LECTURE 3
EXCITABLE TISSUES
Objectives
a) To be able to describe the structure and function of the different types of
muscle.
b) To be able to comment on their appearance in section.
c) To describe the fibre-types of skeletal muscle.
d) To be able to define a motor unit.
e) To be able to describe the structure (whole and in section) and function of a
neurone.
f) To be able to describe myelinated and nonmyelinated nerve axons.
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Muscle & Nerve: together the Excitable Tissues, characterised by
response to external stimuli; part of response is electrical.
MUSCLE
Muscle is adapted for contraction. There are three types of muscle:
smooth, cardiac and skeletal. They can be considered as a series of
increasing specialisation for contraction.
Smooth Muscle
Consists of small spindle-shaped cells with central nuclei; the cells are
not striated. Arranged in sheets or bundles (e.g. gut).
Innervation — complex. Dual innervation with both excitatory and
inhibitory inputs. The balance between them gives rise to a partial
state of contraction or tone.
In single-unit muscle (e.g. gut, uterus) not all cells are directly
innervated; instead they are coupled more or less indirectly, so
that excitation spreads from the directly-innervated cells to those
that do not receive innervation. In multi-unit muscle (e.g. airways,
large arteries) each cell is innervated.
Function. Relatively weak slow, spreading contraction (obviously
related to the above properties. The contraction of each cell is
graded (cf. ‘all-or none’ of cardiac /skeletal muscle).
Cardiac Muscle
Consists of large cells or fibres joined end-to-end at intercalated discs
(modified Z discs). The fibres are striated indicating an elaborate
contractile apparatus. Fibres may branch and have electrical continuity;
may be binucleate. Nuclei are central.
Innervation. Within the heart pacemakers control the rate of
contraction and excitation spreads via the left and right branch bundles
of Purkinje fibres: modified cardiac muscle cells containing glycogen.
Function — non-fatiguing, powerful, co-ordinated contraction; rich in
mitochondria; rich blood supply.