Structure of the Nervous System

Organization of the Nervous System•Nervous system can be classified in terms of information flow: Afferent neurons (sensory neurons) send signals into the central nervous system (CNS) for processing. The processed signal is sent out along efferent neurons to activate the required cellular response in effector cells.

•The afferent and efferent neurons form the peripheral nervous system (PNS).

•The efferents can be divided into the somatic motor division and the autonomic division. The autonomic divisions can be further divided into sympathetic and parasympathetic divisions.

•The enteric nervous system is viewed as the 3rd division of the nervous system.

Structure of a Neuron• The nervous system is composed of primarily neurons and glia.

•Neuron is the functional unit of the nervous system.

•Information flow is from the cell body to the axon terminals via axons.

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Cell Bodies & Dendrites•Cell Body: Makes up approx. 1/10 of the cell volume. It contains the nucleus where genes are transcribed.

•It maintains the well being of the cell and protein synthesis takes place in the cell. Proteins are packaged into vesicles and transported out to other regions along microtubules.

•A number of important organelles are found there: rough endoplasmic reticulum (ER), mitochondria, Golgi complex.

•Dendrites: are highly branched structures which receive incoming signals from other neurons. They increase the surface area of the neuron, allowing multiple inputs from several neurons.

•They contain rough ER, mitochondria and other organelles.

Fiala et al (2002)

Pathologies of Spine Distribution And Shapes

Axons

•Slow axonal transport can move material at a rate of 0.2- 5 mm/day. Carries material not consumed rapidly (eg. enzymes and cytoskeletal proteins).

• Fast axonal transport can move material at a rate of 400 mm/day. Moves material consumed rapidly, such as, synaptic vesicles. Ships back old membrane components for recycling to the cell body.

•Axons propagate electrical signals along their length to their terminals.

•Axons contain mitochondria (source of ATP) and microtubules (act like railroad tracks, enabling transport of organelles and molecules (packaged into secretory vesicles) to and from axon terminals).

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Synapses

Small Synaptic Boutons in the Mammalian Brain

Mouse Neuromuscular Junction

Parasympathetic Varicosities formed by Preganglionic Axons Innervating Cell Bodies of Ganglion Cells

•At synapses, the neuron which delivers the signal is called a presynaptic cell, and the cell which receives the signal is termed a postsynaptic (or target) cell.

•The electrical signal transmitted along the axon is translated into a chemical signal before its relayed across the synapse onto the target cell. In the target cell, the signal is again electrical.

Three Functional Types of Neurons

Sensory NeuronsMotor Neurons

CNS Interneurons

Neural Circuits

Definition: A functional group of neurons which process a special kind of information.

•Neurons link together to form neural circuits which perform special tasks. Many of these are reflexes.

•Signaling within these circuits gives rise to higher cognitive functions, such as thinking.

•Since circuits are needed for even the most basic function, it has been suggested that the functional unit of the nervous system is a group of neurons, rather than an individual neuron.

•How do these circuits link together to create behaviour? How do they adapt to change?

Glial Cells•Glial cells out-number neurons by 10-50:1.

•Play a supportive role in the nervous system. They communicate with neurons and amongst each other.

•Myelin (concentric layer of phospholipid insulating sheath wrapped around portions of a neuron) is formed from oligodendrocytes in the CNS and Schwann cells in the PNS.

•In the CNS myelin wraps around several axons, whereas in the PNS, around a single axon. Nodes of Ranvier are exposed regions of axons.

Brain

•Different regions of the brain subserve different functions. When a single function is carried out by more than one region of the brain, its termed parallel processing.

•Network of neurons can alter their connections based on past experience, thus displaying plasticity.

•Weighs about 1400 g and contains 1012 neurons.

•Each neuron receives about 200, 000 synapses.

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Neurons Are Grouped in Nuclei and Tracts in the Cerebral Cortex

•Cell bodies form layers in parts of the brain or cluster into groups called nuclei.

•Grey matter consists of cell bodies and white matter, of axons.

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Cerebral Cortex

•Within this layer, our higher brain functions arise (eg. Thought).

•Contain 3 functional specializations: (a) sensory areas which direct perception; (b) motor areas which direct movement; (c) associated areas which integrate information and direct voluntary areas.

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Einstein’s Brain

Witelson et al (1999)

•His brain was the same as that of most others, except his lacked a small wrinkle (the parietal operculum) which most have.

•The lack of this region may have provided a compensatory increase in other regions of the brain - the inferior parietal lobes; a region associated with visual imagery and mathematical thinking.

•Einstein’s own words about his thinking process,”…words do not seem to play any role” but there is “associative play” of “more or less clear images” of the “visual and muscular type”.

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Cerebral Lateralization or Left Brain-Right Brain Dominanance

Split-Brain Experiments

Spinal Cord

•Contains major pathways which shuttle information between the brain and the periphery of the body.

•Contain interneurons which do not extend out of the CNS. They modify information passing through them.

•Severed spinal cord leads to paralysis.

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•Cell bodies of sensory neurons are in the dorsal root ganglia. They form synapses in the dorsal horn.

•Cell bodies of somatic motor neurons are found in the ventral horns.

CNS is Protected by the Skull and Vertebral Column

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Meninges Stops Neural Tissue From Bruising

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Blood-Brain Barrier Protects the Brain From Harmful Substances

•Protects the brain from harmful substances entering the interstatial fluid.

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•The endothelial cells which form the walls of the capillaries are connected together by tight junctions, not leaky junctions and pores.

•Paracrines secreted by astrocytes induce the formation of tight junctions.

•Some areas are not protected, such as the vomiting centre of the medulla.

Autonomic Nervous System

Symapthetic & Parasympathetic Divisions

•In fight-or-flight situations, digestion is of low priority. Heart and skeletal muscle prepare for high level of activity.

•The maintenance of homeostasis in the body involves balancing sympathetic and parasympathetic activity.

References

1. Tortora, G.J. & Grabowski, S.R (2003). Principles of Anatomy & Physiology.New Jersey: John Wiley & Sons. Ch.12: pp.385-395; Ch.13: pp.419-425, Ch.14: relevant sections.

2. Silverthorn, D.U (1998). Human Physiology: An Integrated Approach. New Jersey: Prentice Hall. Ch.8: pp.208-224; Ch.9: pp.235-244, pp.248-250; Ch.11: pp.307-310.

3. Witelson, S.F. et al., (1999). The exceptional brain of Albert Einstein. Lancet 353: pp.2149-2153.

Einstein’s Brain Removed Within 7hrs of His Death in 1955

•Cell bodies of sensory neurons are in the dorsal root ganglia. They form synapses in the dorsal horn.

•Cell bodies of somatic motor neurons are found in the ventral horns.

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