Lecture 7



Lecture 9

Nervous Tissue and the Spinal Cord

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

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