Gross Organization I The Brain. Reading: BCP Chapter 7

Transcription

1 Gross Organization I The Brain Reading: BCP Chapter 7

2 Layout of the Nervous System Central Nervous System (CNS) Located inside of bone Includes the brain (in the skull) and the spinal cord (in the backbone) Interprets sensory input, initiates movement, and mediates complex cognitive processes Peripheral Nervous System (PNS) Located outside of bone Includes nerves Serves to bring sensory information into the CNS (called afferents) and carry motor signals out from the CNS (efferents)

3 Major Parts of the Nervous System External environment Internal environment Sensory input Motor output A system of twos

4 Anatomical References All vertebrates (including humans) have the same basic body plan they are strictly bilaterally symmetrical in early embryonic stages and largely bilaterally symmetrical in adulthood. Thus, there are standard anatomical terms of location for all vertebrates (and many invertebrates). Positional descriptive terms are with respect to the organism in its standard anatomical position: anterior/rostral towards the front/nose posterior/caudal towards the back/tail dorsal towards the top ventral towards the bottom medial towards the middle lateral towards the side Structures on the same side of the head are said to be ipsilateral; if on opposite sides of the head, then contralateral.

5 Anatomical Directions in Humans In humans, the directions in the cerebral hemispheres are rotated by 90 in comparison to those in the spinal cord (and brain stem) because of the unusual upright posture of humans. Thus, for example, the top of the head and the back of the body are both dorsal even though the directions are different.

6 Planes of Section To view the internal structures of the brain, it is usually necessary to slice it up. A slice is called a section, and there are three standard perpendicular planes of section: sagittal left vs right (a midsagittal cut separates the left and right halves of the brain) horizontal top vs. bottom (parallel to the ground) frontal (coronal) front vs back A cross-section is cut at a right angle to a long narrow structure (e.g., the spinal cord)

7 The Brain The brain weighs about 3 lbs. Visual inspection (lateral view) reveals three parts that are common to all mammals: the cerebrum top-most, split into two cerebral hemispheres that each receive sensory input from and control motor output to the contralateral side of the body. the cerebellum behind/below the cerebrum, primarily a motor control center, two hemispheres each concerned with movement of the ipsilateral side of body. the brain stem forms the stalk from which the cerebrum and cerebellum sprout, fibers of passage, cranial nerves, basic functions (e.g., breathe rate). cerebrum brain stem cerebellum

8 The Cerebrum 1 The cerebrum is noteworthy for its convoluted surface (to allow more cortical surface area to exist in the confines of a smaller cranium): fissures: large grooves sulci: small grooves gyri: bumps Three major fissures (longitudinal, central and lateral) partially divide each hemisphere into four lobes: frontal; parietal; temporal and occipital Three large gyri: precentral; postcentral and superior temporal Lobes are not functional units Frontal Temporal Parietal Occipital

9 The Cerebrum 2 The cerebral hemispheres are connected by several fiber tracts called commissures. Dorsal view The largest commissure is called the corpus callosum. It is visible if the dorsal surfaces of the two hemispheres are gently pulled apart at the longitudinal fissure. The medial view (midsagittal cut) of the brain shows the callosum in cross section. Medial view

10 The Cerebrum 3 A frontal section of the cerebrum reveals two additional features of the gross organization of the brain. First, the cerebrum comprises three different areas: outer area of neuronal cell bodies (called gray matter or cerebral cortex) inner area of myelinated axons (called white matter) subcortical areas of gray matter Second, the brain contains fluidfilled caverns and canals, called the ventricular system. The cerebrum surrounds the paired, lateral ventricles.

11 The Cerebellum The cerebellum (Latin for little brain ), like the cerebrum, is a highly folded structure consisting of two hemispheres, each of which is divided into lobes. Each ridge or gyrus is called a folium, with gray matter at the edge and white matter inside. Medial view Fourth ventricle

12 The Brain Stem 1 The brain stem consists of four divisions: diencephalon (either side of third ventricle) -- thalamus -- hypothalamus midbrain (cerebral aqueduct) -- tectum -- tegmentum pons (below 4 th ventricle) medulla (below 4 th ventricle) The medulla is continuous with the spinal cord. Cerebral aqueduct Fourth ventricle

13 The Brain Stem 2 The underside of the brain shows the ventral aspects of the brain stem including the hypothalamus (diencephalon), tegmentum (midbrain), pons and medulla (hindbrain). In addition, 12 pairs of cranial nerves can be observed, most of which emerge from the brain stem. The cranial nerves provide sensory and motor innervation mainly to structures in the head and neck. Note that the axons from the eyes (optic nerve) cross (or decussate) as necessary at the optic chiasm prior to entering the brain.

14 Imaging the Brain 1 The gross structural organization of the brain, can now be obtained in the living brain. The are two main methodologies to detect the structure (and changes to it) in the living brain: computer tomography (CT) -- measures opacity to X-rays magnetic resonance imaging (MRI) -- measures hydrogen atom response to magnetic fields MRI series (top to bottom of brain) diffusion tensor imaging (DTI) used to visualize large bundles of axons

15 Imaging the Brain 2 Imaging methods can also be used to reveal functional organization in the living brain. Such techniques are based on the premise that cerebral blood flood and neuronal activation are tightly coupled. Two techniques are in widespread use. Positron emission tomography (PET) inject a radioactive substance have subject perform behavior (active cells take up substance) scan a horizontal slice of brain Functional magnetic resonance imaging (fmri) no substance injected have subject perform behavior (active cells need more oxygen/glucose) scan brain for de/oxygenated hemoglobin Paired image subtraction