NERVOUS SYSTEM

SCHOOL OF ANATOMICAL SCIENCES

Peripheral nerve fibre, non-myelinated (diagram)In this diagram of non-myelinated nerve fibres NOTE:

• Schwann cells associated with several axons (A).

• The nucleus (N), mesaxons (M) and cytoplasm of the Schwann cell.

•The basal lamina surrounding the Schwann cell.

QuestionsName the structures (filaments, cytoskeletal components & organelles) found in the axon.

In what direction do impulses in the axon travel?

(Bloom & Fawcett, 1975)

Peripheral nerve fibre, non-myelinatedIn this EMG of the non-myelinated nerve fibres NOTE:

• The Schwann cells, each associated with more than one axon (only one Schwann cell nucleus is shown in this section).

• The connective tissue fibres (in transverse section) of the endoneurium surrounding the Schwann cells.

• The spherical mitochondria visible within some of the axons.

QuestionsWhat is the connective tissue (CT) surrounding bundles/fascicles of myelinated/non-myelinated fibres called?

What type of CT is it?

What is the CT surrounding the nerve called?

What type of CT is it?

(Toner & Carr, 1971)

Peripheral nerve fibre, myelinated (diagram)This diagram illustrates the formation of myelin layers from the plasma membrane of the Schwann cell (S). NOTE:

• The major dense line (thick black line) of myelin formed by the fusion of the two opposing inner (cytoplasmic) leaflets of the plasma membrane.

• The intraperiod line (thinner black line) of myelin formed by the fusion of the two opposing outer leaflets of the plasma membrane.

• The Schwann cell (S) surrounding the axon fibre (A) in the centre.

• The peripherally displaced cytoplasm and nucleus of the Schwann cell and the inner and outer mesaxon (IM & OM).

• The surrounding basal lamina (BL) of the Schwann cell.

QuestionWhat are the components of the neurilemma?

(Toner & Carr, 1971)

Peripheral nerve fibre, myelinatedEach functional unit (fibre) of a peripheral nerve has three components:

• The axon of a neuron.

• Many Schwann cells which envelope the length of the axon (may or may not form myelin sheaths).

• A loose connective tissue sheath called the endoneurium surrounding the basal lamina of the Schwann cell.

In this EMG of a myelinated nerve fibre cut in transverse section NOTE:

• The collagen fibres of endoneurium in transverse section.

• The large Schwann cell nucleus peripherally displaced in the cell.

• The inner and outer mesaxons (arrow).

QuestionsWhat is the function of endoneurium?

What type of cell is the Schwann cell?

(Fawcett, 1966)

Peripheral nerve fibre, myelinatedThis EMG is a higher magnification of the myelinated axon in the previous EMG. NOTE:

• The alternating major dense and narrower intraperiod lines.

• The outer and inner mesaxon.

• The centrally placed axon.

QuestionsWhat is a mesaxon?

How does the process of myelination occur?

How many fibres can a Schwann cell myelinate?

(Fawcett, 1966)

Peripheral nerve fibre, myelinated & non-myelinatedIn this EMG of peripheral nerve NOTE:

• That only one Schwann cell nucleus (N) is shown in this section.

• One Schwann cell is associated with more than one unmyelinated axon within which are neurofilaments (NF).

• One Schwann cell (SC) is associated with only one myelinated axon.

• The endoneurium (CT) surrounding the Schwann cell basal laminae.

• The electron dense myelin sheath (MY).

QuestionsWhy is myelin electron dense?

What are Schmidt-Lanterman clefts?

(Toner & Carr, 1971)

Peripheral nerve fibre, node of Ranvier (diagram)In this longitudinal section of a myelinated axon NOTE:

• The node of Ranvier (X) between two Schwann cells.

• The peripherally displaced cytoplasm (perinodal cytoplasm) of the Schwann cells (S).

• The major dense and intraperiod lines of myelin.

• The axon with organelles and neurofilaments in the cytoplasm.

• The basal lamina of the Schwann cell.

QuestionsWhat is a node of Ranvier?

Describe its structure.

What is the function of a node of Ranvier?

(Porter & Bonneville, 1964)

Peripheral nerve fibre, node of RanvierIn this EMG of a longitudinal section through a myelinated axon NOTE:

• The node of Ranvier (X) between two Schwann cells (SC).

• The processes (Pr) of the Schwann cells over the node of Ranvier.

• The neurilemmas around peripherally displaced cytoplasm and myelin sheath (My) of the Schwann cells.

• The smooth endoplasmic reticulum (ER), long slender mitochondria (M) and neurofilaments (Nf) in the axon.

• The basal lamina (BM) of the Schwann cells which continues over the node and fuse.

• The endoneurium (CT) surrounding the fibre.

QuestionWhat role do nodes of Ranvier play in impulse conduction?

(Porter & Bonneville, 1964)

Neuromuscular junction (diagram)In this EMG of a neuromuscular junction NOTE:

• The Schwann cell (SC) (with a nucleus) and the connective tissue fibres of the endoneurium accompanying the axon to its synapse on the muscle fibre.

• The numerous mitochondria and synaptic vesicles in the nerve ending (NE).

• The junctional folds (JF) of the sarcolemma of the skeletal muscle at the synapse.

• The synaptic cleft/gutter (Gu) between the neurilemma of the nerve endings and the sarcolemma of the junctional folds.

• The many peripherally placed oval nuclei of the muscle fibre.

• The dark and light striations on the fibrils of the muscle fibre.

QuestionsWhat is a synapse?Is the axon myelinated at the neuromuscular junction?What is contained in the vesicles in the nerve ending?How are the contents of the vesicles released into the synaptic cleft?Explain the many mitochondria in the nerve endings.How is the presynaptic membrane depolarised?

(Porter & Bonneville, 1964)

Neuromuscular junctionIn this EMG of a neuromuscular junction NOTE:

• The Schwann cell (SC) and the connective tissue (CT) fibres of the endoneurium accompanying the axon to its synapse on the muscle fibre.

• The basal lamina (BM) of the Schwann cell.

• The numerous mitochondria (M) and synaptic vesicles (V) in the nerve endings (NE).

• The junctional folds (JF) of the sarcolemma of the skeletal muscle at the synapse.

• The synaptic cleft/gutter (Gu) between the neurilemma of the nerve endings and the sarcolemma of the junctional folds.

• The peripherally placed oval nucleus (N) of the muscle fibre.

• The striations on the fibrils (*) of the muscle fibre.

QuestionsWhy is the sarcolemma folded (JF) at the neuromuscular junction?

Give two diagnostic criteria of the muscle in this EMG.

How is the postsynaptic membrane depolarised?

(Porter & Bonneville, 1964)