Nervous System: Tissue

Question 1

What is nervous tissue made of?

Answer 1

• Neurons and glial cells

Question 2

What is the CNS?

Answer 2

• Includes the brain and spinal cord
• The brain is protected and enclosed by the skull
• The spinal cord is protected and enclosed within the vertebral canal

Question 3

What is the PNS?

Answer 3

• Includes cranial nerves (extend from brain), spinal nerves (extend from spinal cord), and ganglia (clusters of neuron cell bodies located out of the CNS)

Question 4

What do sensory and motor divisions of the nervous system do?

Answer 4

• Collect information through PNS receptors
• Process and evaluate information in which CNS determines if any response is required
• Initiate response to information by CNS producing motor output to effectors

Question 5

What are PNS receptors?

Answer 5

• Collect information
• The dendrite endings of sensory neurons or separate cells
• Detect changes in internal or external environment (stimuli) and pass them on to the CNS sensory input

Question 6

What is CNS motor output?

Answer 6

• After selecting an appropriate response, CNS initiates specific nerve impulses
• They are rapid movements of an electrical charge along the neuron’s plasma membrane of an axon
• Travels through structures of the PNS to effectors

Question 7

What is the sensory nervous system?

Answer 7

• Or afferent nervous system
• Responsible for receiving sensory information from receptors and transmitting this information to the CNS
• Responsible for sensory input
• Contains both PNS and CNS components
• PNS nerves: transmit sensory information
• CNS nerves: in brain and spinal cord interpret the information
• Has 2 components: somatic sensory and visceral sensory

Question 8

What are the somatic sensory components?

Answer 8

• General somatic senses: touch, pain, pressure, vibration, temperature and proprioception (sensing position or movement of joints and limbs)
• Also special senses: taste, vision, hearing, balance and smell
• These functions are considered voluntary because we have some control over them and we tend to be conscious of them

Question 9

What are the visceral sensory components?

Answer 9

• Transmit nerve impulses from blood vessels and viscera to the CNS
• Visceral receptors detect chemical composition of blood or stretch an organ wall
• These functions are considered involuntary because most of the time you don’t have voluntary control over them and are not conscious of them
• You may become aware of visceral sensations when they are extreme (eaten too much and your stomach is bloated)

Question 10

What is the motor nervous system?

Answer 10

• Also known as efferent; conducting outward
• Responsible for transmitting motor impulses from the CNS to effectors (muscles or glands)
• Responsible for output of nerve impulses, transmitted from CNS
• Has CNS and PNS components
• Divided into somatic motor and autonomic motor components

Question 11

What is the somatic motor component?

Answer 11

• Conducts nerve impulses from the CNS to the skeletal muscles, causing them to contract
• Is the voluntary nervous system, because the contractions of the skeletal muscles are under conscious control

Question 12

What is the autonomic motor component?

Answer 12

• Is the autonomic nervous system, because it innervates internal organs and regulates smooth muscle, cardiac muscle and glands without our control
• Also known as the visceral motor system or involuntary nervous system

Question 13

What are neurons?

Answer 13

• The basic structural unit of the nervous system
• Neurons conduct nerve impulses from one part of the body to another
• The have: high metabolic rates, extreme longevity, nonmitotic (unable to divide and produce new neurons), excitable and conductive

Question 14

What is the cell body? (neuron)

Answer 14

• Also called a soma
• Serves as the neurons control center and is responsible for receiving, integrating and sending nerve impulses
• Inclosed by a plasma membrane and contains cytoplasm surrounding the nucleus

Question 15

What is the nucleolus? (neuron)

Answer 15

• Within the nucleus

- Reflects the high metabolic activity of neurons, which require the production of many proteins

Question 16

WHat do the mitochondria do? (neuron)

Answer 16

• Numerous are present within the cytoplasm to produce large amounts of ATP needed by the neuron

Question 17

What is chromatophilic substance? (neuron)

Answer 17

• Both free and bound ribosomes

- Together, with dendrites and cell bodies, account for the gray colour of gray matter

Question 18

What are dendrites? (neuron)

Answer 18

• Tend to be shorter, smaller processes that branch off the cell body
• Some neurons have only one dendrite , whereas others have many
• Conduct information as electrical signals from other cells toward the cell body
• Receive input and transfer it to the cell body for processing
• The more dendrites a neuron has, the more info that neuron can receive from other cells
• Some dendrite surface areas are increased by small knoblike protuberances called dendritic spines
• The more surface area a neuron has, the more interactions it will have with other cells

Question 19

What is the axon? (neuron)

Answer 19

• Typically a longer nerve cell process emanating from the cell body to make contact with other neurons, muscle cells or gland cells
• Neurons either have one, or none at all
• Neurons without axons are called anaxonic (found in CNS as interneurons)
• Transmits nerve impulses away from the cell body, transmitting output info to other cells

Question 20

What is the axon hillock? (neuron)

Answer 20

• Triangular region of the cell body
• Where the axon extends from
• A devoid of chromatophilic substance

Question 21

What is a axon collateral? (neuron)

Answer 21

• Side branches of the axon
• Branch extensively at the distal end into an array of fine terminal extensions called terminal arborizations
• The tips of terminal arborizations have expanded regions called synaptic knobs

Question 22

What is perikaryon? (neuron)

Answer 22

• Cytoplasm within the cell body

Question 23

What are neurofibrils? (neuron)

Answer 23

• The aggregation of neurofilaments (form bundles)
• Extend as a complex network into both the dendrites and axons, where their tensile strength provides support for those processes
• Can be described as neuron structures

Question 24

What are unipolar neurons?

