipolar: These neurons have many cell processes. These are found in CNS. 

Cranial nerves

There are 12 pairs of cranial nerves. These arise from ventral side of brain. There names types and functions are given below in table.

No.	Cranial nerve	Types of fibre	Organs innervated	Function
I	Olfactory nerve	Sensory	Mucosa in nose	Smell
II	Optic nerve	Sensory	Retina of eye	Vision
III	Occulomoror nerve	Motor	Eye muscles, Ciliary muscles	Eye movement, accommodation
IV	Trochlear nerve	Motor	Superior oblique muscles of eye ball	Eye movement
V	Trigeminal nerve	Mixed	Skin teeth, mucosal membrane of mouth	Sensation head face
VI	Abducens nerve	Motor	Eyeball muscles	Eyeball movement
VII	Facial nerve	Mixed	Taste buds, salivary glands, facial and neck muscles	Facial expression, saliva secretion, taste
VIII	Auditory nerve	Sensory	Internal ear	Equilibrium Hearing
IX	Glossopharyngeal	Mixed	Pharynx, tongue, salivary glands	Taste, swallowing and saliva secretions
X	Vagus nerve	Mixed	Pharynx to viscera	Visceral reflexes
XI	Spinal accessory	Motor	Thoracic and abdominal viscera	Visceral reflexes, Shoulder movement
XII	Hypoglossal nerve	Motor	Muscles of tongue	Movement

Spinal Nerves

There are 31 pairs of spinal nerves in human immersed from either side of spinal cord from intervertebral foramina.

These nerves are:

Cervical                                  8 pair in neck

Thoracic                                  12 pair in thorax

Lumber                                   5 pair in upper abdomen

Sacral                                      5pair in lower abdomen

Coccygeal                               1 pair in tail region

Total spinal fibers     =            31pairs

So the spinal formula is ---    C₈ Th₁₂ L₅ S₅ Co₁

Each spinal nerve is a mixed nerve. It originates by two roots-Dorsal or sensory or afferent roots and Ventral or motor or efferent root from spinal cord. The two roots join within the neural canal of vertebral column. Each spinal nerve immediately divides into three branches:

·        Ramus dorsalis: It supplies muscles and of dorsal side.

·        Ramus ventralis: It supplies muscles and skin of ventral and lateral sides.

·        Ramus comminicans: it joins sympathetic ganglion of ANS.

Q. Write difference between cranial nerves and spinal nerves.

Autonomic Nervous Sys tem (ANS)

Autonomic nervous system controls and co-ordinates the various activities of visceral organs. Hence it is also called visceral nervous system. Actually ANS is not autonomous or independent because it is regulated by higher nerve centre of brain. It consists of two antagonistic (opposite in function) systems-

1.      Sympathetic nervous system

2.      Parasympathetic nervous system

Sympathetic nervous system

It consists of sympathetic chains, preganglionic fibres, collateral ganglia and postganglionic sympathetic fibres.

i.                    Sympathetic chains: There are long lateral chains of sympathetic ganglia (21=3 cervical, 12thoracic, 5 lumbar and 1 sacral) present on either side of vertebral column.

ii.                  Preganglionic sympathetic fibres: There are short sized axons originate from the grey matter of spinal cord.

iii.                Collateral ganglia: There are three collateral ganglia-coeliac ganglion, superior mesenteric ganglion and inferior mesenteric ganglion.

iv.                Postganglionic sympathetic fibres: These are long sized axon of neurons originated from collateral ganglia. These nerves supply the visceral organs and iris and ciliary muscles.

The sympathetic nerves stimulate the adrenal glands to secrete adrenalin or nor adrenaline, so these are called adrenergic nerve fibres.

Diagram……

Parasympathetic nervous system

It consists of parasympathetic fibres, parasympathetic ganglia and postganglionic parasympathetic fibres.

i.                    Preganglionic parasympathetic fibres: These are long sized axons of neurons present in midbrain brain stem and sacral region of spinal cord. These are emerging from cranium and sacrum form cranio-sacral out flow.

ii.                  Parasympathetic ganglia: These ganglia are present either close or inside the muscles of visceral organs. These are isolated ganglia. These ganglia join the preganglionic fibres and post-ganglionic fibres.

iii.                Postganglionic parasympathetic fibres: These are short sized axons of neurons arising from the parasympathetic ganglia and supply smooth muscle and glands or visceral organs.

Diagram……

Q. Write differences between sympathetic and parasympathetic nervous system.

Synapse

Synapse is an area of functional contact between one neuron and another to transfer information. Synapses are usually found between the fine terminal branches of the axon of a neuron and the dendrites or cell body of another.

Diagram….

Structure of synapse

A typical synapse consists of a bulbous expansion of a nerve terminal called a pre-synaptic knob close to the membrane of a dendrite. Cytoplasm of synaptic knob contains mitochondria, smooth endoplasmic reticulum, micro-filaments and numerous synaptic vesicles. Each vesicle contains neurotransmitter responsible for transmission of nerve impulse across the synapse. The membrane of synaptic knob nearest the synapse is thickened and forms the pre synaptic membrane. These membranes are separated by a gap, the synaptic cleft. The post-synaptic membrane contains large protein molecules which act as receptor sites for neurotransmitter and numerous channels and pores.

The two neruro-transmitters in vertebrate nervous are acetacholine (Ach) and noradrenalin although other neurotransmitters also exist. Neurons which release the neurotransmitters are called cholinergic neurons and adrenergic neurons.

Diagram….

Stimulus

Stimulus is a sudden change in the external or internal environment, which excite the nerve or organism or muscle as whole. The stimulus which capable to just excite given tissue is called threshold stimulus.

