NEET Biology Notes Neural control and chemical coordination
Nervous System and Coordination
Coordination is the process through, which two or more organs interact and complement the functions of one another. In our body, the neural system and the endocrine system jointly coordinate and integrate all the activities of the organs and their functions. The nervous system provides an organised network of point-to-point connections for a quick coordination. The endocrine systojn provides chemical regulation through hormones in animals.
It controls the body by using a series of tissues through out the body formed by a network of electrically conducting cells called neurons or nerve cells.
Neurons and Nerves
- A neuron is a microscopic structure composed of three major parts, i.e. cell body, dendrites and axons.
- Cell body or soma contains cytoplasm with typical cell organelles and certain granules called Nissl’s granules.
Dendrites Cytoplasmic processes from cell body. - Axon A single long extension of cell body which is protected by a neurilemma. The distal end of axon is branched which terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemical called neurotransmitters.
- Neurons are bundled together to form nerves. These nerves connect nerve centres to specific organs. The nerves can be insulated or non-insulated. The insulated nerves have coating of myelin sheath. At certain places, the insulation or myelin sheath is interrupted and such places are called nodes of Ranvier.
- Neurons are categorised into three groups
- Multipolar
It is with one axon with two or more dendrites and found in cerebral cortex. - Bipolar
It is neuron with one axon and one dendrite. It is found in the retina of eye. - Unipolar
The cell body is with one axon only and found usually in the embryonic stage.
Human Nervous System
Nervous system in human beings develops frdm the ectoderm. In mammals, nervous system consists of Central Nervous System (CNS) and Peripheral Nervous System (PNS).
The Autonomic Nervous System (ANS) comes urider PNS in which conduction of impulse from CNS to cardiac muscle, smooth muscles and glands takes place. The study of nervous system including receptors is called neurology.
- Central Nervous System (CNS)
The central nervous system oonsists of brain and spinal cord. In humans both of them are surrounded by three tissue coverings called meninges (two in amphibians, i.e. frog) which are as follows: - Duramater (outermost)
- Arachnoid mater (middle)
- Piamater (innermost) tox.
The fluid present in around brain and spinal cord is called cerebrospinal fluid, produced by choroid plexus.It is a colourless fluid similar to the plasma with volume of about 80-150 mL in an adult.
The main functions of cerebrospinal fluid are as follows:
- It absorbs shock or jerk to protect brain and spinal cord.
- It serves as medium for exchange of nutrients and waste products between blood and nervous tissue.
- It serves as endocrine medium for transport of hormones.
- The CNS has two types of tissues. These are
- Grey matter, containing cell bodies, dendrites and axon terminals of neurons (synapses) and white matter that contains axons.
Brain
The human brain is composed of more than 100 billion neurons and 10-15 trillion neuroglia that support and nourish neurons. The different regions of brain are forebrain, midbrain and hindbrain.
Forebrain
It is the largest part of brain and is divided into :
- Cerebrum The cerebrum is the most developed part in human and is divided into right and left cerebral hemispheres connected by corpus callosum (unique feature of mammals).
The right lobe controls the left body and vice-versa. The cerebrum is composed of subregions cerebral cortex, basal ganglia and limbic system. - Cerebral cortex forms 80% of the total brain mass. It has folds or convolutions consisting of small grooves called sulci, large grooves called fissures and bulges between two (gyri), which help in increasing surface area of cortex.
- The lobes and sulci of cerebrum are tabulated as below:
- Basal ganglia Deep within cerebral hemisphere a collection of scattered grey matter with three subcortical nuclei is present which is termed as basal ganglia. It has three members, i.e. globus pallidus, putamen and caudate nucleus.
Together (putamen and caudata nucleus) is called as corpus striatum. The basal ganglia helps to regulate initiation and termination of movements and also controls subconscious movements of skeleton. - Limbic system or emotional brain It is a loop of cortical structures surrounding corpus callosum and thalamus, including amygdala and hippocampus, on median side of temporal lobe.
It plays crucial role in wide range of emotions, i.e. pain, pleasure, affection and anger and also controls memory and olfaction. The amygdala is concerned only with expression of fear and aggression.
The hippocampus functions as index for recall of an event, associated with memory. - Olfactory lobe It is anterioventrally positioned and functionally related to smell.
- Diencephalon It consists of three major parts:
- Thalamus It forms the 4/5th of the diencephalon and consists of oval mass of grey matter. All types of sensory input and other information going to cerebrum passes through synapses in the thalamus. It is also connected with the limbic system, hence controls the emotion and memory.
- Hypothalamus It nestles at the base of thalamus, integrating and controlling the visceral activities. It also helps to maintain homeostasis, controls thirst, hunger, temperature and influences, respiration and heartbeat.
- Epithalamus It consists of pineal gland and habenular nuclei. Pineal gland secretes melatonin and habenular nuclei which is associated with olfaction.
