Nervous Tissue and the Spinal Cord

Lecture 9

Neurons – nerve cells  Universal properties of neurons o ______– all neurons have the ability to respond to environmental changes o Conductivity – Neurons produce traveling electrical signals that quickly reach other cells at ______locations o Secretion – when the electrical signal reaches the end of a nerve fiber, the neuron usually secretes a chemical called a ______that stimulates the next cell  Functional classes of neurons o Sensory (afferent) neurons – neurons specialized to detect ______such as light, temperature, or pressure and to transmit information about them towards the CNS o Interneurons (association neurons) – neurons located entirely within the brain or spinal cord . These neurons carry out the integrative function of the nervous system – they process, store and ______information, and “make decisions” about how the body responds to stimuli o Motor (efferent) neurons – neurons that send signals to muscle or gland cells, sending signals ______the central nervous system  Structure of a neuron o Soma (cell body) – control center of the neuron . Has a ______and cytoplasm containing organelles like other cells o Dendrites - processes of a neuron that receives information from other cells or from environmental stimuli . They conduct signals ______the soma . Dendrites are usually shorter, more branched, and more numerous than the axon o Axon – process of a neuron that conducts action potentials ______the soma . Each neuron has only one axon . The ______end of an axon usually has terminal arborization  Terminal arborization is an extensive complex of fine branches . Each branch ends in a synaptic knob  Synaptic knob (terminal button) o Swollen tip of the distal end of an axon; the site of synaptic ______and neurotransmitter release  Neuronal variety o Multipolar neurons – Have one axon and ______or more dendrites . This type is common in the brain and spinal cord o Bipolar neurons – have one axon and one dendrite . Includes ______cells and sensory cells of the inner ear o Unipolar neurons – have only one process leading away from the soma . Also called pseudounipolar neurons . Includes neurons that carry signals to the ______o Anaxonic neurons – have multiple dendrites but no axon . This type is found in the brain and ______

Supportive Cells  Types of Neuroglia o Oligodendrocytes – Forms a myelin sheath around nerve fibers in the ______nervous system o Astrocytes – form the blood-brain barrier that controls which substances are able to get from the bloodstream into the ______tissue o Ependymal Cells – ______cerebrospinal fluid and help to circulate cerebrospinal fluid o Microglia – develop from ______blood cells and phagocytize dead nervous tissue, microorganisms, and other foreign matter o Schwann Cells – envelop peripheral nervous system fibers with myelin and assist in the regeneration of damaged fibers  Myelin o Insulating layer around a nerve fiber o Formed by oligodendrocytes in the CNS and Schwann cells in the PNS o Axons are covered in segments . Gaps between segments are called ______of Ranvier . Myelin-covered areas between nodes of Ranvier are called internodes  Myelin and signal conduction o In an unmyelinated nerve fiber, the signal spreads by diffusion of sodium and potassium ions through the plasma membrane at every point along the fiber . The ion movement creates a sudden voltage change called an action potiential at each point . Each action potential triggers another one just ______of it . The nerve signal consists of a wave of action potentials traveling down the axon . This signal travels at about .5 to 2 m/sec o In a myelinated nerve fiber, the ion movements through the membrane occur only at the ______of Ranvier (gaps between segments of myelin) . In the internodes (myelin covered portions), signals travel by a much faster process of ion diffusion along the length of the nerve fiber immediately under the plasma membrane . Since most of the fiber is covered with myelin, the signal can travel as fast as ______m/sec

Synapses and Neural Circuits  Synapses – the meetings between ______and any other cells o Chemical synapses – junctions in which the presynaptic neuron releases a neurotransmitter to stimulate the ______cell . At a chemical synapse, a terminal branch of a presynaptic fiber ends in a swelling called the synaptic ______. Between the synaptic knob and the next cell there is a 20-40 nm gap called the synaptic ______. A nerve signal arrives at the end of the presynaptic neuron and triggers the release of neurotransmitters that either excite or inhibit the postsynaptic cell o Electrical synapses – junctions in which adjacent cells are joined by gap ______. Ions diffuse directly from one cell to the next for quick transmission

