Nervous coordination Neurones, Spinal cord and the Spinal Reflex

Starter How many cell type can name that involved in a reflex arc?

Objectives To describe the structure and function of sensory, relay, motor neurones and parts played in the reflex arc. Understand the role of Schwann cells and myelination. Be aware of the histology of the spinal cord as seen using the light microscope. Some students may be able to interpret electron micrographs of nervous tissues.

Anatomy of the nervous system The central nervous system consist of the brain and spinal cord The peripheral nervous system consist of cranial nerves arising from the brain and spinal nerves arising from the spinal cord Nerves are bundles of nerve fibres, each fibre an extensions of a single neurone (nerve cell). Most cranial and spinal nerves are mixed nerves, containing sensory fibres carrying impulses from receptors into the CNS, and motor fibres carrying impulses from the CNS to effectors.

Schematic overview of the nervous system Receptor Sensory fibre (dendron) Part of cranial or spinal nerve Cell body of sensory neurone Ganglion Cell body of motor neurone Motor fibre (axon) Part of cranial or spinal nerve Effector Note that sensory neurones have one main input but many output connections Note that motor neurones have many input connections but one main output Relay or inter- mediate neurones provide very many pathways between sensory and motor neurones Synapse

Neurones Thin section of cat brain, stained by the Golgi method: this stains 1- 5% of neurones black, leaving the rest invisible. A similar preparation at higher power: these are multipolar pyramidal neurones A single multipolar neurone at very high power. Note the dendritic spines on the fibres, increasing the surface area for synaptic connections.

Neurones Sensory neurone Dendron (may be very long) Cell body (soma) Axon Terminal arborisation Receptor Myelin sheathNode of Ranvier Multipolar (relay) neurone Most of the cells in the CNS are like this, with numerous inputs and outputs

Neurones Axon (may be very long) Motor neurone Dendrites (many, short, highly branched) Myelin sheath Node of Ranvier Muscle fibres Motor end plate (neuro- muscular junction Cell body (soma) Nissl granules (polyribosomes)

Glial cells and myelination Neurones are the conducting cells of the nervous system: the CNS also contains glial cells which surround and support the neurones Specialised glial cells called Schwann cells form the insulating myelin sheath around the axons and dendrons of vertebrate neurones

Glial cells and myelination Myelin is a lipid material composed of many closely pressed cell membranes: its fatty nature gives the CNS’s white matter its characteristic appearance Myelin is an electrical insulator that speeds the transmission of impulses in myelinated fibres

Glial cells and myelination Invertebrate nerve fibres are non- myelinated and usually slower-conducting than those of vertebrates Speed of conduction of non-myelinated fibres can be increased by increasing their diameter: giant axons in the nervous system of squid are about 1 mm in diameter, and have been used extensively in research on impulse conduction

Glial cells and myelination In the CNS extensions of one glial cell can enwrap several neighbouring fibres In the PNS long axons or dendrons are myelinated by numerous Schwann cells, each Schwann cell wrapping itself around a small length of the fibre.

Schwann cells and myelination Axon (ts) Schwann cell Schwann cell grows around axon … … becoming thinner and thinner … … until many layers of membrane are left wrapped around the fibre

Electron micrograph of an axon (A) in the process of myelination. Schwann cell nucleus In the lower left a single glial cell has wrapped itself around two other fibres.

Animation of myelination

A myelinated axon (ts)

Schwann cells and myelination Scanning electron micrograph of a t.s. peripheral nerve. Cut ends of several myelinated axons can be seen. Identify: MS myelin sheath SN Schwann cell nucleus

Node of Ranvier Each Schwann cell myelinates a few  m of axon. Between Schwann cells there is a short region of exposed axon called the Node of Ranvier: these are important in impulse conduction. S.e.m. of myelinated fibres in a peripheral nerve Node Myelin sheath Axon Neurilemma Axolemma Basement membrane Diagram of a single node lie

The spinal cord Spinal cord Spinal nerve Vertebra (cut open) Meninges Dorsal root ganglion

The spinal cord The photograph shows a cross-section of the spinal cord in the neck. Each spinal nerve divides into two branches where it enters the spinal cord: a dorsal root at the back and a ventral root at the front. Vertebra Spinal cord Ventral root of spinal nerve Dorsal root of spinal nerve: the swelling (X) is the dorsal root ganglion

The spinal cord Within the spinal cord we can see the grey matter (consisting of the cell bodies of nerve cells), surrounded by white matter (consisting of the fibres that connect nerve cells with each, and with receptors and muscles). White matter Grey matter

A spinal reflex arc Grey matter Dorsal root ganglion Spinal nerve Ventral root Dorsal root Central canal Sensory fibre – from a receptor Motor fibre – to a muscle Sensory cell body – one of thousands in this ganglion Relay neurone Motor neurone Synapses

A spinal reflex arc Spinal cord Dorsal root Sensory neurone Grey matter Note that the brain is not involved in carrying out this withdrawal reflex. However, withdrawal reflexes like this can sometimes be ‘overridden’ by the brain. See again Move on

Over-riding a spinal reflex arc Spinal cord Dorsal root Sensory neurone Grey matter Fast-conducting fibres to and from brain Brain The sensory neurone synapses with a relay neurone in the grey matter, but also with fast- conducting fibres that carry the impulses to the brain. The information is quickly processed by the brain, and if it is more advantageous not to let the reflex action occur, fast- conducting descending fibres carry impulses to the synapse between the relay and the motor neurones, producing an inhibitory post-synaptic potential (IPSP) and preventing the motor neurone from being triggered.

Plenary Name cell types studied today and add a function for each Outline the order of cells used in a reflex arc