Nervous tissue Flashcards
What are the two classes of cells in nervous tissue?
Neurons and glia
What is the CNS?
Central nervous system is contained within the spinal column and skull: 1. Brain 2. Spinal chord
What is the PNS?
Peripheral nervous system is primarily outside of spinal columns and skull: 1. nerves 2. ganglia
What is the difference between afferent and efferent?
Afferent = sensory = incoming – from receptor to CNS Efferent = motor = outgoing – from CNS to effector
What is the difference between somatic and autonomic divisions?
Based on targets of efferent information and sources of afferent information: Somatic = part of the nervous system having to do with the body (soma = body) Autonomic = monitors and controls functions that are internal (i.e. blood pressure, heart rate, gut motility, oxygenation, etc)
What do somatic efferent fibers do?
ALWAYS and ONLY innervate skeletal muscle (AKA: General Somatic Efferent)
What do Autonomic efferent fibers do?
Innervate: Cardiac muscle, smooth muscle, glands, and fat NEVER innervate: skeletal muscle (AKA General Visceral Efferent)
What do Somatic afferent fibers do?
Collect information regarding the body. Includes: - Pain - Temperature - Touch - Pressure - joint position - information from golgi tendon organs and muscle spindles Most of which we can be concious of and can pinpoint (AKA General Somatic Afferent)
What do Autonomic afferent fibers do?
Collect information such as degree of stretch and chemical composition from internal organs and blood vessels (AKA General Visceral Afferent)
What is the embryonic origin of the cells int he CNS?
Both neuronal and non-neuronal CNS cells are derived from the neural tube. - The neural tube originates from epithelial cells in the ectoderm.
What characteristics do epithelial tissue and nervous tissue share?
Because nervous tissue originates from epithelial tissue in the neural tube, it has: - No connective tissue except that which surrounds the CNS and the blood vessels that penetrate the CNS - Entire CNS is surrounded by basal lamina (that began as basal lamina of neural tube) - Brain and spinal cord have fluid filled cavities lined by ependyma (epithelial cells)
Describe the two sections shown
- White matter: contains oligodendrocytes, nerve processes
- Gray matter: Made up of dorsal and ventral horns; contain neuron cell bodies and microglial cells
Where would you find spinal nerves?
They are formed where the dorsal and ventral roots meet.
- less than an inch long
- extend distally only to the point where the nerve divides into dorsal and ventral rami
What type of nerve fibers exist in spinal nerves?
Spinal nerves are mixed nerves
- contain both afferent and efferent fibers
- also contain elements of both the somatic and autonomic systems
How many spinal nerves are there in a human?
31
What would result from cutting a dorsal root?
Dorsal roots are completely sensory
- cutting root results in loss of sensation, both somatic and visceral
What is the dorsal root ganglion?
Near the distal end of the dorsal root, right where it exits an intervertebral foramen, is the DRG
- contains the cell bodies of the dorsal root neurons
- Each cell body has a peripheral process that receives information from receptors and a central process taht enters the spinal cord
- There are no synapses in the dorsal root ganglion
What would result from cutting a ventral root?
Ventral roots are completely motor
- cutting this root results in paralysis
- both somatic and autonomic fibers are involved
- there are no ganglion along the ventral root; cell bodies are located in spinal cord
What is the general principal of Autonomic Efferents? What makes them differnt from Somatic Efferents?
- All Autonomic pathways from CNS to target organ (heart, smooth muscle, glands) have TWO neurons (somatic have one)
- Preganglionic neuron = cell body in CNS
- Postganglionic neuron = cell body in ganglion
- Autonomic ganglia have synapses; somatic do not
- Autonomic ganglion = ALWAYS motor/efferent; Somatic = ALWAYS sensory/afferent
Fill in the chart:
What can be found in the gray matter of the spinal cord?
neuronal cell bodies and their dendrites, along with axons and central microglia
- synapses occur here
What can be found in the white matter of the spinal cord?
- Myelinated and unmyelinated axons
- Oligodendrocytes
Explain the connective tissue of the CNS
Three sequential connective tissue membranes, called the meninges, cover the brain and spinal cord:
- Dura mater = outermost layer
- arachnoid mater = layer beneath the dura
- pia mater = delicate layer resting directly on surface of brain and spinal cord
What is a Nissl body?
Prominent in neuron cell bodies with toluidine blue stain, Nissl bodies are stacks of rER.
- They are so prominent b/c neurons are constantly making large amounts of proteins
What is the embryonic origin of the PNS?
