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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Neural System Human Neural System Neuron as structural and functional unit of neural system Central Neural System Reflex Action and Reflex Arc Sensory Reception and Receptors Smell Receptors (Olfactoreceptors) Taste Receptors (Gustatoreceptors) Eye Ear Digestion, respiration, transportation, excretion, movement and locomotion are the major physiological processes occurring in the body. These complex processes work in a controlled and regulated manner. Regulation means adjustment of all those variables that determine the nature of a physiological function. The variables may be an amount, a concentration, a rate or so on. Regulation means keeping these variables at some optimum or desired level. Coordination is the process through which two or more organs interact and complement the functions of one another. The main purpose of maintaining coordination among the various organs is homeostasis. In our body two co- ordination systems, neural system and endocrine system, function in a synchronised way to integrate all activities. The neural system provides an organised network of point-to-point connections for a quick coordination. The neural coordination is fast because there is a point-to-point connection between the neural system and the body organs. This 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 coordination and integration has to be provided. This function is carried out by chemicals called hormones. The endocrine system provides chemical coordination through hormones. Organisation at neural level became evident for the first time in members of phylum cnidaria. NEURAL SYSTEM The neural system of all animals is composed of highly specialised cells called neurons which can detect, receive and transmit different kinds of stimuli. The neural organisation is very simple in lower invertebrates. For example, in Hydra it is composed of a network of neurons. The neural system is better organised in insects, where a brain is present along with a number of ganglia and neural tissues. The vertebrates have a more developed neural system. Chapter 8 Neural Control and Coordination
40 Neural Control and Coordination NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 HUMAN NEURAL SYSTEM The nervous system consists of two main parts : (A) Central nervous system : It consists of brain and spinal cord. The brain is lodged in the skull while spinal cord is enclosed by the vertebral column. (B) Peripheral nervous system : The nerves arising from the central nervous system, constitute the peripheral nervous system. The nerves originating from the brain and spinal cord and are known as cranial nerves and spinal nerves respectively. It is further differentiated into somatic neural system and autonomic nervous system. Visceral nervous system is that part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system. Types of Nerve Fibres of the PNS The nerve fibres of the PNS are of two types : (a) Afferent fibres : The afferent nerve fibres transmit impulses from tissues/organs to the CNS. The afferent nerve fibres are also called the sensory nerve fibres. The function of these nerve fibres is to carry the sensory information from various tissues/organs of the body towards the CNS so that the CNS can accordingly respond and control the body. (b) Efferent fibres : The efferent nerve fibres transmit the regulatory impulses from the CNS to the concerned peripheral tissues/organs. The efferent nerve fibres are also called the motor nerve fibres. The function of these nerve fibres is to carry the regulatory information from CNS to the concerned peripheral tissue/organ through which the response of the body is shown. Type of Nerves based on their Function (a) Sensory nerves: Some cranial nerves contain only sensory (afferent) nerve fibres, and are thus called sensory nerves. First (olfactory nerve), second (optic nerve) and eighth (auditory nerve) cranial nerves are sensory. (b) Motor nerves: Some cranial nerves contain only motor (efferent) nerve fibres, and are thus called motor nerves. Third (occulomotor nerve), fourth (trochlear nerve), sixth (abducens nerve), eleventh (accessory spinal nerve) and twelfth (hypoglossal nerve) cranial nerves are motor in function. (c) Mixed nerves: The remaining cranial nerves contain both sensory and motor nerve fibres and are referred to as mixed nerves. Fifth (trigeminal nerve), seventh (facial nerve), ninth (glossopharyngeal nerve) and tenth (vagus nerve) cranial nerves are mixed in nature. Each spinal nerve contains both afferent and efferent nerve fibres and is thus mixed in nature. NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF NEURAL SYSTEM Nerve cells also called neurons are structural as well as functional unit of neural system. On an average, the human brain is made up of more than 100 billion (1010) neurons. Similarly, other parts of the neural system, whether of CNS or of PNS, are structurally made up of a large number of neurons. These neurons are further interlinked with each other or are linked with other cells of the body forming complex nerve connections for the exchange of neural informations. Neurons are called the functional units of the neural system because the various functions of the neural systems are basically the functions of the neurons. Structure of a Neuron A neuron is a microscopic structure composed of three major parts: (a) Cell body or soma (b) Dendrites (c) Axon