There are many types of stimuli which can excite the tissue:

a.       Mechanical stimuli- These include touch muscular stress etc.

b.      Physical stimuli-These include heat and humidity.

c.       Chemical stimuli-The electrical stimuli is able to excite tissue.

Properties of a nerve fibre are:

i. Excitability                                               ii. Conductivity

iii. Refractory period                                   iv. Summation v. All or none rule.

i.         Excitability-When a nerve fibre is stimulated by stimuli of physical, mechanical or chemical means and impulse is formed, it is called excitability.

ii.    Conductivity-When the stimulated nerve transmits the impulse in a particular     direction is called conductivity.

iii.   Refractory period-After excitation or transmission of nerve impulse nerve         regain the original state is called refractory period.

iv.   Summation-When stimulus applied to a nerve fibre is below the threshold          stimulus then it fails to stimulate any response. However, if the same              stimulus is continuously applied, stimulation occurs. This is called summation.

v.   All or none rule-When the organism gives response by stimulation, it cannot     be increased or response is always maximum. It is called all or none rule.

Nerve impulse

A nerve impulse is defined as wave of depolarization (wave of reverse polarity) of the membrane an axon of nerve cell. Nerve impulse is generated in nerve fibre is an electrical phenomenon.

Extra cellular fluid and intracellular fluid

Nerve is bathed in side the fluid. Outside the nerve is extracellular fluid (ECF) and inside the nerve fibre is the intracellular fluid (ICF).

·        Extra cellular fluid contains a large amount of sodium chloride, bicarbonates, oxygen, carbon dioxide and other metabolic wastes.

·        Intracellular fluid contains a large amount of potassium and magnesium phosphates along with proteins and organic molecules.

      Most of the solutes in extracellular fluid and intracellular fluid are electrically charged particles or ions (K⁺, Na⁺).

Differential permeability: Extra cellular fluid contains ten times more Na⁺ ions than inside the membrane of nerve cell which make outer side electropositive. K⁺ ions are 25 times more in intracellular fluid which makes inner side membrane electronegative. It is due to the differentially permeable to Na⁺, K⁺, Ca⁺⁺ , and Cl^-  ions. Cell membrane of neuron is less permeable for Na⁺ but it is more permeable for K⁺ (about 50 times more than Na⁺).this phenomenon is called differential permeability. It is maintained by Na-k pump.

Transmission of nerve impulse

It is described in two steps.

A. Transmission of nerve impulse along the nerve fibre.

B. Transmission of nerve impulse across the synapse.

Transmission of nerve impulse along the nerve fibre is explained by Hodgkin and Huxley in the late 1930 in three steps.

1. Polarization or resting potential.

In the rest state the inside of membrane has –ve electrical potential compared to outside.

This difference in potential is called resting potential. Which is about -40mv to-90mv.Na⁺ and K⁺ are transported across the membrane against their concentration gradient by carrier protein. Which is called Na-k pump and energy is used through ATP. The sodium channel and potassium channels on the membrane of neuron are closed.

2. Depolarization or action potential.

A nerve impulse can be initiated by mechanical, chemical and physical stimulation. Sodium channels are opened but potassium channels are closed and Na⁺ ions flood in through cell membrane and create a positive charge of +40mv. It is very short periods that change in potential and last for 3 milliseconds. When an action potential occurs, the axon is said to be depolarized.

Diagram(consult transmission of nerve impulse)

3. Repolarization.

Sodium channels are closed potassium channel are opened and K⁺ ions diffuse out along their concentration gradient. This start repolarization and resting potential going to reestablish. At the same time nerve become less permeable for Na⁺ than K⁺ .So many K⁺ flow out and inside charge become more negative than that it was originally. Na-k pump starts and normal concentrations of Na and K ions are reestablished. Each pump actively transports two K⁺ ions into the cell to every three Na⁺ ions transported out. The membrane is once again at its resting potential.

Transmission of nerve impulse along a non medullated nerve fibre is slower than in medullated or myelinated nerve fibre.

During this conduction of nerve impulse negative charge present on outside of a depolarized area attracts the +ve charge from outer surface of next polarized area. While +ve charge present on inner surface of depolarized area is attracted by –ve charge on inner surface of next polarized area.so that depolarized area become repolarized and nest polarized area become depolarized. Hence the impulses move towards synapse.

Transmission of nerve impulse along medullated nerve fibre is 20 times faster than non medullated fibre. The ionic exchange or depolarization occurs only at nodes because medullary sheath is impermeable to ions. The action potential is conducted from node to node in a jumping manner. This also called saltatory conduction of nerve impulses.

B. Mechanism of transmission of nerve impulse across the synapse.

It is explained by henry in 1936.Following are the steps for the process:

1. When an impulse arrives at the pre-synaptic knob of axon Ca⁺⁺ ions concentrate at the synapse.
2. Ca⁺⁺ions pass from the synaptic cleft into synaptic knob and cause the movement of synaptic vesicles towards to the surface of knob. Vesicles discharge their neurotransmitters chemicals acetylcholine in to the synaptic cleft and return to the cytoplasm of synaptic knob to refill neurotransmitters.
3. The neurotransmitter binds with protein receptor molecules in synaptic cleft. Na⁺ ions enter into the cell of another neuron and action potential generates on it. Thus nerve impulse transferred to the next synapse.
4. The acetylcholine is hydrolyzed by an enzyme acetyl cholinesterase into acetic acid and choline in the cleft which are reabsorbed into synaptic knob and resynthesized into acetylcholine using energy from ATP.  

FIG:mechanism of synapses