chapter 7: the structure of the nervous system

Chapter 7: The Structure of the Nervous System

Korbinian Brodmann (1868-1918) - responsible for establishing the

science of comparative cytoarchitecture of the mammalian cortex

Comparative morphology of the CNS studies the interrelationships between different species of animals and the brain and spinal cord.

This comparative approach reveals significantly similarities and several differences between species.

Gross Organization of the Mammalian Nervous SystemGross Organization of the Mammalian Nervous System

The Central Nervous System of the Rat

– Cerebrum, cerebellum, brain stem– Spinal Cord

The Spinal Cord

– Location: Attached to the brain stem– Conduit of information (brain body)– Spinal nerves

– Dorsal root – Ventral root

The Peripheral Nervous System

– Nervous system outside of the brain and spinal cord

– Somatic PNS: Innervates skin, joints, muscles

– Visceral PNS: Innervates internal organs, blood vessels, glands

– Dorsal root ganglia: Clusters of neuronal cell bodies outside the spinal cord that contain somatic sensory axons

– Ventral roots: neuronal cell bodies outside the spinal cord that contain motor neurons

Afferent and Efferent Neurons– Afferent (carry to): Carry information toward a particular point– Efferent (carry from): Carry information away from a point

The Cranial Nerves– 12 nerves from brain stem– Mostly innervate regions of the head

Note that these cranial nerves will be further discussed in the anatomy section.

Meninges – Three

membranes that surround the brain

• Dura mater• Arachnoid mater• Pia mater

This view of the meninges helps to illustrate the net-like arrangement of the arachnoid mater.

Brain floats in cerebrospinal fluid (CSF)

Liquor cerebrospinalis, is a clear bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord. Essentially, the brain "floats" in it.

– Ventricles: CSF-filled caverns and canals inside brain

– Choroid plexus: specialized tissue made of ependymal cells in ventricles that secretes CSF

CSF serves four primary purposes:

1.Buoyancy: the human brain is about 1400 grams; however, the net weight of the brain suspended in the CSF is equivalent to a mass of 25 grams. The brain therefore exists in near neutral buoyancy, which allows the brain to maintain its density without being impaired by its own weight, which could cut blood supply and kill neurons in the lower sections without CSF.

2.Protection: CSF protects the brain tissue from injury when jolted or hit. In certain situations such as auto accidents or sports injuries, the CSF cannot protect the brain from forced contact with the skull case, causing hemorrhaging, brain damage, and sometimes death.

3.Chemical stability: CSF flows throughout the inner ventricular system in the brain and is absorbed back into the bloodstream, rinsing the metabolic waste from the central nervous system through the blood-brain barrier. This allows for homeostatic regulation and distribution of neuroendocrine factors.

4.Prevention of brain ischemia: made by decreasing the amount of CSF in the limited space inside the skull. This decreases total intracranial pressure and facilitates blood perfusion.

Hydrocephaly in a small child.

Hydrocephaly - known as "water on the brain," is a medical condition in which there is an abnormal accumulation of cerebrospinal fluid (CSF) in the ventricles, or cavities, of the brain.

The most performed treatments for hydrocephalus, the cerebral shunt, was first developed in 1960. The shunt must be implanted through neurosurgery into the patient's brain.

Computed Tomography (CT)– Hounsfields and Cormack developed the device and received the

1979 Nobel Prize for this work

– Generates an image of a brain slice much like histology of cadaver brains

– X-ray beams are used to generate data that generates a digitally reconstructed image

Hounsfields and Cormack

Magnetic Resonance Imaging (MRI)

– Advantages of MRI over CT• greater detail• not a significant source of irradiation• Brain slice image in multiple angles

– Uses iinformation on how hydrogen atoms respond in the brain to perturbations of a strong magnetic field – signals mapped by computer

A classic MRI device.

Functional Brain Imaging Techniques– Positron emission tomography (PET) - a nuclear medicine imaging technique

which produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.

- Functional MRI (fMRI) - a type of specialized MRI scan. It measures the hemodynamic response (change in blood flow) related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging.

Basic Principles• Detect changes in regional blood flow and metabolism within the brain.• Active neurons demand more glucose and oxygen, more blood to

active regions, techniques detect changes in blood flow.

Ventricular System and the CNS– The CNS forms from the walls of a fluid-filled neural tube– The inside of the tube becomes ventricular system– The neural tube

• Endoderm, mesoderm, ectoderm• Neural plate neural groove• Fusion of neural folds

– Neural tube (forms CNS neurons)– Neural crest (forms PNS neurons)

Understanding CNS Structure Through Development

• Formation of the Neural Tube

Anencephaly - a fatal neural tube defect in which the cerebral hemispheres do not develop.

Typically some brainstem tissue is present, but it is anatomically highly disorganized.

Spina bifida – literally means cleft spine, which is an incomplete closure in the spinal column. In general, the three types of spina bifida (from mild to severe) are: Spina Bifida Occulta, Meningocele, & Myelomeningocele

• Putting the Pieces Together

Special Features of the Human CNS Compared to that of a Rodent

– Many similarities in rat and human brain

• Basic arrangement of various structures

Yet, differences are seen in:

•Special Features of the Human CNS•Differences•Convolutions on human cerebrum surface called sulci and gyri•Size of olfactory bulb•Growth of cerebral hemisphere: Temporal, frontal, parietal, occipital

Comparison of Three Different Mammalian Brains