Biological Basis of Behaviour PDF

**Introduction** Your nervous system plays a role in everything you do. The three main parts of your nervous system are your brain, spinal cord and nerves. It helps you move, think and feel. It even regulates the things you do but don't think about like digestion. It contains the central nervous sy...

**Introduction** Your nervous system plays a role in everything you do. The three main parts of your nervous system are your brain, spinal cord and nerves. It helps you move, think and feel. It even regulates the things you do but don't think about like digestion. It contains the central nervous system and the peripheral nervous system. **What is the nervous system?** Your nervous system is your body's command center. It's made up of your brain, spinal cord and nerves. Your nervous system works by sending messages, or electrical signals, between your brain and all the other parts of your body. These signals tell you to breathe, move, speak and see, for example. Your nervous system keeps track of what's going on inside and outside of your body and decides how to respond to any situation you're in. Your nervous system regulates complicated processes like thoughts and memory. It also plays an essential role in the things your body does without thinking, like blushing, sweating and blinking. **What does the nervous system do?** Your nervous system's main function is to send messages from various parts of your body to your brain, and from your brain back out to your body to tell your body what to do. These messages regulate your: Thoughts, memory, learning and feelings. Movements (balance and coordination). Senses (how your brain interprets what you see, hear, taste, touch and feel). Wound healing. Sleep. Heartbeat and breathing patterns. Response to stressful situations, including sweat production. Digestion. Body processes, such as puberty and aging. **How does the nervous system work?** Your nervous system uses nerve cells called neurons to send signals, or messages, all over your body. These electrical signals travel among your brain, skin, organs, glands and muscles. The messages help you move your limbs and feel sensations, like pain. Your eyes, ears, tongue, nose and the nerves all over your body take in information about your environment. Then, nerves carry that data to and from your brain. **There are different types of neurons. Each type of neuron has a different job:** **Motor neurons** take signals from your brain and spinal cord to your muscles. They help you move. They also assist with breathing, swallowing and speaking. **Sensory neurons** take information from your senses (what you see, touch, taste, etc.) to your brain. **Interneurons** communicate between motor and sensory neurons. These neurons regulate your movement in response to sensory information (like moving away from a hot surface) and play a role in how you learn, think and remember. **Anatomy** What are the parts of the nervous system? The nervous system has two main parts: **Central nervous system (CNS):** Your brain and spinal cord make up your CNS. Your brain reads signals from your nerves to regulate how you think, move and feel. **Peripheral nervous system (PNS):** Your PNS is made up of a network of nerves. The nerves branch out from your spinal cord. This system relays information from your brain and spinal cord to your organs, arms, legs, fingers and toes. There are two parts to your peripheral nervous system: The somatic nervous system guides your **voluntary movements**. The autonomic nervous system regulates the activities you do without thinking about them **(involuntary movements**). **What does the nervous system look like?** Nerve cells (neurons) are the basis of your nervous system. There are 100 billion neurons in your brain. These cells connect throughout your entire body. Imagine your nervous system as a tree. Your central nervous system is the trunk of the tree that contains your brain and spinal cord. The tree branches are your peripheral nervous system (nerves). The branches extend from the truck (brain and spinal cord) to reach all parts of your body. **Central Nervous system** **The CNS consists of the brain and the spinal cord, which lie within the skull and within the** spinal column, respectively; both are bathed in a special fluid called the cerebrospinal fluid (CSF) and are protected by enclosing sheaths called meninges. The meninges consist of three layers: the outer layer (dura mater), the middle layer (arachnoid layer), and the inner layer(pia mater). Below the arachnoid layer and above the pia mater is a space called the subarachnoid space, which is filled with cerebrospinal fluid. The Brain Three cavities, called the primary brain vesicles, form during the early embryonic development of the brain. These are the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain(rhombencephalon). During subsequent development, the