Cellular elements

2 categories

1. Nerve cells/neurons- concerned with information processing and signalling

2. Glial cells- supporting role 100 billion neurons; maybe more glial

cells

If we took 1 second to count one neuron, counting 100 billion neurons would take more than 3000 years

All neurons are variations on the same theme

Convey information by combined electrical and chemical signaling mechanisms

Electrical signals- rapid transmission of information from one part of neuron to another

Chemical messengers carry information between neurons

Anatomically specialized areas for collecting, integrating, conducting and transmitting information

Components of neuron

Cell body= soma/perikaryon [karyon=nucleus]

Supports metabolic and synthetic needs of the rest of the neuron

Processes of neurons

Dendrites –series of branching tapering processes

Receive information from other neurons via synaptic contacts/synapses

Axon

One long cylindrical process Conducts information away from cell

body Gives rise to a series of terminal

branches, forming synapses on other neurons

Neurons are anatomically and functionally polarized, with electrical signals travelling in only one direction under ordinary physiologic circumstances

Anatomical classification

Depends on number of processes Multipolar- vast majority-multiple

dendrites Bipolar- 2 dendrites Pseudounipolar -

Unipolar neurons are actually pseudounipolar

They start out as bipolar, but during development, cell body expands asymmetrically, leaving behind a stalk from which both processes emerge

Located in dorsal root and cranial nerve ganglia

Formation of a pseudounipolar neuron

Motor neurons and interneurons are multipolar

Bipolar neurons are located in retina and CN VIII ganglia

Functional classification

Sensory neurons

Directly sensitive to various stimuli [e.g. touch or temperature] or receive direct connections from non-neuronal receptor cells

Sensory neurons

Their processes are included in somatic and visceral afferents

Somatic afferents convey pain, temperature, touch, pressure, proprioception

Visceral afferents convey pain and other sensations from mucous membrane, glands and blood vessels

Most sensory and motor neurons live partly in CNS and partly in PNS

The words sensory and motor are often used in a broader sense to refer to cells and axons carrying information related to sensory stimuli and the responses generated

Motor neurons

Convey impulses from CNS/ganglia to effector cells

Their processes are included in efferent nerve fibres

Somatic efferents → skeletal muscles Visceral efferents → smooth muscle,

heart, glands

Interneurons

≥ 99% of all neurons Form a communicating and integrating

network between sensory and motor neurons

Local interneurons have all there processes confined to a single area od CNS

Probably not more than 20 million sensory fibres in all of spinal cord and cranial nerves combined

No more than few million motor neurons

Projection neurons

Have long axons connecting different areas, such as a neuron in cerebral cortex whose axon reaches spinal cord

Strictly speaking, human nervous system is almost entirely composed of interneurons and projection neurons

More than 99% are interneurons or projection neurons

GRAY MATTER AND WHITE MATTER

CNS is easily divisible into gray matter and white matter

Gray matter- preponderance of cell bodies and dendrites. In life it is pinkish gray due to abundant blood supply

White matter

preponderance of axons; many of whom have myelin sheath

Myelin sheath is mostly lipid- hence the white appearance

Nuclei

Specific areas of gray matter in CNS whose neurons are functionally related- similar areas in PNS are called ganglia

Cortex

An area where gray matter forms a layered surface covering some part of CNS

Subdivisions of white matter[collections of axons] Variety of names in CNS- fasciculus,

funiculus, lemniscus, peduncle- most commonly tracts

Collections of axons in PNS are called nerves

Fasciculus = ‘little bundle’ Funiculus = ‘string’ Lemniscus = ‘ribbon’- tracts flattened out

in cross section Peduncle= ‘little foot’- site where tracts

funnel down into a compact bundle

Features of a neuron

Synthesizes

1. neuronal enzymes,

2. structural proteins,

3. membrane components,

4. organelles and

5. some of its chemical messengers [neurotrnsmitters]

Nucleus; large, pale staining with dispersed chromatin

Abundant RER, free ribosomes, stacks of Golgi apparatus

Many mitochondris

Nissl bodies/Nissl substance- ribosomes, stained intensely with basic dyes, appear as clumps- prominent in large neurons

