NERVOUS TISSUE
- Cells specialized for conductivity and irritability = very responsive to stimuli
- Nervous system = communication system of body; receptor collects stimuli,
transforms to electrical signal, passes signal to CNS (interpretation), finally passes to
effectors (response)
- Divisible into 3 parts:
1) Central Nervous System (CNS) = brain and spinal cord
2) Peripheral Nervous System (PNS) = spinal and cranial nerves and ganglia
3) Autonomic Nervous System = viscerosensory nerves, ganglia and visceromotor
nerves; two divisions: sympathetic and parasympathetic
UNIT OF STRUCTURE = Neuron (nerve cell)
- Consists of: 1) Cell Body - contains nucleus
2) Cell Processes - the greatest part of the cell is in the cell processes
a) Dendrites = stimulus receiving and impulse-generating component, carries
impulse to cell body
b) Axon = propagates impulse, carries impulse away from cell body
- NEURON TYPES:(SEE HANDOUT)
1) Unipolar = cell body + axon only (photoreceptors of eye, embryonically)
2) Bipolar = single axon + single dendrite (retina of eye, ganglia of auditory nerve)
3) Pseudounipolar = proximal regions of axon and dendrite fuse to form single
common segment leaving cell body (spinal ganglia)
4) Multipolar = numerous dendrites + one axon (by far the most common)
- NEURON STRUCTURE:
1) Cell Body usually large (4_m in retina, 120_m in motor neurons); nucleus lies near
center of cell body and is pale-staining but has single dark-staining nucleolus;
within cytoplasm of cell body and proximal cell processes are Nissl Bodies = ribosome-
rich regions active in protein synthesis; many mitochondria are also present
indicating energetically active cell
2) Axon usually longest process of neuron (1-20_m diameter); joins cell body at Axon
Hillock = pale-staining region; contains mitochondria but no ribosomes
3) Dendrites are generally shorter and highly branched, multiple most often; larger
regions with same subcellular components as cell body
NERVE FIBERS (AXONAL TRANSPORT)
- Nerve fibers may be myelinated (vast majority, including most nerves and white
matter of CNS) or unmyelinated (CNS gray matter, nerve fibers innervating intestinal
smooth muscle)
1) Axons encased in tough inelastic membrane (neurilemma) formed by associated
interstitial cells; sheath is not present near cell body or axon terminal; Neurilemma
composed of Schwann Cells (in PNS) = lay down myelin coating on axon, myelin
laid down as Schwann cell membranes wrap around axon (SEE HANDOUT).
Myelin coat laid down by glial cells in CNS. Myelin is a lipid and acts as an insulator,
increasing the efficiency of conduction.
2) Myelin Sheath is interrupted at regular intervals = Node of Ranvier - these act to
increase velocity of conduction by allowing nerve impulse to jump from node-to-node
(saltatory conduction) rather than proceed slowly down the plasma
membrane as in the unmyelinated neuron.
- Clefts in the myelin coat represent breaks between membrane layers
ARRANGEMENT OF FIBERS INTO NERVE
THE SYNAPSE = sites where nerve impulse passes from one neuron to another
Invertebrate Synapses = electrical, gap junctions thru which ions may freely pass
Vertebrate Synapses = chemical, involves chemical mediator = neurotransmitter
Structure of Synapse:
1) Presynaptic Terminal = portion which delivers impulse, usually is a terminal
expansion of axon
2) Postsynaptic Terminal = portion which receives impulse, usually a dendrite or less
commonly a cell body and very rarely another axon; also can be an effector cell
membrane (as in a nerve-muscle junction)
3) Synaptic Cleft = narrow space between pre- and postsynaptic terminals (20nm)
4) Neurotransmitters = may have excitatory (depolarizing) or inhibitory
(hyperpolarizing) effects on postsynaptic membrane; 2 major types: a) Cholinergic
Fibers = release acetylcholine, b) Adrenergic Fibers = release norepinephrine; many
minor types of neurotransmitters also exist
- For Physiology of Conduction, SEE HANDOUT
To Lecture 12