NEET Neural Control and Coordination 41 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 (a) Cell body : Cell body is the controlling part of the neuron. The cell body contains cytoplasm with a nucleus in the centre. The cytoplasm of cell body contains typical cell organelles like Golgi apparatus, endoplasmic reticulum, ribosomes, mitochondria, etc. The cytoplasm also contains certain granular bodies called Nissl’s granules. These granular bodies gives a slight gray colour appearance to the cytoplasm of cell body. The function of Nissl’s granules is supposed to be the protein synthesis in the body of neurons. Dendrites Nissl’s granules Cell body Nucleus Schwann cell Axon Myelin sheath Node of Ranvier Axon terminal Synaptic knob Fig. : Structure of a neuron (b) Dendrites : From the cell body, short fibres arise. These short fibres which arise from the cell body are called the dendrites. The dendrites branch repeatedly to further give rise to smaller fibres. The dendrites also contain the Nissl’s granules, like the cell body. The dendrites of one neuron receive electrical impulses from different sources, i.e., other neurons and then transmit these impulses towards the cell body of the neuron to which they belong. (c) Axon : It is a long fibre which arises from the cell body. Each neuron may have many dendrites but has only one long fibre-like axon. That end of the axon which is attached to the cell body is called proximal end whereas the end of the axon which is away from the cell body is called the distal end. The distal end of the axon shows branching. The distal end of the axon is divided into certain branches which are called the axon terminals. Each axon terminal terminates as a bulb-like structure called the synaptic knob. These bulb-like structures are called the synaptic knobs because these knobs participate in synapse formation which will be discussed later. The cytoplasm present inside the axon is called the axoplasm. The synaptic knobs also possess the axoplasm. In the axoplasm present inside the synaptic knobs, numerous tiny, rounded bag-like or sac-like structures are present called as synaptic vesicles. Each synaptic vesicle contains neurotransmitter which are involved in the transmission of the nerve impulse from one neuron to other neuron or other cell. The dendrites transmit the nerve impulses towards the cell body while the axons transmit the nerve impulses away from the cell body to a synapse or to a neuromuscular junction depending upon whether the information is being sent to another neuron or to a muscle fibre, respectively. Hence, the axons take away the information from the cell body of the neuron to which they belong.
42 Neural Control and Coordination NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Types of Neurons on the basis of number of Axon and Dendrites Various types of neurons are present in the human body. The neurons are divided into three major types on the basis of the number of axon and dendrites present in them: (a) Multipolar neurons : These possess a cell body with one axon and two or more dendrites. These neurons are called the multipolar neurons because they possess many poles. The dendrites of these neurons transmit nerve impulses towards the cell body whereas the axon transmits nerve impulses away from the cell body. The multipolar neurons are present in the cerebral cortex. (b) Bipolar neurons : These possess cell body with one axon and one dendrite. These are called the bipolar neurons because they possess only two poles. Here also the dendrite transmits the nerve impulse towards the cell body and axon transmits the nerve impulse away from the cell body. Bipolar neurons are found in the retina layer of the eye. (c) Unipolar neurons : These possess cell body with one axon only. No dendrites are present. These are called the unipolar neurons because they possess single pole, i.e., axon. Unipolar neurons are found usually in the embryonic stage. These unipolar neurons later develop the other poles and become bipolar multipolar. (d) Pseudounipolar nerve cells : These are T-formed nerve cells derived from branching of a single process which grows out from the cell (e.g., neuron of dorsal root ganglion of spinal cord). Types of Axons based on Presence or Absence of Myelin Sheath (a) Myelinated axons : Many neurons in the human body are enveloped by a lipid-rich sheath called the myelin sheath. Such axons are found in spinal and cranial nerves. The myelin sheath is produced by neuroglial cells called Schwann cells in the PNS. This is not a continuous sheath. Certain gaps/intervals are present called Nodes of Ranvier. Oligodendrocytes myelinate axons in the CNS. Function of myelin sheath and nodes of Ranvier : The myelin sheath acts as a biological electrical insulator and permits the flow of electrical impulse through an axon. However, at the areas called the nodes of Ranvier, impulse/action potential jumps from one node to another. This is called saltatory conduction of impulse. (b) Non-myelinated axons/unmyelinated axons : Unmyelinated nerve fibres are also enclosed by Schwann cells. However, these do not produce a myelin sheath and form neurilemma (Sheath of Schwann). Neurilemma is not found in neurons of CNS. Nodes of Ranvier are absent in the unmyelinated axons. Unmyelinated nerve fibres are commonly found in autonomous and somatic neural systems. Generation and Conduction of Nerve Impulse Neurons are excitable cells because their membranes are in a polarised state. The Nerve Impulse Nerve cells have polarised membrane i.e., have electrical potential difference or membrane potential. This is because of a variety of ion channels (pores formed by proteins) specific for particular types of ions. Resting Membrane Potential In a resting nerve fibre, the cytoplasm just beneath its membrane is electronegative relative to the layer of extracellular fluid (ECF) just outside the membrane. The inner side of membrane is seen to possess a negative potential of about 70 mV relative to the outer side. This is called resting membrane potential. This results due to two factors: (i) The resting membrane has poor permeability for Na+ although it has a higher permeability for K+. Therefore, K+ can cross more easily while Cl– and Na+ have more difficulty in crossing. (ii) Negatively charged protein molecules inside the neuron cannot cross the plasma membrane.