Topic 9: CNS tissue Flashcards
What does the central nervous system occupy?
Brain and spinal cord occupy the cranium and the vertebral canal.
Peripheral nervous system
- outside the PNS
- includes CRANIAL NERVES (extend from brain and carry signals to and from the brain) and SPINAL NERVES (extend from the spinal cord and carry signals to and from the brain)
PNS: What are ganglia?
Clusters of neuronal cell bodies located outside of CNS.
Afferent signals
Sensory signals picked up by sensory receptors throughout body and are carried by nerve fibers of PNS to CNS
Efferent signals
Motor signals are carried from CNS by nerve fibers of the PNS to innervate muscles to contract and/or glands to secrete
What regions do the sensory input and motor output serve?
- Somatic body region: structures external to the ventral body cavity: outer tube (skin, skeletal muscle, axial bones) and appendicular bones and muscles
- Visceral body region: contains viscera within the ventral body cavity: inner tube (digestive tube, lungs, heart, spleen, bladder etc.) as well as smooth muscle and glands throughout
What is integration?
When the nervous system processes and interprets sensory input and makes decisions on what should be done then dictating effector organs to produce a motor output repsonse
Neurons
excitatory cells
Neuroglial cells
non-conducting cells that wrap around, nourish, insulate, and protect neurons
Cell body of the neuron
- 5-140 μm
- single nucleus surrounded by cytoplasm
- mostly in CNS but found in ganglia of PNS
Within cell body:
chromataphyllic bodies
AKA Nissl bodies
clusters of rough endoplasmic reticulum and free ribosomes
Within cell body:
neurofibrils
bundles of intermediate filaments which form a network between chromatophyllic bodies and prevent the cell from being pulled apart when subjected to tensile forces
Neuron processes;
dendrites
- Function as receptive sites for receiving signals from other neurons and transmitting the signal towards the cell body
- Extensively branched
Neuron processes:
axon
- One axon arises from the cone-shaped AXON HILLOCK
- Impulse generator and conductor
- Transmits impulses away from the cell body
What supports the axon structure?
Neurofilaments, actin microfilaments, and microtubules aid in AXONAL TRANSPORT of substances to and from the cell body
As axon diameter increases…
…the faster the impulse it carries because resistance decreases
Axon branching
- Branching is less frequent than dendrites
- Branches usually occur at terminus of axon, the AXON BRANCHES
- The end is called AXON TERMINAL, or end bulbs or boutons
Name of a nerve cell
neuron
Nerve fiber refers to…
…long axon of neuron
Nerve refers to…
…collection of parallel running nerve fibers in PNS
Nerve impulses
- Generated where the axon extends from the axon hillock
- Release neurotransmitters from axon terminal into extracellular space, SYNAPTIC CLEFT, and excite neurons or target organs
- Axon terminal of pre-synaptic neuron -> pre-synaptic membrane -> synaptic cleft -> post-synaptic membrane -> post-synaptic dendrite of post-synaptic neuron
* passing in one direction
Synapse
- Usually transmit info via chemical messengers
2. Most synapses in CNS function with both presynaptic and postsynaptic neurons
Synaptic vesicles
- On pre-synaptic side
- Membrane-bound sacs containing neurotransmitter
- High concentration of mitochondria at axon terminal, provide energy for secretion of neurotransmitter
Synaptic cleft
Separates the plasma membrane of the two neurons
Multipolar neurons
- Consists of more than 2 neurons
- Interneurons & motor neurons
- 99% of neurons in body
- Typically multiple dendrites, 1 axon
- Most are interneurons (association neurons) that conduct impulses within CNS
Bipolar neurons
- Possess 2 processes that extend from opposite sides of the cell body; 1 fused dendrite, 1 axon
- Found in some special sensory organs (inner ear, olfactory epithelium, retina of eye)
Unipolar (pseudounipolar) neurons
- 1 short process near neuron cell body
- This process divides into two longer branches: axon extends to CNS via CENTRAL PROCESS; DENDRITIC AXON extends peripherally to receptors via PERIPHERAL PROCESS