Many mitochondria

Cytoskeleton composed of microtubules, neurofilaments [aggregates of these are called neurofibrils] and microfilaments

Dendrites

Tapered extensions of neuronal body Collectively provide a great increase in

surface area available for synaptic inputs

In spinal cord, dendritic surface area may be 30 or more times that of cell body

Dendrites of many neurons are studded with small protuberances called dendritic spines

These are preferred sites of some synaptic contacts

Dendrites and dendritic spines

Axons

Different from dendrites Cylindrical process- arises abruptly from

an axon hillock on one side of neuronal body

Initial segment has bundles of microtubules, neurofilaments and mitochondria- no Nissl substance

It is most electrically excitable part of a neuron

Beyond initial segment, many axons are encased in spiral wrapping of a membrane called myelin sheath- greatly increases speed of propagation of electrical impulses

Transport of macromolecules and organelles synthesized by cell body occurs away from soma[anterograde] and towards it [retrograde]

It can be slow or fast Microtubules act as ‘railroad tracks’ for

fast transport

Many peripheral nerves are myelinated- resemble a string of sausages

Each link of sausage corresponds to a length of axon wrapped in myelin with adjacent links separated by a gap in myelin

At this site axon is separated from extracellular space only by fingerlike processes from Schwann cells

Myelin sheath between 2 nodes is called internode- formed by s single Schwann cell

Most of smaller axons in peripheral nerves are unmyelinated- slow conductors of electrical signals

Glial cells

Glia =Gr. Glue Cells are so named because they fill up

most of the spaces between neurons- appear to hold them in place

Some do provide structural support Play a wide variety of additional roles

Neuroglial cells, collectively known as the neuroglia or simply as glia, have important ancillary functions.

The neuroglial cells of the normal CNS are astrocytes, oligodendrocytes, ependymal cells (derived from neural tube ectoderm), and microglia (derived from mesoderm)

Astrocytes occur throughout the brain and spinal cord

Oligodendrocytes produce myelin and are also found next to the cell bodies of some neurons.

Microglial cells become phagocytes when local injury or inflammation is present.

The neuroglial cells of the peripheral nervous system are Schwann cells in nerves and satellite cells in ganglia.

Synapse

Neurons, neuropil, and the common glial cells of the CNS

Neurons, neuropil, and the common glial cells of the CNS

Supporting cells

Glia/neuroglia in CNS Schwann cells and supporting cells in

PNS Schwann cells surround neurites,

isolating them from adjacent cells and extracellular matrix

In PNS ganglia, supporting cells are Satellite cells- surround nerve cell bodies [nucleus containing part]- analogous to Schwann cells

In development, glial cells serve as scaffolding that directs neuronal migration to appropriate sites

Functions

Physical support for neurites Electrical insulation for nerve cell bodies

and processes Metabolic exchange pathway between

vascular system and neuronsS

Oligodendrocytes

Processes extend around several axons in CNS

Predominant glial cell in white matter

Astrocytes

Form a network of cells within CNS Communicate with neurons- modulate

and support their activities Fibrous astrocytes- few, long processes-

found in white matter Protoplasmic astrocytes- many short

branched processes- found in gray matter

Important role in proper formation of CNS in fetal and embryonic development

Control ionic environment of neurons Form scar tissue in CNS damage

Perivascular feet contribute to blood- brain barrier Regulate vasodilatation Regulate transfer of oxygen, ions and

other substances from blood to neurons

Ependymal cells

Low cuboidal/columnar Line ventricles of brain and central canal

of spinal cord In some places they are ciliated to

facilitate movement of CSF- in others they have long microvilli

Microglia

5% of all glial cells Part of MPS- monocyte precursors Proliferate and become actively

phagocytic in regions of injury and disease

Remove debris of apoptotic cells during CNS development

Synapse

Types of synapses

Nerve injury/regeneration Nerve cells, unlike neuroglial cells,

cannot proliferate but can regenerate their axons, located in the PNS