Neural Control and Coordination

We have studied that a number of physiological processes take place in the body of animals. Digestion, respiration, transportation, excretion, movement and locomotion are major physiological processes occurring in the body. Now, you all must be curious to know that how these complex processes work in a controlled and regulated manner i.e about neural control and coordination..

Regulation means adjustment of all those variables that determine the nature of a physiological function.

The various physiological processes occurring in our body are not independent events. The function of the different organs or organ system must be coordinated to maintain a steady and stable environment inside the body. The maintenance of steady and stable internal environment is called homeostasis.

Coordination is the process through which two organs interact and complement the functions of one another.

In our body, two coordination systems, neural system and endocrine system. For maintaining the coordination among these organs of the body, these controlling systems in our body ensures that various organs work harmoniously to maintain a smoothly operating body of an organism. In our body the neural system and the endocrine system jointly coordinate and integrate all the activities of the organs so that they function in a synchronized fashion.

The neural system provides an organized network of point to point connections for a quick coordination. The neural coordination is fast because the connection is with the help of a network of nerves in the body. The neural coordination is fast and short lived.

To continuously regulate the cellular functions in the body, a special kind of cordination and integration has to be provided. This function is carried out by chemicals called hormones. The endocrine system provides chemical cooordination through hormones.

In this context, you will learn about the neural system of human, mechanism and transmission of nerve impulse,impluse conduction across the synapse, physiology of a reflex action, sensory reception and processing and sense organs.

NEURAL SYSTEM

The neural system of all animals is composed of highly specialised cells called neurons. The neurons are also called as the nerve cells. The bit of information are called stimuli. Touch, pain, heat, cold, sound, light, smell, taste are some of the stimuli. There are some structures which can detect and receive these different kinds of stimuli. This function of performed by the specialised cells called neurons which can detect, receive and transmit different kinds of stimuli. These detecting and receiving neurons are present in sensory organs which are also called receptors. The vertebrates have more developed neural system having several division of neural system.

HUMAN NEURAL SYSTEM

Humans has an elaborate and well developed neural system. The ability of man to control his body is largely because of specialised neural system. The human neural system is divided into two parts:

• The central neural system
• The peripheral neural system

The central neural system- It lies along the central axis of the body. It includes the brain and the spinal cord. All the information is received by various sensory receptors present in the body. Thereafter, this information is transmitted to CNS. The CNS is the site of processing of information reaching it. The processing the information means interpretation and evaluation of the sensory information.

The peripheral neural system- the PNS lies along the peripheries of the body. The PNS comprises of all the nerves of the body associates with the CNS. A nerve is formed by the collection of many nerve fibres. The nerves are the components of the PNS.

TYPE OF NERVE FIBRES

AFFERENT FIBRES: the afferent fibres transmit impulses from tissue/organs  to the CNS. the afferent nerve fibres are called as sensory nerve fibres. the function of these nerve fibres is to carry the sensory information forms various tissue/ organs of the body towards the CNS so that CNS can respond accordingly.

EFFERENT FIBRES: the efferent fibres transmit the regulatory impulses  from CNS to concerned peripheral tissues/ organs. they are also called as motor nerve fibres. the function is to carry the regulatory informatio from CNS to concerned peripherla tissue/organ through which the response is shown.

DIVISIONS OF PNS

They are divided into two:

1. SOMATIC NEURAL SYSTEM

2. AUTONOMIC NEURAL SYSTEM

The somatic relays impulses from CNS to skeletal muscle of the body. They consist of efferent nerve fibres which transmit the regulatory impulse from the CNS to converned skeletal muscle of the body.

The autonomic neural system trasmits impulses from CNS to involuntary organs, smooth muscle and glands of the body.They consist of efferent nerve fibre which transmits the regulatory impulse from CNS to concerned involuntary muscle, organs of the body.

AUTONOMIC NEURAL SYSTEM

It plays important role in tuning of internal environment of the body. It is classified into two sub-systems:

1. SYMPATHETIC NEURAL SYSTEM

2. PARASYMPATHETIC NEURAL SYSTEM

Sympathetic and parasympathetic neural system have antagonistic effects on the organs. In the parasympathetic neural system the neurotransmitter between the axons of the neurons and target organ is acetylcholine whereas in sympathetic neural system the neurotransmitter between the axons of the neurons and target organ is adrenaline/noradrenaline. Adrenaline and noradrenaline have an effect opposite to that of acetylcholine.

