Nervous Tissue Lecture PDF

Nervous tissue Specialised to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus Controls and integrates the activity of different parts of the body Is made up of specialised nerve cells called neurons and supporting cell...

Nervous tissue Specialised to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus Controls and integrates the activity of different parts of the body Is made up of specialised nerve cells called neurons and supporting cells called neuroglia Neurons are easily stimulated and transmit electrical impulses very rapidly Nervous tissue makes up the nervous system  Nervous system Anatomically divided into: – Central nervous system (CNS) – Peripheral nervous system (PNS) Central nervous system: Collects information from all areas of the body, process information and sends nerve impulses that control all bodily movements Includes brain and spinal cord Peripheral nervous system: Connects CNS to the rest of the body Includes cranial nerves, spinal nerves and associated ganglia  Nervous system - Functional classification Somatic and autonomic nervous system Somatic nervous system – regulates body activities that are under conscious control, such as the movement of skeletal muscles – Composed of somatic parts of the CNS and PNS, provides sensory and motor innervation to all parts of the body, except the viscera in the body cavities, smooth muscle, and glands Autonomic nervous system – regulates activities that are involuntary, or not under conscious control (organs, glands and smooth muscles) – When you start to run, the autonomic nervous system speeds up your heart rate and blood flow to the skeletal muscles, stimulates the sweat glands, and slows down the contractions of smooth muscles in the digestive system  Cells of nervous system Neurons and neuroglia - Neuron: Is the structural and functional unit of nervous system Transmits electrical impulses Size varies from 4-20 microns Has a cell body and processes (axon and dendrite) - Neuroglia: Do not transmit electrical impulses Support and protect neurons  Classification of Neurons Based on Morphology: depending on the number of processes (neurites) Multipolar neuron Bipolar neuron Pseudounipolar neuron Based on function: Motor neuron Sensory neuron Pseudounipolar Interneuron Functional classification of neurons: based on type of information & direction of information transmission - Motor (efferent) neurons Transmit motor information from the CNS to effectors (muscles/glands) in the periphery of the body All are multipolar neurons - Sensory (afferent) neurons Transmit sensory information from receptors of PNS towards the CNS Most of the neurons are pseudounipolar and few are bipolar - Interneurons Transmit information between neurons within the CNS, connect sensory and motor neurons, communicate with each other Are all multipolar neurons and most common type of neuron  Multipolar neuron Situated in the brain, spinal cord, sympathetic and parasympathetic ganglia Has a cell body, one axon and many dendrites Cell body (soma or perikaryon): Star shaped or irregular shaped Has a cell membrane Has cytoplasm Has a nucleus  Cytoplasm contains cell organelles like Golgi apparatus, mitochondria Also contains Nissle granules (bodies) and neurofibrils Nissle granules are rough endoplasmic reticulum, responsible for protein synthesis Neurofibrils are thread like structures give support to the cell body and processes of neuron Part of cell body which lies adjacent to the initial part of axon is called axon hillock, lacks Nissle granules  Dendrites: Short processes, many in number Branch profusely and thickness diminishes as these divide Surface is not smooth, but it bears many small spine-like projections for making synaptic contacts with the axons of other nerve cells Contain Nissle granules Carry nerve impulses towards the cell body Axon: One long process Carry nerve impulse away from the cell body Contains all the cell organelles except Nissle granules Axon terminates by dividing into branches called telodendria  Telodendria end by supplying the muscles/gland or end by synapsing with the dendrites or cell bodies of other neurons The cell membrane of axon – axollema The cytoplasm of axon – axoplasm The axons are surrounded by Schwann cells or oligodendrocytes Part of the Schwann cell closely surrounding the axon forms myelin sheath  Outer part of Schwann cell is called neurilemma Part of axon covered by myelin sheath is called internode Part of axon not covered by myelin sheath is called node of Ranvier These neurons are called myelinated neurons Myelin sheath is composed of phosopholipids Unmyelinated axon Unmyelinated neurons – without myelin sheath  Bipolar neuron: Situated in the retina of eyeball, spiral ganglia and vestibular ganglia of inner ear Has – A cell body – One axon – One dendrite Pseudounipolar neuron: Situated in the spinal (dorsal root) ganglia of spinal nerves Has a cell body Has one dendrite and one axon which are united together initially and then separate  Neuroglia (glial cells) The cellular connective tissue of nervous system Types of cells of neuroglia are – Astrocytes Schwann cells Oligodendrocytes Microglia Ependymal cells Satellite or capsular cells  Astrocytes: Star shaped cells with multiple processes -2 types of astrocytes Protoplasmic astrocytes – found in the gray matter Fibrous astrocytes – found in the white matter Create supportive framework for neurons Create “blood-brain barrier” Monitor & regulate interstitial fluid surrounding neurons Microglia: Smallest of the neuroglia, present both in gray and white matter Act as scavenger cells Show phagocytic activity in trauma or other destructive lesions of CNS  Schwann cells: Responsible for forming myelin sheath in the peripheral nerves Passage of nerve impulse is faster due to the presence of myelin sheath Oligodendrocytes: Present both in grey and white matter Make up the three-fourth of the glial cells Responsible for forming myelin sheath around the axons within the brain and spinal cord May enclose several axons  Ependymal cells: Ciliated columnar cells Line the ventricles of brain and central canal of spinal cord Involved