three primary brain vesicles develop into five secondary brain vesicles. The names of these vesicles and the major adult structures that develop from the vesicles follow (see Table 1): The telencephalon generates the cerebrum (which contains the cerebral cortex, white matter, and basal ganglia). The diencephalon generates the thalamus, hypothalamus, and pineal gland. The mesencephalon generates the midbrain portion of the brainstem. The metencephalon generates the pons portion of the brainstem and the cerebellum. The myelencephalon generates the medulla oblongata portion of the brainstem. The cerebrum consists of two cerebral hemispheres connected by a bundle of nerve fibers, the corpus callosum. The largest and most visible part of the brain, the cerebrum, appears as folded ridges and grooves, called convolutions. The following terms are used to describe the convolutions: A gyrus (plural, gyri) is an elevated ridge. A sulcus (plural, sulci) is a shallow groove. A fissure is a deep groove. The deeper fissures divide the cerebrum into five lobes (see Figure 1; most lobes are named after bordering skull bones): the frontal lobe, the parietal lobe, the temporal lobe, the occipital lobe, and the insula. All but the insula are visible from the outside surface of the brain. A cross section of the cerebrum shows three distinct layers of nervous tissue (see the list below and Figure 2): The cerebral cortex is a thin outer layer of gray matter. Such activities as speech, evaluation of stimuli, conscious thinking, and control of skeletal muscles occur here. These activities are grouped into motor areas, sensory areas, and association areas. The cerebral white matter underlies the cerebral cortex. It contains mostly myelinated axons that connect cerebral hemispheres (commissural fibers), connect gyri within hemispheres (association fibers), or connect the cerebrum to the spinal cord (projection fibers). The corpus callosum is a major assemblage of commissural fibers that forms a nerve tract that connects the two cerebral hemispheres. Basal ganglia (basal nuclei) are several pockets of gray matter located deep inside the cerebral white matter. The major regions in the basal ganglia---the caudate nuclei, the putamen, and the globus pallidus---are involved in relaying and modifying nerve impulses passing from the cerebral cortex to the spinal cord. Arm swinging while walking, for example, is controlled here. The diencephalon connects the cerebrum to the brainstem. It consists of the following major regions: The thalamus is a relay station for sensory nerve impulses traveling from the spinal cord to the cerebrum. Some nerve impulses are sorted and grouped here before being transmitted to the cerebrum. Certain sensations, such as pain, pressure, and sensitivity to temperature, are also evaluated here. The epithalamus contains the pineal gland. The pineal gland secretes melatonin, a hormone that helps regulate the biological clock (sleep‐wake cycles). The hypothalamus regulates numerous important body activities. It controls the autonomic nervous system and regulates emotion, behavior, hunger, thirst, body temperature, and the biological clock. It also produces two hormones (antidiuretic hormone or ADH, and oxytocin) and various releasing hormones that control hormone production in the anterior pituitary gland. The following structures are either included or associated with the hypothalamus: The mammillary bodies relay information related to eating, such as chewing and swallowing. The infundibulum connects the pituitary gland to the hypothalamus. The optic chiasma passes between the hypothalamus and the pituitary gland. Here, portions of the optic nerve from each eye cross over to the cerebral hemisphere on the opposite side. The brainstem connects the diencephalon to the spinal cord. The brainstem resembles the spinal cord in that both consist of white matter fiber tracts surrounding a core of gray matter. The brainstem consists of the following four regions, all of which provide connections between various parts of the brain and between the brain and the spinal cord. (Some prominent structures of the brainstem regions are listed in Table 2 and illustrated in Figure 3, which also illustrates the relationship of the cranial nerves to the brainstem.) **Structure and functions of neuron** **Neurons** Behavior and mental processes result from activities in the body's nervous system and other physiological systems. Structure. The basic unit of the nervous system is a cell known as the neuron (Figure 1). It is estimated that the nervous system contains over 11 billion neurons. The neuron, which is covered by a cell membrane, consists of Dendrites, branched appendages that carry information to the cell body A cell body (soma), which contains the nucleus An axon, which conveys information away from the cell body **. Neurons are surrounded by glial cells, which n** Some axons are covered with a myelin sheath (interspersed with spaces called nodes of Ranvier), which aids in neural transmission nourish the neurons and hold them in place; these cells are the basis of the myelin sheaths. Axon terminals are branched and contain terminal buttons, tiny swellings that in turn contain synaptic vesicles (Figure 2). Synaptic vesicles are filled with chemicals called neurotransmitters, which assist in transmission of information to other neurons. **Types of neurons.