- Start as bipolar neurons and develop into pseudounipolar
- Found in sensory ganglia of PNS functioning as sensory neurons
- Common in dorsal root ganglia along spinal cord and sensory ganglia of cranial nerves
Sensory (afferent) neurons
- Impulse TOWARD CNS
2. Unipolar neurons
Motor (efferent) neurons
- Impulse AWAY from CNS to form junctions with muscles and glands
- Mostly multipolar
- Cell body is within CNS
Interneurons (association neurons)
- Confined to CNS
- Mostly multipolar
- A single neuron or a chain of neurons lie between motor and sensory neurons integrating sensory input and motor output
- 99.98 of neurons in body indicative of the amount information processed in CNS
3 characteristics of neurons
- Extreme longevity, live for a lifetime
- Most cannot divide; some neural stem cells are located in some areas of CNS and can divide
- High metabolic rate, high O2 and glucose consumption, dying within a minute of deprivation
Neuroglial (glial) cells
Provide support to neurons
General information about CNS neuroglial cells
- Smaller than neurons, but outnumber them 10:1
- Have branching processes and a central body
- Half of mass of brain
- Can divide mitotically
Ependymal cells
CNS
- Epithelium that lines central cavity of spinal cord and brain
- Fairly permeable layer between the cerebral spinal fluid (CSF) and tissue fluid that bathes the cells of CNS
- Cilia circulate CSF
Microglia
CNS
- Smallest and least abundant glial cell
- Elongated cell body with cell processes with many pointed projections
- Derived from WBC, MONOCYTES, migrate to brain during embryonic and fetal development and become phagocytes, macrophages of the CNS engulfing dead neurons and microorganisms
- Support maturation of synaptic clefts
Astrocyte
- Most abundant
- Bulbous ends of radiating processes cling to neurons or capillaries to extract glucose from glucose
- Regulate neurotransmitter levels by increasing uptake in regions of high neuronal activity
- Signal increased blood flow through capillaries in active regions of brain
- Regulate ions to influence ionic environment around neurons
- Help synapses form in developing neural tissue
- Produce molecules necessary for neural -development, i.e. BDTF, brain-derived trophic factor
- Propagate calcium signals that may be involved in memory
Unmyelinated axons in CNS
Thinnest axons in CNS and covered by the processes of glial cells, i.e. astrocytes
Oligodendrocytes
CNS
- Produces processes that insulate coverings, MYELIN SHEATHS that prevents leakage of electrical current from axon
- NODES OF RANVIER increase speed of impulse conduction
- Surround thicker axons
- Plasma membrane rolled in concentric layers around axon
- 1 oligodendrocyte for many axons
Multiple sclerosis
Attacks myelin around CNS axons
More prevalent in women but faster development in men
Satellite cells
PNS
Surround neuron cell bodies within ganglia
Schwann cells
PNS
- Actual cell wraps in concentric layers around axon to produce a tightly paced coil of membranes called MYELIN SHEATH
- NEURILEMMA is outermost, nucleated, cytoplasmic layer of Schwann cells
- Nodes of Ranvier
- Develop during fetal period and 1st year of post natal life
Thick axons in PNS
Thick axons are myelinated and conduct impulses faster than their thin unmyelinated counterparts
Thin axons in PNS
- Unmyelinated conducting impulses more slowly
- Surrounded by Schwann cells but do not form myelin sheath and can surround multiple axons
- Found in autonomic nervous system and some sensory fibers
Nerve fassicle
Groups of axons bound into bundles by connective tissue wrapping within a nerve
Nerve layers
- Each axon is surrounded by Schwann cells
- ENDONEURIUM layer of connective tissue that covers Schwann cell layer
- PERINEURIUM cover nerve fascicle
- EPINEURIUM tough fibrous sheath that surrounds nerve and associated blood vessels
Gray matter in spinal CNS
- Gray colored and surrounds hollow central cavity of CNS
- Forms butterfly-shaped region in spinal cord
- Site where neuron cell bodies are clustered