Answer 24

• Have a single, short process that emerges from the cell body and branches like a ‘T’
• Start out as bipolar neurons during development, but their 2 processes fuse into one
• The combined peripheral process and central process denote the axon because they generate and conduct impulses and are myelinated
• Most sensory neurons of the PNS are unipolar

Question 25

What are bipolar neurons?

Answer 25

• Have 2 processes that extend from the cell body: one axon and one dendrite
• These tend to be uncommon in humans and primarily limited to some of the special senses
• Located in the olfactory epithelium of the nose and retina of the eye for examples

Question 26

What are multipolar neurons?

Answer 26

• The most common type of neuron
• Have multiple processes, many dendrites and a single axon
• Examples include motor neurons that innervate muscle and glands

Question 27

What are sensory neurons?

Answer 27

• Also known as afferent neurons
• Transmit nerve impulses from sensory receptors to the CNS
• Specialized to detect changes in their environment called stimuli (touch, pressure, heat, light or chemicals)
• Most are unipolar although a few are bipolar
• The cell bodies of unipolar sensory neurons are located outside of the CNS and housed in posterior root ganglia

Question 28

What are motor neurons?

Answer 28

• Also known as efferent neurons
• Transmit nerve impulses from the CNS to muscles and glands
• Most extend to muscle cells and nerve impulses they transmit cause those cells to contract
• The muscle and gland cells that receive nerve impulses from motor neurons are called effectors (their stimulation produces a response or effect)
• The cell bodies of most motor neurons lie in the spinal cord, whereas their axons primarily travel in cranial or spinal nerves to muscles and glands
• All motor neurons are multipolar

Question 29

What are interneurons?

Answer 29

• Also known as association neurons
• Lie entirely within the CNS and are multipolar structures
• Receive nerve impulses from many other neurons and carry out that integrative function of the nervous system
• They retrieve, process and store information and decide how the body responds to stimuli
• Facilitate communication between sensory and motor neurons
• It is estimated that 99% of our neurons are interneurons
• The number of interneurons involved during processing and story information, increases dramatically with the complexity of the response

Question 30

What are glial cells?

Answer 30

• Sometimes referred to as neuroglia
• Located within both the CNS and PNS
• Differ from neurons in that they are smaller and capable of mitosis
• Do not transmit nerve impulses, but do assist neurons with their functions
• They physically protect and help nourish neurons and provide an organized, supporting framework for all nervous tissue
• During development, glial cells form the framework that guides young migrating neurons to their destinations
• Also play a role in learning and memory through their interaction with synapses between neurons
• Collectively, glial cells account for roughly half the volume of the nervous system

Question 31

What are the glial cells of the CNS?

Answer 31

• 4 types: astrocytes, ependymal cells, microglial cells and oligodendrocytes
• Can be distinguished by the basis of size, intercellular organization and the presence of specific cytoplasmic processes
• All glial cells except for microglia are derived from neural ectoderm
• Microglia are derived from stem cells within red bone marrow that become monocytes (a type of white blood cell)

Question 32

What are astrocytes? (CNS glial)

Answer 32

• Exhibit a starlike shape due to many projections from their surface
• The numerous cell processes touch both capillary walls and different parts of neurons
• Most abundant glial cell in the CNS and constitute over 90% of the nervous tissue in some areas of the brain
• Subpopulations are fibrous and protoplasmic astrocytes
• Help form the blood-brain barrier (perivascular feet wrap around and cover the outer surface of the brain capillaries)
• Regulate tissue fluid composition
• Replace damaged neurons
• Assist neural development
• Help regulate synaptic transmission (2 way communication between astrocytes and neurons at the synapse)
• Change synapse numbers

Question 33

What is the blood-brain barrier? (astrocyte)

Answer 33

• Strictly controls substances entering the nervous tissue in the brain from the blood
• Protects the delicate brain from toxins
• Allows needed nutrients to pass through
• Sometimes it is detrimental when some meds cannot exit capillaries and enter the nervous tissue of the brain

Question 34

What are ependymal cells? (CNS glial)

Answer 34

• Cuboidal epithelial cells that line the internal cavities of the brain and the central canal of the spinal cord
• Have slender processes that branch extensively to make contact with other glial cells in the surrounding tissue
• These cells and blood capillaries together form a network called the choroid plexus, which produces cerebrospinal fluid that bathes the CNS and fills its cavity
• Ependymal cells have cilia on their apical surfaces that help circulate the fluid