Effect on heart beat: the sympathetic neural system accelerates the heart beat whereas the parasympathetic neural system slows down the heart beat.

Effect on secretions of salivary glands: the sympathetic neural system inhibits the secretions of saliva whereas parasympathetic neural system stimulates the secretions of saliva.

NEURON

Structural and functional unit of neural system.

Neurons are also called as nerve cells. On an average, the human brain is made up of  more than 100 billion neurons. These neurons are interlinked with each other forming complex nerve connections for exchange of neural information.

STRUCTURE:

 It is a microscopic structure. It is majorly composed of three parts:

• CELL BODY

It is the controlling part of the neuron. It contains cytoplasm with nucleus in the centre. The cytoplasm contains various cell organelles like golgi apparatus, endoplasmic reticulum, ribosomes, mitochondria etc. The cytoplasm contains granules called as nissl’s granules. These granules gives slightly grey color to the cytoplasm. Its function is protein synthesis in the body.

• DENDRITES

The short fibres arise out from the cell body is called as dendrites. They branched repeatedly to give rise to smaller fibres. They also contain nissl’s granules. They receives the electrical impulse and transmit to its own cell body.

• AXON:

It is called as nerve fibre. The axon is the long fibre which arises from the cell body. The end of axon which is attached to the cell body is called proximal end whereas the end of axon which is away from the cell body is called distal end. The distal end is divided into certain branches which are called as axon terminals. Each axon terminates as a bulb-structures called synapticknob.  The cytoplasm present inside the axon is called as axoplasm. The sac like structures are present in the synaptic knob called synapticvesicles. Each synaptic vesicles contains neurotransmitter. The axons take away the information from cell body of neuron and transmit away from cell body to a synapse or to neuromuscular junction.

TYPES OF NEURONS ON THE BASIS OF NUMBER OF AXON AND DENDRITES

TYPES OF AXONS ON THE BASIS OF PRESENCE OR ABSENCE OF MYELIN SHEATH

GENERATION AND CONDUCTION OF NERVE IMPULSE

POLARIZED STATE: neurons are excitable when their membranes are in the polarized state. The neurons are surrounded by the fluid outside them are called extracellular fluid. The fluid inside and outside the neurons possess ions like sodium ions, potassium ions, chloride ions, calcium ions etc. some are positively charged or negatively charged. Because of the differential concentrations of the ions, the membranes of neurons are said to be polarized. Hence, development of positive or negative charge makes the membrane polarized.

ION CHANNELS:  the neural membrane contains variety of passages called ion channels. These ion channels are selectively permeable to different ions. The passage only allows the ion such as Na⁺ or K⁺ or Ca²⁺ or Cl^– and resists others.

The ion channels that always remains open are called leak channels while the channels remain closed but open due to specific stimulus are called gated channels.

THE RESTING POTENTIAL:

• When the charge separation across the axonal membrane is maintained, the neuron is said to be at rest. This condition is called resting potential.
• A resting neuron is negatively charge on the inside and positively charge on the outside. Such a charge separation is called polarization and thus resting neuron is said to as polarized.
• At resting potential, the axonal membrane is comparatively more permeable to potassium ions and nearly impermeable to sodium ions.
• The ionic gradients across the resting membrane are maintained by active transport of ions by sodium – potassium pump which transports 3Na⁺ outwards for 2K⁺ into the cell on the cost of one ATP.
• Thus the outer surface of axonal membrane possesses a positive charge while its inner surface becomes negatively charged.

THE ACTION POTENTIAL:

The transmission of a nerve impulse is called an action potential to differentiate it from resting potential occurs in 4 phases.

1. Initiation of action potential by sufficient large stimulus called threshold stimulus.
2. Its transmission along the neuron
3. Its transfer to target cell which can be neuron or a muscle cell
4. Its effects on target cell.
5. Action potential temporarily disrupts the polarization of a neuron resulting in depolarization.
6. When stimulus is applied to a site of neuron, the closed voltage gated Na+ channels suddenly opens, allowing Na+ ions to enter into the neuron.
7. This sudden movement of positive ions into neuron causes the neuron’s interior to develop a positive charge relative to exterior.
8. Next, the voltage gated Na⁺ channels close and voltage gated K+ channels open and K+ ions swept out, thus restoring the resting potential called repolarization.