- Dorsal half contains cell bodies of interneurons
- Ventral half contains cell bodies of motor neurons
- Primarily composed of neuronal cell bodies, dendrites, short unmyelinated axons of interneurons and neuroglia
White matter of CNS
- Lacks neuronal cell bodies, predominantly myelinated axons
- TRACTS are bundles of axons passing between specific regions of CNS
- Either spinal cord to brain or vice versa
- Connect various gray matter areas of CNS to each other
Positional relationship of white and gray matter
- CNS gray matter surrounds hollow central cavity, which is surrounded by white matter
- Brain has additional thin layer of gray matter external to white matter (cortex)
CNS gray matter interneurons
- Receive and process sensory information
- Direct information to specific CNS regions or to cell bodies of motor neurons in ventral spine
- Initiate appropriate motor response
CNS white matter interneurons
Transport information (sensory and motor) from one are of CNS to another
Reflex arcs
- Simple chains of neurons
- Responsible for REFLEXES, rapid involuntary automatic motor responses to stimuli
- Somatic (skeletal muscle) or visceral (smooth muscle, cardiac muscle, glands)
Essential components to the reflex arc
- Receptor at terminal end of sensory nerve
- Sensory neuron, transmits afferent impulses to CNS
- Integration center, consists of one or more synapses in gray matter of CNS
- Motor neuron conducts efferent impulses from integration center to an effector
- Effector, muscle or gland that responds to efferent impulses by contracting or secreting
Integration center of reflex arc
- Monosynaptic in simple reflex arcs between sensory neuron and motor neuron, NO INTERNEURON, very fast (i.e. upright posture and knee jerk) stretch reflexes are fastest of all body reflexes
- Polysynaptic in more complex reflexes, more common, one ore more interneurons involve
Simple polysynaptic reflex arcs: pricking finger involves one interneuron -> 3 neurons, meaning 2 synapses
Diverging circuit
1 presynaptic neuron synapses with multiple other neurons
Example
Stretch of a muscle stimulates a lot of sensory neurons which synapse with neurons in gray matter
a. motor neurons that directly innervate stretch muscle and stimulate contraction
b. interneurons that act to inhibit activity of antagonistic muscle group
c. interneurons that project sensory information to brain
Converging circuit
Many neurons synapse on a single postsynaptic neuron and a single motor neuron may receive both excitatory and inhibitory messages
Reverberating circuit
One neuron in circuit receives feedback from another in the same circuit; a branch off the axon of one neuron circles back and synapses with a previous neuron in the circuit
involved in rhythmic activities like breathing
Serial processing
Neurons pass a signal to a specific destination in sequence along a single pathway from one neuron to the next
reflex arc or long chain of interneurons carrying a sensory signal to brain
Parallel processing
- Simultaneously, nerve impulses travel in axon branch that extends into the spinal white matter and extends up to the brain as an ascending pathway
Example
withdrawal reflex: painful stimulus triggers nerve impulses in a sensory neurons, which initiate withdrawal reflex
integration in gray matter: multiple interneurons process nerve impulses to localize stimulus, identify source and plan response
voluntary (non-reflexive) motor response: initiated in cerebral gray matter and is transmitted down descending fiber in the white matter to stimulate motor neurons i.e. run cold water over a burned finger
Input processing: Parallel processing
- Information from a single neuron is sent along two or more parallel pathways allowing a single sensory stimulus to result in multiple perceptions
- Allows brain to rapidly evaluate stimuli and enables information to be processed and integrated along multiple pathways
Example
You see a dog approaching, multiple pathways triggered in parallel by sensory visual stimulus including shape, color, spatial location, movement and memory of past dogs