The Spinal Cord  Functions o Conduction – the ______contains fibers that conduct information up and down the body . It enables sensory information to reach the brain . It enables motor commands to reach the receptors . Input received at one level of the spinal cord can affect output at ______level o Locomotion – the simple repetitive muscle contractions that put one foot in front of another are controlled by central pattern generators in the spinal cord . The spinal cord does not control the ______or direction of locomotion (those are under control of the motor neurons in the brain) o Reflexes – the spinal cord is responsible for involuntary stereotyped responses to stimuli  Surface Anatomy o ____ pairs of spinal nerves over five regions . 8 Cervical (C1-C8) . 12 Thoracic (T1-T12) . 5 Lumbar (L1-L5) . 5 Sacral (S1-S5) . 1 Coccygeal o Enlargements . The diameter of the spinal cord is relatively constant except for the cervical ______and lumbar enlargement o Conus medullaris . Location in which the cord tapers to a ______below the lumbar enlargement o Cauda equina . Bundle of nerves resembling a horse’s tail that innervates the pelvic organs and lower ______ Meninges of the Spinal Cord o Dura mater – outermost meninx that forms a dural sheath around the spinal cord . Epidural space is found between the sheath and the vertebral bone  It is a space filled with blood vessels, loose connective tissue, and adipose tissue  It is a site where anesthetics are sometime introduced to block ______signals o Arachnoid- middle meninx that adheres to the inside of the dura mater composed of a loose mesh of collagenous and elastic fibers . Subarachnoid space is the gap between the arachnoid and the pia mater  Filled with cerebrospinal ______. Lumbar cistern is a (subrachnoid) space occupied by the cauda equina below the medullary cone o Pia mater – innermost layer of the meninges that closely follows the contours of the spinal cord  Cross-Sectional Anatomy o Gray matter – has a dull color because it contains very little myelin . Contains the somas, dendrites, and proximal parts of the axons of neurons o White matter – has a pearly white color because it contain ______. Composed of axons that carry signals from one part of the CNS to another  Spinal Tracts o Ascending tracts – those that carry signals ___ the spinal cord o Descending tracts – those that carry motor signals ______the brainstem and spinal cord The Spinal Nerves  General Anatomy of Nerves and Ganglia o Nerve – cordlike organ composed of axons bound together by ______tissue . Mixed nerve – consists of both sensory and motor fibers and transmits signals in ______directions (but any one fiber transmits in only one direction) . Sensory nerve – consists of sensory axons, including those of the olfactory and optic nerves . ______nerve – consists of motor fibers only  Many motor nerves are actually mixed nerves because they also carry sensory signals from muscles back to the CNS o Ganglion – a ______of cell bodies outside the CNS (resembling a knot).  Spinal Nerves o Proximal Branches . Dorsal root- ______signals . Ventral root- efferent signals o Distal Branches . Dorsal ramus – innervates the muscles and joints in the region of the spine the nerve is in. It also innervates and the skin of the ______. Ventral ramus – innervates the ventral and lateral skin and muscles of the trunk and gives rise to the nerves of the ______ In the thoracic region, it forms the intercostal ______ In other regions, it forms nerve plexuses  Nerve Plexuses o Except in the thoracic region, the ventral rami form web-like nerve plexuses which carry signals from bones, joints, muscles, and the ______o Cervical Plexus (C1-C5) – . Great Auricular nerve (sensory nerve of skin of and around the ear) . Transverse ______nerve (sensory nerve of skin of ventral and lateral neck) . Ansa Cervicalis (motor nerve of omohyoid, sternohyoid, and sternothyroid) . ______nerve (motor nerve of the diaphragm) o Brachial Plexus (C5-T1) . Axillary nerve . Radial nerve . Musculocutaneous nerve . Median nerve . ______nerve o Lumbar Plexus (L1-L4) . Ilioinguinal nerve . Lateral femoral cutaneous nerve . ______nerve . Obturator nerve o Sacral Plexus (L4-S4) . Superior gluteal nerve . Inferior gluteal nerve . ______nerve  Tibial nerve  Common fibular (peroneal) nerve  Cutaneous Innervation and Dermatomes o Each spinal nerve receives sensory input from a specific area of skin called a ______o Dermatomes overlap at their edges by as much as 50%, so severing one sensory nerve root does not entirely deaden sensation from a dermatome Somatic Reflexes  Reflexes have 4 properties o They require stimulation – they are responses to sensory input o They are quick – they involve few if any interneurons and ______synaptic delay o They are involuntary – they occur without intent, often without our awareness, and they are difficult to suppress o They are ______– they occur in essentially the same way every time, in a predictable manner  Visceral vs. Somatic o Visceral reflexes are responses of ______muscle, cardiac muscle, or glands o Somatic reflexes are responses of skeletal muscle, such as the quick withdrawal of your hand from a hot stove . Somatic reflexes use simple neural pathways called reflex arcs that send signals from the sensory nerve ending to the spinal cord or brainstem and back to a skeletal muscle  Monosynaptic reflex arc – simplest type of reflex arc, consisting only of a sensory neuron and a motor neuron (with just ______synapse between neurons)  Polysynaptic reflex arc – reflex arc containing one or more association neurons  Ipsilateral reflex – CNS input and output are on the ______side of the body  Contralateral reflex – sensory input enters the spinal cord on one side of the body and the motor output leaves from the opposite side  Intersegmental reflex – Sensory signal enters the spinal cord at one level, and the motor output leaves the cord from a ______or lower level. o Example – stepping on something sharp influences trunk muscles that flex the waist, so that as the foot is lifted, the center of gravity is shifted, so that you don’t fall over