4- Nervous Tissue Part 1.pdf

Part 1 Nervous tissue Human Anatomy and Physiology II Summer semester 2024 Peripheral nervous system ❑ Nerve: is a bundle of axons with the associated connective tissue and blood vessels that lies outside the brain and spinal cord. ❑ Twelve pairs of cr...

Part 1 Nervous tissue Human Anatomy and Physiology II Summer semester 2024 Peripheral nervous system ❑ Nerve: is a bundle of axons with the associated connective tissue and blood vessels that lies outside the brain and spinal cord. ❑ Twelve pairs of cranial nerves emerge from the brain and thirty-one pairs of spinal nerves emerge from the spinal cord. ❑ Ganglia: are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord. ❑ Enteric plexuses: are networks of neurons located in the walls of organs of the gastrointestinal tract. ❑ The sensory receptor is a structure of the nervous system that monitors changes in the external or internal environment. Examples: touch receptors in the skin, photoreceptors in the eye, and olfactory receptors in the nose Peripheral nervous system (PNS) The PNS is divided into a somatic nervous system (SNS), an autonomic nervous system (ANS), and an enteric nervous system (ENS) (intestines). The Somatic nervous system (SNS) consists of: (1) Sensory neurons that convey information to the CNS from somatic receptors in the head, body wall, and limbs and from receptors of the special senses (vision, hearing, taste, and smell). (2) Motor neurons that conduct impulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part is voluntary. Peripheral nervous system ❑ The autonomic nervous system (ANS) consists of: (1) Sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs. (2) Motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. It consists of two branches, the sympathetic and parasympathetic divisions. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary. Functions of the nervous system 1. Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves. 2. Integrative function. The nervous system processes sensory information by analyzing it and making decisions for appropriate responses. 3. Motor function. Once sensory information is integrated, the nervous system make an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete hormones. Histology of Nervous tissue Nervous tissue have two types of cells: 1- Neurons: ✓ Most neurons are unable to divide. 2- Neuroglia: ✓ Support, nourish, and protect neurons. ✓ Continue to divide throughout life. ✓ Smaller than neurons, but they are more. 1- Neurons (Nerve cells) Possess electrical excitability, the ability to respond to a stimulus and convert it into an action potential. An action potential (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels. 1- Parts of the Neuron The receiving and the : input portions of a neuron (Cell body) 1- Neurons (Nerve cells) Nissle bodies Also called Nissl substance. They are granules in the cytoplasm of nerve cell bodies. They consist of aggregates of free ribosomes and rough endoplasmic reticulum. ** The ribosomes are the site of protein synthesis.  Axon hillock Cone-shaped elevation that joins the axon to the cell body. The part of the axon closest to the axon hillock is the initial segment. In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment (trigger zone) from which they travel along the axon to their destination. Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon. The cytoplasm of an axon, called axoplasm, is surrounded by a plasma membrane (axolemma). ▪ The axon ends by dividing into many fine processes called axon terminals ▪ The tips of some axon terminals swell into bulb-shaped structures called synaptic end bulbs Synapse The site of communication between two neurons or between a neuron and an effector cell. Pre-synaptic Post-synaptic Synapse ▪ Synaptic end bulbs contain many tiny membrane-enclosed sacs called synaptic vesicles that store a chemical called a neurotransmitter. ▪ A neurotransmitter is a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell. (Ex: Ach = Acetylcholine) ▪ Axonal transport that occurs in an anterograde (forward) direction moves organelles and synaptic vesicles from the cell body to the axon terminals. ▪ Axonal transport that occurs in a retrograde (backward) direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycle. Structural Classification of Neurons 1. Multipolar neurons usually have several dendrites and one axon. Found in: most of the brain and spinal cord neurons, and all motor neurons. 2. Bipolar neurons have one main dendrite and one axon. Found in the retina of the eye, the inner ear, and the olfactory area (smell) of the brain. 3. Unipolar neurons have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. These neurons are also called pseudounipolar neurons. Structural Classification of Neurons  Functional Classification of Neurons 1. Sensory or afferent neurons: convey sensory information (impulse) from sensory organs to CNS. Contain sensory receptors at their distal ends (dendrites). Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure. 2. Motor or efferent neurons: convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves. Motor neurons are multipolar in structure. 3. Interneurons or association neurons: are mainly located within the CNS between sensory and motor neurons. Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure. Functional Classification of Neurons 2- Neuroglia (glia): ▪ CNS glial cells: 1. Astrocytes 2. Microglia 3. Oligodendrocytes 4. Ependymal cells. ▪ PNS glial cells: 1. Schwann cells 2. Satellite cells. 2- Neuroglia (glia) of CNS 1. Astrocytes: The largest and the most common. Play a role in neuron's development. Aid in nutrition exchange. Help form the blood brain barrier. 2. Microglia: Immune cells for the nervous system, that act as macrophages (phagocytosis).  Blood brain barrier (BBB) ▪ Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from potentially harmful substances. ▪ Blood–brain barrier restricts the movement of substances between the blood and intersteial fluid 2- Neuroglia (glia) of CNS 3. Oligodendrocytes: Produce the myelin sheath around the neurons in the CNS. Each cell produces myelin for multiple axons. 4. Ependymal cells:: Line the cavities of the CNS. They have cilia and microvilli. Produce and circulate the cerebrospinal fluid (CSF). 2- Neuroglia (glia) of PNS 1. Schwann cells: Form Myelin sheath in the PNS. Help with regeneration. Each cell myelinate a single axon. 2. Satellite cells: Protects and cushion the neurons of the PNS. Surround the cell bodies not the axons.  Myelin sheath ❑Myelin sheath: lipid and protein in multiple layers, covering some axons to insulate them and increase the speed of nerve impulse conduction. ❑Axons surrounded by a myelin sheath, are said to be myelinated. ❑Axons without a covering are said to be unmyelinated. ❑Gaps in the myelin sheath, called Nodes of Ranvier, appear at intervals along the axon.  Remember! ✓ Ganglion is a cluster of neuronal cell bodies located in the PNS. ✓ Nucleus is a cluster of neuronal cell bodies located in the CNS ✓ Nerve is a bundle of axons that is located in the PNS ✓ Tract is a bundle of axons that is located in the CNS  Remember! 1.White matter is composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. 2.Gray matter contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies have a gray color and there is little or no myelin in these areas Grey matter = butterfly H shape The End