in secretion of cerebrospinal fluid Satellite or capsular cells: Satellite cell Surround the neurons of spinal and autonomic ganglia Support and protect the neurons of these ganglia  Nerve trunk Nerve is made up of group of axons (nerve fibers) outside CNS Nerve trunk is surrounded by connective tissue sheath called epineurium Nerve trunk contains bundles or fascicles of nerve fibres Epineurium Each fascicle is surrounded by a Endoneurium connective tissue sheath called perineurium Perineurium Individual nerve fiber in a fascicle is surrounded by a connective tissue sheath called endoneurium Connective tissue sheath carries blood capillaries in them Most of the nerve fibers are covered by a sheath of schwann cells A nerve fiber consists of an axon, its neurilemma and surrounding endoneurium Nerve Cross Section – slide 28 Nerve – longitudinal section -28  Cranial nerves: There are 12 pairs They take origin from the brain Leave the skull by passing through the foramina of skull All supply the structures in the head and neck region (except 10th cranial nerve) 10th cranial nerve (vagus) in addition to head and neck structures also supplies structures in the thorax and abdomen  Cranial nerves: Olfactory – 1st cranial nerve Optic – 2nd cranial nerve Oculomotor – 3rd cranial nerve Trochlear – 4th cranial nerve Trigeminal – 5th cranial nerve Abducent – 6th cranial nerve Facial – 7th cranial nerve Vestibulo-cochlear – 8th cranial nerve Glossopharyngeal – 9th cranial nerve Vagus – 10th cranial nerve Accessory – 11th cranial nerve Hypoglossal – 12th cranial nerve  Spinal nerves: There are 31 pairs of spinal nerves Attached to the spinal cord They are named according to the region Cervical spinal nerves (1st 8 nerves) Thoracic spinal nerves (next 12 nerves) Lumbar spinal nerves (next 5 nerves) Sacral spinal nerves (next 5 nerves) Coccygeal spinal nerve (last nerve)  Typical spinal nerve: Parts: roots, trunk and rami Roots: Each spinal nerve contains 2 roots – ventral and dorsal Spinal nerve is attached to the spinal cord by 2 roots Ventral root: Contains motor fibres These fibres leave the spinal cord and supply the muscles  Dorsal root of spinal nerve with spinal ganglion Dorsal ramus Spinal cord in the vertebral canal of spinal nerve Ventral ramus of spinal nerve Ventral ramus Grey and white of spinal nerve rami communicantes Sympathetic ganglion Ventral root of spinal nerve Lateral cutaneous Trunk of spinal nerve nerve Muscles Anterior cutaneous nerve Typical spinal nerve  Dorsal root: Contains sensory fibers which enter the spinal cord These fibers carry sensation from the skin, muscle, bone and joints Presents an enlargement – dorsal root (spinal) ganglion Ganglion contains pseudounipolar neurons  Dorsal and ventral roots terminate at the intervertebral foramen by joining each other to form trunk of spinal nerve Trunk of spinal nerve: Formed by the union of ventral and dorsal roots Located in the intervertebral foramen As it leaves the intervertebral foramen it terminates by dividing into ventral and dorsal rami  Dorsal ramus: Is the smaller division of spinal trunk It runs backwards Divides into medial and lateral branches Both supply the muscles and skin of the back  Ventral ramus: Larger division of spinal trunk Runs laterally and forwards Gives lateral cutaneous nerve which supplies the skin of lateral side of body (trunk) Gives muscular branches which supply the lateral and Anterior cutaneous front muscles of the body Lateral nerve Terminates as anterior cutaneous nerve cutaneous nerve which supplies the skin of front of body  Ventral rami of all spinal nerves are connected to the sympathetic ganglia by gray rami communicantes Ventral rami of all thoracic spinal nerves and 1st and 2nd lumbar spinal nerves are connected to sympathetic ganglia by gray and white rami communicantes Ventral rami of lower cervical spinal nerves also supply upper limb Ventral rami of lumbar and sacral nerves also supply lower limb  Ganglia Collection of neurons outside the central nervous system Ganglion is covered by a connective tissue capsule Ganglion contains cell bodies of neurons, nerve fibres, satellite cells and blood capillaries 2 types – sensory and autonomic  Sensory ganglia eg. Spinal ganglia Covered by a connective tissue capsule Contain cell bodies of pseudounipolar neurons Cell bodies of neurons – Are large and rounded – Are in groups – Are surrounded by a layer of flattened cells called satellite or capsular cells Contains bundles of nerve fibers Groups of cell bodies of neurons are separated by bundles of nerve fibers Pseudounipolar Contains few blood capillaries Nerve fibers neurons  Spinal ganglia Nerve fibres Cell body of pseudounipolar neuron apsule Blood capillary CB – cell bodies of pseudounipolar neurons Satellite cells Pseudounipolar Nerve neurons fibers  Autonomic ganglia eg. sympathetic and parasympathetic ganglia Contain cell bodies of multipolar neurons Cell bodies of neurons – Are small and irregularly shaped Capsule – Are scattered – Are surrounded by few flattened cells called satellite or capsular cells The cell bodies of neurons are separated by scattered nerve fibers Contains many blood capillaries Autonomic/ Sympathetic ganglia - slide 36 Cell body of multipolar neuron Nerve fibres – scattered  Concerned with innervation of involuntary structures Autonomic nervous system like heart, smooth muscle and glands Nerve fibers of autonomic nervous system begin at the brain and spinal cord and these fibers are distributed through peripheral nervous system (cranial and spinal nerves) Hypothalamus controls the autonomic nervous system Divided into 2 parts – sympathetic and parasympathetic  Sympathetic nervous system: – Prepares the body for an emergency – increases heart rate, constriction of peripheral blood vessels, raises blood pressure – Redistributes blood – less blood to the skin, intestine and more to brain, heart, skeletal muscle – Inhibits peristalsis of intestinal tract and closes the sphincters Parasympathetic nervous system: – Concerned with conserving and restoring energy – reduce heart rate, increase peristalsis of the intestine, increase glandular activity, open sphincters