Midbrain
It extends from pons to diencephalon. A canal called cerebral aqueduct, passes through the midbrain. Cerebral peduncles (a part of tracts) are present in the anterior part of midbrain, whereas posterior part of the midbrain (tectum) contains four little lobes, the corpora quadrigemia with two superior colliculi (involved in visual activities) and two inferior colliculi (involved in auditory pathway).
The midbrain have several nuclei including large and darkly pigmented nuclei (substantia nigra). Dopamine releasing neurons extend from substantia nigra to basal nuclei/ganglia and loss of the function of these neurons causes Parkinson’s disease. Midbrain also contains red nuclei, which look reddish due to the rich blood supply and iron containing pigment.
Hindbrain
It consists of cerebellum, pons and medulla oblongata.
- Cerebellum The cerebellum or little cerebrum is second largest part of brain with a central constricted area called vermis and lateral lobes or wings called cerebellar hemispheres.
- The flocculonodular lobe on inferior surface contributes to equilibrium and balance. Deep inside the grey matter, arbor vitae are present which resemble branches of tree.
- Purkinje cells are distinctive neurons of cerebellum. These are unusually large, globular neurons with tremendous profusion of dendrites.
- The bundles of myelinated nerve fibres called cerebellar peduncles forms communication between cerebellum and other parts of the CNS.
- The three pairs of peduncles are given below:
- Superior cerebellar peduncles connects cerebellum to red nuclei.
- Middle cerebellar peduncle largest of three which communicates with pons.
- Inferior cerebellar peduncle pathway between cerebellum, medulla oblongata and spinal cord.
The major functions of cerebellum includes, maintaining equilibrium of the body, modulating motor commands and controlling muscular and involuntary movements. - Pons Varolii It forms the floor of brain stem and serves as neuronal link between cerebral cortex and cerebellum. It consist of pontine nuclei, pneumotaxic area and apneustic area. Pontine nuclei are sites at which signals for voluntary movements are relayed into cerebellum. Both pneumotaxic and apneustic area control respiration.
- Medulla oblongata It is the triangular part of brain its roof is associated with overlying piameter to form the posterior choroid plexus. It contains cardiac centres (control rate and force heartbeat), vasomotor cehtres (control blood pressure by adjusting” blood vessel diameter) and two respiratory centres (control rate and depth of breathing).
Ventricles of Brain
The four cavities within brain are called cerebral ventricles. These are given below:
Ventricle I, II and III are connected by foreman of monro, while ventricle III and IV are connected by cerebral aqueduct.
- Peripheral Nervous System (PNS)
The PNS comprises of all the nerves of the body associated with the CNS. These are
- Afferent fibres
- Efferent fibres
Afferent fibres transmit impulses from tissues/organs to the CNS and the efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues. The PNS consists of somatic nervous system and autonomic nervous system. The autonomic nervous system is further divided into sympathetic nervous system and parasympathetic nervous system. It is composed of 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial nerves emerge from brain, while spinal nerves arises from spinal cord. 10 pairs of cranial nerves are present in fishes and amphibians. The number of cranial and spinal nerve present in rabbit is 12 pairs and 37 pairs respectively. Thirty one pairs of spinal nerves includes eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pair of lumber nerves, five pair of sacral nerves and one pair of coccygeal nerves in human.
PNS is composed of two parts
It was discovered by Langley. It is entirely motor and operates without concious control.
- Autonomic nervous system It was discovered by Langley. It is entirely motor and operates without concious control. Autonomic nervous system consists of two divisions:
- Sympathetic nervous system It increases the defence system of body against adverse conditions. It is active in stress condition, e.g., pain, fear and anger.
- Parasympathetic nervous system It provides relaxation, comfort, pleasure at the time of rest. It helps in the restoration and conservation of energy.
- Somatic Nervous System
It relays impulses from the CNS to skeletal (voluntary) muscles.
Generation and Conduction of Nerve Impulse
When a neuron is not conducting any impulse, i.e. resting, the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium (Na+) ions.
The axoplasm inside the axon contains high concentration of K+ and negatively charged proteins and low concentration of Na+. In contrast, the fluid outside the axon contains a low concentration of K+, a high concentration of Na+ and form a concentration gradient.
These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump, which transports of 3Na + outwards for 2K * into the cell. In this condition, the electrical potential difference across the resting plasma membrane is called as the resting potential. When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to Na+. This leads to a rapid influx of Na + followed by the reversal of the polarity at that site.
The polarity of the membrane at the site A is thus, reversed and, hence depolarised. The electrical potential difference across the plasma membrane at the site A is called the action potential or nerve impulse.
At sites, immediately ahead, the axon (e.g. site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to B. On the outer surface, current flows from site B to site A to complete the circuit of current flow. Hence, the polarity at the site is reversed and an action potential is generated at site B. Thus, the impulse (action potential) generated at site A arrives at site B.
This sequence is repeated along the length of the axon and consequently, the impulse is conducted.
The rise in the stimulus-induced permeability to Na+ is quickly followed by a rise in permeability to K+. Within a fraction of a second, K+ diffuses outside the membrane and restores the resting potential of the membrane.