REPOLARIZATION:as the Na+ channels close after 0.5 seconds the membrane becomes extra permeable to K+ ions due to opening of K+ ion gates. With pumping out of K+ ions the interior of neuron becomes negative and the potential falls back to resting potential. The phenomenon of change of membrane potential from excited state to resting state is called repolarization. However, K+ ion channels remain open for a bit longer period so that membrane potential becomes more negative than -70 mV. It is called hyperpolarization. It takes about 1-5 sec for repolarization.

TRANSMISSION OF IMPULSES

The nerve impulses generated and conducted through axons needs to be transferred and transmitted.

SYNAPSE: a nerve impulse is transmitted from one neuron to another through junctions called synapses. It is formed by the membranes of a pre synaptic neuron or a post synaptic neuron, which may or may not be separated by the gap called synaptic cleft.

PRE SYNAPTIC NEURON:the neuron which is transmitting the impulse to other neuron and is present before the synaptic cleft is called pre synaptic neuron.

POST SYNAPTIC NEURON: the neuron which is present after the synaptic cleft and is receiving the impulse from other neuron. Is called post synaptic neuron.

SYNAPTIC CLEFT: the space present between the pre synaptic membrane and the post synaptic membrane is called synaptic cleft.

Mechanism

The cytoplasm of synaptic knob contains tiny, round sacs called synaptic vesicles. Each vesicle contains 10,000 molecules of neurotransmitters. Neurotransmitters are the chemicals which are involved in transmission of nerve impulses across chemical synapses.

These chemicals do not participate in the transmission of nerve impulses at electrical synapses.

These different kinds of neurotransmitters permit different kinds of responses either excitatory or inhibitory in nature.

Steps in the mechanism of transmission of nerve impulse through chemical synapse:

1. Wave of depolarization reaches presynaptic membrane
2. Voltage gated calcium channels open, Ca++ ions diffuses into the axon terminal from the surrounding fluid.
3. Ca++ stimulates fusion of synaptic vesicle with pre synaptic membrane and release of neurotransmitters by exocytosis into synaptic cleft
4. Neurotransmitter binds with specific receptor molecules  of post synaptic membrane.
5. The binding opens sodium ion channels allowing the entry of Na+ ions which can generate a new potential in the post synaptic neuron.
6. The new potential developed may be either excitatory or inhibitory depends upon neurotransmitter and post synaptic membrane.

CENTRAL NERVOUS SYSTEM

It is the site of processing of information and control. Brain and spinal cord comprises the central nervous system.

HUMAN BRAIN

The brain is the central information processing organ of our body. It acts as command and control system. This is because all the systems in the body follow the commands given to them by brain. The commands are given through the nerves.

STRUCTURE:

Brain has a soft and delicate structure. It has wrinkled surface. The human brain is well protected by the bony skull. Inside the skull, the brain is covered by protective layers called cranial meninges that are:

1. Duramater- outer layer; thick and tough
2. Arachnoid- middle layer; delicate
3. Piamater- inner layer; thin but highly vascular

FUNCTIONS

• VOLUNTARY MOVEMENTS: the movements that are performed according to our will or desire of the organism are called voluntary movements such as walking, running, talking etc.
• BALANCE OF OUR BODY:  Both static and dynamic movements requires the body balance to be maintained which is done by human brain.
• FUNCTIONING OF VITAL ORGANS: the functioning of involuntary organs such as heart, kidney, liver, digestive tract etcare controlled by the brain. The activities such as heart beat, digestion of food, excretion etc.
• THERMOREGULATION: the control of body temperature is called thermoregulation.
• HUNGER AND THIRST: they are also controlled by human brain.
• CIRCARDIAN RHYTHMS OF OUR BODY: it maintains 24 hour rhythm of sleeping, feeling hungry, awakening etc.
• ACTIVITIES OF ENDOCRINE GLANDS: the secretions of hormones are also controlled by the human brain.
• HUMAN BEHAVIOR: the creative ideas, memory, ability of decision making, emotions and thought; all the cognitive functions are controlled by the brain.
• PROCESSING OF VARIOUS SENSATIONS: brain is the site for processing various sensations like vision, hearing, smell, taste, touch, heat etc.

FOREBRAIN

The forebrainforms the major part of brain and consists of cerebrum, thalamus and hypothalamus.

1. CEREBRUM
2. It is most prominent portion as well as form the major part of the human brain.
3. It is made up of two large, deeply folded parts called cerebral hemispheres.
4. These hemispheres are separated by a deep cleft forming right and left cerebral hemispheres.
5. Each hemisphere divides into 4 lobes frontal, parietal, occipital and temporal. Hence, there are 2 frontal, 2 parietal, 2 occipital and 2 temporal lobes in our cerebrum.

