A&P 1: Nervous System Tissue Anatomy

Nervous system functions

Collect (receptor), process & evaluate, initiate response to information (effector)

Central nervous system

Brain & spinal cord

Peripheral nervous system

Nerves & ganglia

Ganglion

Cluster of neuron cell bodies located along nerves within PNS

Results in swelling/enlarged portion often visible to naked eye

Includes poster root

Afferent nervous system

Sensory nervous system; brings sensory input from receptors

After sensation

Somatic sensory component

Detects conscious stimuli (5 sense, proprioception)

Visceral sensory component

Detects unconscious stimuli in internal structures (e.g. stretch)

Efferent nervous system

Motor nervous system; controls effectors (muscles & glands)

Somatic motor component

Initiates and transmits motor output from CNS to skeletal muscle

Autonomic motor component

Initiates and transmits motor output from CNS to involuntary effectors (cardiac/smooth muscle, glands); function without conscious control

Nerve

Cable-like bundle of fascicles/axons in PNS

Fascicle

Bundle of axons

Epineurium

Thick layer of dense connective irregular tissue enclosing entire nerve to protect and support

Perineurium

Layer of dense irregular connective tissue around each fascicle to protect and support each fascicle; also supports blood vessels

Endoneurium

Delicate layer of areolar connective tissue surrounding each axon

Separates and electrically insulates each axon

Vascularity

Extensive network of blood vessels branch and extend through epineurium, perineurium

Capillaries associated with endoneurium for exchange of oxygen, glucose, waste and other substances between axons and blood

Cranial nerves

Extend from brain

Spinal nerves

Extend from spine

Sensory nerves

Nerves with only neurons to relay information toward CNS

Motor nerves

Nerves with only neurons that relay information away from CNS

Mixed nerves

Nerves with sensory and motor neurons, each type still only transmits one type of information

Nervous tissue

Primary tissue of nervous system composed of neurons and glial cells

Neuron

Basic structural unit of nervous system

Excitable: responsive to stimulus; stimulus causes RMP change

Conductive: electric change an propagate action potential

Secretary: release neurotransmitters in response to conductive activity

Longevity: neurons formed during fetal development persist through adulthood

Amitotic: not able to divide except in hippocampus and olfactory

Neurotransmitter

Molecule stored in vesicles and released to bind to excitable cell for excitatory or inhibitory effect

Axon hillock

Triangular, cone-shaped region of cell body; no chromatophilic substance

Cell body

Houses nucleus with chromatin and prominent nucleolus and perikaryon composed of organelles with many ribosomes in large ER or cytosol

Controls cell metabolism and transmits graded potentials toward dendrites

Dendrites

Relatively short, small, tapering processes that branch off the cell body and transmit graded potentials toward cell body

Neurons may have many or one

Axon

Longer process used to make contact with other neurons, muscle cells or gland cells

Contains axoplasm, axolemma, axon collaterals

Axon collaterals

Side branches of axon that branch extensively at distal end into array of fine terminal extensions to form synaptic knob (synaptic/end bulb, terminal bouton)

Function in initiation and propagation of action potentials to release neurotransmitter vesicles from synaptic knob

Myelin sheath

Neurofibril nodes

Uninsulated regions of axon between myelin sheaths

Neurofilament

Type of intermediate filament which composes cytoskeleton with microtubules and microfilaments (actin).

Extend into dendrites and axon from cell body to maintain neuron shape and provide structural support forming neurofibrils

Neurofibrils

Parallel bundles of neurofilaments with microtubules embedded in parallel clusters in axon to participate in axonal transport, stabilized by tau protein

Anterograde transport

Movement of materials from cell body toward synaptic knob

Retrograde transport

Movement of materials from synaptic knob toward cell body

Fast axonal transport

Motor proteins (kinesin, dynein, etc…) split ATP to move substances along microtubules

Anterograde: vesicles, organelles, glycoproteins

Retrograde: used vesicles for recycling, potentially harmful agents

400 mm/day

Slow axonal transport

Movement of enzymes, cytoskeletal components, new axoplasm from anterograde axoplasmic flow

.1-.3mm/day

Multipolar neuron

Neuron with many dendrites and single axon extending from cell body

Most common neuron type

Bipolar neuron

Neuron with two processes extending from cell body; one dendrite, one axon

Relatively limited locations such as special sense neurons (retinal, olfactory)

Unipolar neuron

Neuron with single, short neuron process emerging from cell body with T-shaped branches.

