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 cordNerves 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 sheath Node 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 myelinationIn 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)Schwan

n cell

Schwann cell grows

around axon …

… becoming

thinner and

thinner …

… until many layers of

membrane are left wrapped around the

fibre

Electron micrograp

h of an axon (A) in

the process of myelinatio

n.

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 RanvierEach 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

NeurilemmaAxolemma

Basement membran

e Diagram of a single node

lie

The spinal cord

Spinal cord

Spinal nerve

Vertebra (cut open)

Meninges

Dorsal root ganglion

The spinal cordThe 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 matterDorsal root ganglion

Spinal nerveVentral 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