Both neurons and glia develop from the neural crest
Describe the cell body of a neuron
large and euchromatic nucleus, prominent nucleolus, stacks of rER (Nissl bodies), visible in light microscope
Describe the dendrites of a neuron
there are multiple per neuron attached to the cell body
- they contain ribosomes
Describe the axon of a neuron
- One per neuron
- originating from the axon hillock
- contains microtubules, neurofilaments for axoplasmic transport and mitochondria
- generally surrounded by myelin for insulation of action potential
Describe the terminals of neuron axons (nerve endings)
- nerve terminals contain neurotransmitters and form the synapses with other cells
- terminals may be branched to form several synaptic endings attached to different postsynaptic cells
–> lesser branched or non-branched endings are often called boutons
What is a neuromuscular junction?
- Elaborate, specialized synaptic endings of neurons on muscle cells
- axons split into several synaptic endings
- narrow intercellular gap (synaptic cleft) separates pre- and postsynaptic cells
- presynaptic axon terminal filled with storage organelles called synaptic vesicles that contain neurotransmitters
- synapses release vesicles/neurotransmitters by exocytossi into synaptic cleft and they diffuse to postsynaptic cell receptors
What are sensory receptors?
Cells or complex aggregates of receptive and supportive cells
What is a sensory unit?
Includes the sensory receptor plus all the cells in the pathway of information transfer up to the point o fthe initial synaptic contact within the CNS
What are the connective tissues surrounding neurons?
Epineurium = dense irregular connective tissue surrounds a peripheral nerve and fills the spaces between nerve fascicles
Perineurium = specialized connective tissue surrounding each nerve fascicle
Endoneurium = includes loose connective tissue surrounding each individual nerve fiber; contains blood vessels
What are exteroceptors?
Receptors that react to stimuli from the external environment
i.e. temperature, toch, smell, sound, and vision
What are Enteroceptors?
Receptors that react to stimuli from within the body
i.e. degree of filling or stretch of the alimentary canal, bladder, and blood vessels
What are proprioceptors?
Receptors that react to stimuli from within the body, provide sensation of body position and muscle tone and movement
What are astrocytes (astroglia)?
- Largest glia cells with many cytoplasmic processes; many cytoplasmic organelles such as intermediate filaments
- many processes contact blood vessels and nearly continuously envelop the vessels with “astroglia feet,” providing the basis of the blood-brain-barrier
- Surround most neurons and provide structural support.
- also provide key metabolic support for ionic environment and uptake of metabolic waste and toxic molecules
What occurs when CNS tissue is damaged? (in terms of astrocytes)
A specialized form of astroglia appears (reactive or stellar astrocytes).
- fill the space of killed neurons and eventually form the glia scar
What are oligodendrocytes (oligodendroglia)?
- Most numerous glial cell. Smaller than astrocytes with shorter, fewer processes.
- nuclei and cytoplasm more condensed than astrocytes, contain ribosomes, mitochondria, microtubules - form myelin sheath for CNS
- found mostly in white matter, but also abundant in gray
Why do neurons need a myelin sheath?
Myelin acts as an electrical insulator
- allows for faster action potential conduction than unmyelinated neurons
What are the nodes of Ranvier?
Small gaps between adjacent myelin segments (between myelinating cells)
What is myelin?
Almost pur plasma membrane, is very rich in lipid
- due to high lipid content (which is extracted during conventional paraffin embedding), myelinated axons in paraffin sections are badly distorted by artifacts
What are Schwann Cells?
The myelinating cells of the PNS
- Main supporting cells of PNS
- Even form coating around unmyelinated axons (the smallest axons)
What are microglia?
Smallest glial cells with highly condensed nuclei and short, thorn-like projections
- NOT derived from neuroepithelium
- resident phagocytotic cells of the CNS
- accumulate at sites of injury andphagocytose debris (called Gitter cells)
What are ependymal cells?
Cuboidal/columnar cells with apical microvilli and, frequently, one or more cilia that line the central canal of the spinal cord and ventricles of the brain
- basal lamina has been pushed far away - to the perimeter of the CNS - and so these cells are like the apical cells of very highly stratified epithelium
- joined by modified junctional complexes (no zonulae occludentes); allows CSF to exchange “freely”
What are satellite cells?
Originating from neural crest, satellite cells are flattened supporting cells covering peripheral ganglia
- maintain the environment for the nerve cell bodies, analagous to schwann cells on axons