Question 35

What are microglial cells? (CNS glial)

Answer 35

• Represent the smallest percentage of CNS glial cells
• Typically small cells that have slender branches extending from the main cell body
• Wander through the CNS and replicate in response to infection
• Perform phagocytic activity and remove debris from dead or damaged nervous tissue (janitors)

Question 36

What are oligodendrocytes? (CNS glial)

Answer 36

• Large cells with a bulbous body and slender cytoplasmic processes
• Their processes ensheath portions of many different axons, each repeatedly wrapping around part of an axon like electrical tape around a wire (myelin sheath)
• Can myelinate 1 mm length portion of many axons, not just one
• The cells successive plasma membrane layers form the myelin sheath

Question 37

What are satellite cells? (PNS glial)

Answer 37

• Flattened cells arranged around neuronal cell bodies in ganglia
• Physically separate cell bodies in ganglion from their surrounding interstitial fluid
• Regulate the continuous exchange of nutrients and waste products between neurons and their environment

Question 38

What are neurolemmocytes? (PNS glial)

Answer 38

• Also called schwann cells
• Associated with PNS axons
• Are elongated, flattened cells that wrap around the axons of the PNS, insulating the axon and forming a myelin sheath
• Neurilemma is used to describe the delicate, thin, outer membrane of the neurolemmocyte
• The overlapping inner layers of plasma membrane form the myelin sheath in these cells
• These cells can only myelinate 1 mm length portion of a single axon only

Question 39

What is myelin sheath/myelination?

Answer 39

• The process by which part of an axon is wrapped with a myelin sheath, the insulating covering around the axon consisting of concentric fibers of myelin
• CNS: formed by oligodendrocytes
• PNS: formed by neurolemmocytes
• Myelin consists of the plasma membranes of these glial cells and contain a large proportion of fats and a lesser amount of proteins
• The high lipid content of myelin sheath gives the axon a distinct, glossy, white appearance and serves to effectively insulate it

Question 40

What are unmyelinated axons?

Answer 40

• PNS: associated with a neurolemmocyte, but no myelin sheath covers them
• CNS: not associated with oligodendrocytes

Question 41

What are neurofibril nodes?

Answer 41

• Small spaces that interrupt the myelin sheath on the axon
• These nodes can change the voltage across the plasma membrane and result in the movement of a nerve impulse
• Nerve impulses seem to “jump” from node to node along the axon (saltatory conduction) and this only occurs in myelinated axons
• In unmyelinated axons, continuous conduction occurs across the whole axon

Question 42

Why do myelinated axons produce a faster nerve impulse?

Answer 42

• Only the exposed membrane regions are affected as the impulse moves toward the end of the axon (this is rapid and sends the impulse quickly to skeletal muscles and limbs)
• In an unmyelinated axon, the nerve impulse takes longer because every part of the membrane must be affected by the voltage change (conduct nerve impulses from pain and some cold stimuli)

Question 43

What is PNS axon regeneration?

Answer 43

• These axons are vulnerable to cuts, crashing injuries and other types of trauma
• A damaged axon can regenerate if the cell body remains intact and a critical amount of neurilemma remains
• Its success of regenerating depends on: amount of damage and the distance between the site of the damaged axon and the structure it innervates

Question 44

What are nerves?

Answer 44

• Cable like bundles of parallel axons
• Has 3 successive connective tissue wrappings: endoneurium, perineurium and epineurium
• Components of the peripheral nervous system

Question 45

What is endoneurium? (nerve)

Answer 45

• An individual axon in a myelinated neuron is surrounded by neurolemmocytes and then wrapped in endoneurium
• A delicate layer of areolar connective tissue that separates and electrically isolates each axon
• Within this connective tissue layer are capillaries that supply each axon

Question 46

What is perineurium? (nerve)

Answer 46

• Groups of axons are wrapped into separate bundles called fascicles by a cellular dense irregular connective tissue layer caller perineurium
• This layer supports blood vessels supplying capillaries within the endoneurium

Question 47

What is epineurium? (nerve)

Answer 47

• All of the fascicles are bundled together by a superficial connective tissue covering called epineurium
• This thick layer of dense irregular connective tissue encloses the entire nerve
• Provides both support and protection to the fascicles within the layer

Question 48

What are synapses?

Answer 48

• Axons terminate as they contact other neurons, muscle cells or gland cells at specialized junctions called synapses, where the nerve impulse is transmitted to the other cell
• As the axon approaches the cell onto which it will terminate, it generally branches repeatedly into several unmyelinated terminal arborizations
• The synaptic endings normally have swellings called synaptic knobs at the end of each axon branch
• A typical synapse in the CNS consists of the association between a presynaptic and postsynaptic neuron at a space where their plasma membranes are separated by a space called the synaptic cleft
• 3 common types: axodendritic, axosomatic, axoaxonic

Question 49

What are presynaptic/postsynaptic neurons?

Answer 49

• Pre: transmit nerve impulses through their axons towards a synapse
• Post: conduct nerve impulses through their dendrites and cell bodies away from the synapse