** There are three types of neurons: **Sensory neurons** are located in the body's sense organs (for example, the eye, ear, or nose) and send information from these organs to the brain. **Motor neurons** convey information from the nervous system to the body's organs, glands, and muscles. Interneurons (association neurons) transmit information from one neuron to another within the nervous system. **Neural Transmission** The function of a neuron is to transmit information within the nervous system. Neural transmission occurs when a neuron is activated, or fired (sends out an electrical impulse). Activation (firing) of the neuron takes place when the neuron is stimulated by pressure, heat, light, or chemical information from other cells. (The type of stimulation necessary to produce firing depends on the type of neuron.) The fluid inside a neuron is separated from that outside by a polarized cell membrane that contains electrically charged particles known as ions. When a neuron is sufficiently stimulated to reach the neural threshold (a level of stimulation below which the cell does not fire), depolarization, or a change in cell potential, occurs. **Potentials**. The term potential refers to a difference in electrical charges. Neurons have two types of potentials, a resting potential and an action potential. The neural threshold must be reached before a change from resting to action potential occurs (Figure 1). **Transmission of a Nerve Impulse** **Resting potential** is the potential maintained by the inactive neuron. When unstimulated, a neuron is like a small battery and has a measurable negative electrical charge (about 70 millivolts) called the resting potential. **Action potential** is the potential produced when appropriate stimulation is high enough to reach the neural threshold and causes the neuron to fire, that is, alters the membrane permeability. Alteration of membrane permeability (polarization) allows a change of electrical charges (negative to positive) that runs along the entire cell membrane. The neuron then returns to its resting electrical state, the resting potential, until stimulated again. The rate of neural transmission, however, is independent of the level of stimulation. That is, if the neural threshold is not reached, the neuron will not fire. If the threshold is reached or exceeded, the amplitude of the action potential is the same regardless of the level of stimulation. The relationship between the level of stimulation and the production of a neural impulse is called the all or none principle. Once triggered, the action potential continues the length of the axon without diminishing because the action potential depends upon cell membrane permeability, a cell characteristic, and not upon the strength of the triggering stimulus. After the action potential occurs, however, there is a short period of refractoriness, which affects neuron firing. During the first part of the refractory period (the absolute refractory period), the neuron will not fire again no matter how great the stimulation. The absolute refractory period lasts for only a short time. It is followed by the relative refractory period, during which a stronger than usual stimulus is required to trigger the action potential before the neuron returns to resting state. After the refractory period, the neuron will fire when the neural threshold is reached. **Synaptic transmission**. The synapse is the name given the junction between neurons where information is exchanged. The action potential causes information to be transmitted from the axon of the first neuron (presynaptic neuron) to the dendrites or cell body of the second neuron (postsynaptic neuron) by secretion of chemicals called neurotransmitters. Neurotransmitters are stored in small containers (vesicles) located in knoblike structures (terminal buttons) on the axon tips. The axon of the presynaptic neuron does not actually touch the dendrites of the postsynaptic neuron and is separated from them by a space called the synaptic cleft. Stimulation of the presynaptic neuron to produce an action potential causes the release of neurotransmitters into the synaptic cleft. Most of the released neurotransmitters bind with molecules at special sites, receptors, on the dendrites of the postsynaptic neuron. (Molecules of the neurotransmitter that do not bind to receptors in the postsynaptic neuron are taken up again by the presynaptic neuron, a