Pseudounipolar: start as bipolar during development and processes fuse

Dendrites: short, multi-branched receptive endings

Axon: central (into CNS) & peripheral (to cell body) process branches from single process

Anaxonic neuron

Neuron with only dendrites, no axons

Produce graded potentials but not action potentials

Afferent neuron

Sensory neuron; conducts sensory input from somatic & visceral receptors toward CNS

Mostly unipolar, some like retina & olfactory are bipolar

Efferent neuron

Motor neuron; conducts motor output away from CMS to somatic & autonomic effectors

Always multipolar

Interneuron

Association neuron; neurons in CNS that receive stimulation from many other neurons and carry out integrative functions of nervous system

Receive, process, store info for stimulus response formulation

Facilitate communication between sensory and motor neurons

99% of neurons; multipolar or anaxonic

Chemical synapse

Synapse between presynaptic neuron (signal producer, releases neurotransmitter) axon and any unmyelinated portion (commonly dendrite) of postsynaptic neuron (signal receiver, binds neurotransmitter) separated by a synaptic cleft

Communication via neurotransmitter diffusion to initiate graded potential involves .3-.5ms synaptic delay

Postsynaptic neuron may receive signals from 2+ neurons simultanously

Electric synapse

Presynaptic and postsynaptic neurons are physically bound together with gap junctions present to facilitate from of ions between cells

No synaptic delay

Limited to a few regions within brain & eyes

Glial cell

Neuroglia; mitotic CNS/PNS cell that assists neurons with function

Does not transmit electrical signals

Protects & nourishes; provides supporting scaffold for nervous tissue

Outnumber neurons 10-50:1, half nervous system volume

4 types

Astrocytes

Starlike glial cells with surface projections contacting neurons and capillaries

Most abundant glial cells in CNS

Helps form blood-brain barrier: perivascular feet cover and wrap around capillaries

Regulate interstitial fluid composition: maintains K+ gradient

Form structural support: cytoskeletons strengthens cell to support & organize neurons

Assist in neuronal development: direct fetal brain development by secreting chemicals regulating connection formation

Alter synaptic activity: add and eliminate synapses, influence neuron communication at existing synapses

Occupy dying neuron space

Ependymal cells

Ciliated simple cuboidal or simple columnar cells lining ventricles (internal cavities) of brain and central canal of spinal cord

Slender extensively branched processes to make connections with other glial cells

Form choroid plexus with nearby capillaries

Cilia help circulate CSF

Choroid plexus

Network formed by ependymal cells and capillaries which helps to produce CSF

Microglia

Typically small phagocytic glial cells with slender branches that protect CNS agains microorganismsand other potenitally harmful substances and removign debris from dead or damaged nervous tissue

Wander through NS and replicate in response to infection

5% of glial cells

Oligodendrocyte

Large glial cells with bulbous body and slender cytoplasmic extensions which wrap around and insulate CNS axons forming myelin sheath

Neoplasm

CNS tumor; primary tumor typically originates in supporting brain//spinal cord tissue because neurons are amitotic

Gliomas vary in growth rate and malignancy

Satellite cells

Flattend PNS cells arranged around neuronal cell bodies in a ganglion. Physically separates them from interstitial fluid for electrical insulation and nutrient/waste exchange

Neurolemmocytes

Schwann cells; elongated and flattened cells wrap around and insulate PNS axon forming myelin sheath

Unmyelinated axons

Neurolemmocytes help protect and support axon with axon resting on depressed portion of neurolemmocyte

Guillen-Barre syndrome

PNS disorder with inflammation causing loss of myelin surrounding neuron

Causes muscle weakness (distal > proximal)

Usually preceeded by acute, infections respiratory or GI illness, or rarely immunization

Most recover neurologic function with no/little medical intervention

Axon regeneration

PNS nerve repair system for axon trauma (cut, crush, etc..)

Depends on amount of damage and distance between damage site and innervated structure (> = less chance of repair)