Corpus callosum: the tract of nerve fibres which connects the two cerebral hemispsheres is called corpus callosum. The anterior part is genu and the posterior part is splenium.

Cerebral cortex:the outer surface of cerebrum called the cortex, is a leyr of 2-4 millimetres thick. It is referred to as grey matter. It consist convulsions or folds having sulci, fissures and gyri.

The outer surface of cerebrum is called gray matter due to concentrations of cell bodies in this area. The inner area of cerebral hemisphere is called white matter because presence of tracts in this area.

• DIENCEPHALON CONSISTS OF THALAMUS, HYPOTHALAMUS AND EPITHALAMUS.

EPITHALAMUS: it is smallest part of diencephalon. It includes pineal gland.

THALAMUS: the cerebrum wraps around a structure called the thalamus. It is relay centre and major coordinating centre for sensory and motor signaling.

HYPOTHALAMUS: it lies at the base of thalamus.

It contains the number of centres which controls body temperature, urge for eating and drinking, sexual drive, anger etc. It keeps the water content of body constant by controlling amount of urine and by controlling thirst.

It not only controls nervous activity but also controls the endocrine function. The hypothalamus contains several groups of neurosecretory cells, which secrete hormones.

MIDBRAIN

• It is present between the thalamus of the forebrain and pons of the hindbrain. It consist of four lobes.
• The superior pair receives sensory impulses from the eyes and muscles of the head and control visual reflexes. They control and coordinate the movement of the head and eye at the same time to focus on an object.
• The posterior or inferior part receives the sensory impulses from ears and muscles of the head and control auditory reflexes.
• A canal called cerebral aqueduct passes through the midbrain.

HIND BRAIN

The posteriormostpart of the brain that’s why it is called hindbrain. It comprises of the pons, cerebellum and medulla oblongata.

PONS:

• it consist the fibre tracts that interconnect different regions of the brain.
• It mainy acts as a neuronal link between the cerebral cortex and cerebellum.
• A centre present in pons called pneumotaxiccentre which can moderate the functions of respiratory rhythm.

CEREBELLUM:

• it is the second largest part of the brain after cerebrum.
• It is made up of two cerebral hemispheres and has its gray matter on the outer side and white matter on the inner side.
• The cerebellum does not initiate the movement of the body but reorganizes the motor commands. Its most important function seems to be coordinating locomotor activity in the body which is inititated by the impulses in motor area of the forebrain.

MEDULLA OBLONGATA:

• It is the posteriormost part that connects various part of the brain and the spinal cord.
• It contains the centes which controls respiration, cardiovascular reflexes and gastric secretions.
• It consist a centre present in the medulla which is called respiratory rhythm centre. Its function is to maintain respiratory rhythm.
• Heart is also controlled by the neural centre in the medulla.
• The gastric secretions are also under neural control of medulla.

SPINAL CORD

• It is other major part of CNS. In an adult human being, spinal cord is about 42-45 cm long and 2 cm thick.
• It consistsmyelinated nerve fibres running up and down the spinal cord delivering and receiving the information to and from the brain. This way spinal cord acts as a link between the brain and nerves that stretch throughout the body.
• It connects PNS to brain. All the information is transmitted to the brain and the appropriate action is taken.
• It acts as minor coordinating itself. Simple reflex actions can take place through sole action of the spinal cord.

REFLEX ACTION

The entire process of response to a stimulation that occurs involuntarily without any conscious effort, requires involvement of CNS is called reflex action. It is very rapid, automatic and short-lived.

REFLEX PATHWAY

The stimulus is detected b sensory receptors present in the skin.

These initiate nerve impulses in the sensory or afferent neurons leading from them to the spinal cord.

These impulses enter the spinal cord and initiate impulses in one or more interneurons.

Interneurons initiate impulses in one or more motor or efferent neurons.

The motor neurons then carry these impulses to the effectors or skeletal muscles in which response will be shown.

PERIPHERAL NERVOUS SYSTEM

CRANIAL NERVES

SPINAL NERVES

In man there are 31pairs of spinal nerves. They are classified into 5 groups. They include:

• 8 pairs of cervical nerves
• 12 pairs of thoracic nerves
• 5 pairs of lumbar nerves
• 5 pairs of sacral nerves
• one pair of coccygeal.

ANTAGONISTIC ROLE OF SYMPATHETIC AND PARASYMPATHETIC NERVOUS SYSTEMS

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