process called reuptake). The combination of the neurotransmitter molecules to receptor cell molecules in the postsynaptic cell membrane produces a change of potential in the postsynaptic cell membrane called the postsynaptic potential (PSP). The PSP allows ions to enter or leave the cell membrane of the postsynaptic neuron. The ionic movements increase or decrease the probability of a neural impulse occurring in the postsynaptic neuron. There are two types of PSPs, excitatory (EPSPs) and inhibitory (IPSPs). EPSPs increase and IPSPs decrease the likelihood that the postsynaptic neuron will fire a neural impulse. The rate of firing of a neuron at a particular time depends upon the relative number of EPSPs and IPSPs. **Neurotransmitters**. Many drugs, both therapeutic and recreational, work by affecting the level of neurotransmitters (the chemicals released at the axon terminal buttons of the presynaptic neuron), and some disorders are associated with neurotransmitter deficiencies or excesses. Neurotransmitters are of several types: **Acetylcholine** occurs throughout the nervous system and is the only neurotransmitter found in synapses between motor neurons and voluntary muscle cells. (Degeneration of cells producing acetylcholine is associated with Alzheimer's disease.) **Biogenic amines include three neurotransmitters: norepinephrine, dopamine, and serotonin.** Parkinson's disease is believed to be related to a deficiency of dopamine; certain types of depression are associated with low levels of norepinephrine; levels of serotonin increase with the use of the recreational drug LSD (lysergic acid diethylamide). **GABA** (gamma aminobutyric acid) appears to produce only inhibitory PSPs. Many tranquilizers work by increasing the inhibitory actions of GABA. **Glycine** is an inhibitory neurotransmitter found in the lower brainstem, spinal cord, and retina. **Endorphins** modulate the activity of other neurotransmitters and are called neuromodulators. They seem to function in the same way as opiates such as morphine; "runner's high" is produced by an increase in endorphins. Substance P is a neurotransmitter in many neural circuits involving pain. **Peripheral Nervous system** The nervous system is the controlling system of the body and is composed of nerve cells and organs. It is further classified into the central nervous system and the peripheral nervous system. The central nervous system comprises the brain and the spinal cord. The peripheral nervous system comprises the network of nerves connected to the brain and the spinal cord. Table of Contents Peripheral Nervous System Somatic Nervous System Autonomic Nervous System Peripheral Nervous System Functions Nerve fibers are of two types -- afferent fibers and efferent fibers. The afferent nerve fibers are responsible for the transmission of impulses from the tissues to the central nervous system while the efferent nerve fibers are responsible for the transmission of impulses from the central nervous system to the concerned tissues or organs. Let us study the peripheral nervous system, its definition, types, diagram and functions in detail. The peripheral nervous system has two divisions: **Somatic Nervous System** **Autonomic Nervous System** **Somatic Nervous System** The main function of the somatic nervous system is to transfer impulses from CNS to skeletal muscles. It consists of **1.Cranial Nerves** **2. Spinal Nerves** Cranial nerves are 12 pairs and they emerge from the brain. Some of the examples of cranial nerves are optic, olfactory, etc. Spinal nerves have their point of emergence as the spinal cord. There are 31 pairs of spinal nerves. They emerge from the spinal cords into dorsal and ventral roots. At the junction of these two roots, the sensory fibres continue into the dorsal root and the motor fibres into the ventral root. Autonomic Nervous System The autonomic nervous system relays impulses from the central nervous system to the involuntary organs and smooth muscles of the body. It is divided into two parts -- **Sympathetic Nervous System** **Parasympathetic Nervous System**  The **sympathetic nervous system** consists of nerves arising from the spinal cord between the neck and waist region. It prepares the body for violent actions against abnormal conditions and is generally stimulated by adrenaline. The **parasympathetic nervous system** is located anterior in the head and neck and posterior in the sacral region. It is mainly involved in the re-establishment of normal conditions when violent action is over. **Peripheral Nervous System Functions** Following are the important functions of the peripheral nervous system: The peripheral nervous system connects the brain and the spinal cord to the rest of the body and the external environment. It regulates internal homeostasis. It can regulate the strength of muscle contractility. It controls the release of secretions from most exocrine glands.