Structure and function of the human nervous system - 1 Flashcards
1
Q
The Neural Axes
A
- Rostral (anterior) - ‘towards the beak’
- Caudal (Posterior) - ‘towards the tail’
- Dorsal (Superior) - ‘towards the back’
- Ventral (inferior) - ‘toward the belly’
- Terms denoting directions are used to locate structures within the nervous system. These directs are normally described in terms of neuraxis, which is an imaginary line drawn through the spinal cord toward the front of the brain
- It is easiest to begin with an animal that has a relatively straight neuraxis, as is the case for most four legged animals (e.g. dogs)
2
Q
The Neural Axes - Humans
A
- Because human stand upright, their neuraxis bends
- The top of the head is perpendicular to the back
- In humans the neuraxis is slightly more complicated as it has a bend where the spinal cord meets the brain
- In addition to the terms just introduced, we can add four more that are important for defining positions of structures in the nervous system:
- Lateral = toward the side
- Medial = toward the midline
- Ipsilateral = structures on the same side of the body (in Latin ipse means ‘same’)
- Contralateral = structures on the opposite side of the body (in Latin contra means ‘against’)
3
Q
Major divisions of the nervous system
A
- The nervous system has two major divisions:
- CNS = brain and spinal cord
- PNS = includes cranial nerves, spinal nerves and peripheral ganglia
- PNS further divide into subcomponents:
- Somatic system = connects CNS to voluntary muscles
- Autonomic nervous system = connects the CNS to non-voluntary muscles and glands
- Autonomic nervous system also subdivided into two systems that tend to operate in opposition:
- Sympathetic system = arousing, prepares body for activity and therefore expends energy (flight/fight responses)
- Parasympathetic = calming, prepares body for restoration of energy
4
Q
The Meninges
A
- Three layered sheath surrounding brain and spinal cord (CNS):
- Dura mater - ‘tough mother’ = thick outer layer
- Arachnoid mater - ‘spider-like mother’ = middle layer, has a weblike appearnace due to the protrusions called arachnoid trabeculae, and is soft and spongy
- Pia mater - ‘pious mother’ = delicate inner layer, which follows every fold of brain tissue
- The entire nervous system (CNS and PNS) is covered by a protective sheath of connective issue
- The protective sheaths around the brain and spinal cord are called the meninges
- Lying between the arachnoid mater and the pia mater is the subarachnoid space, which holds the fluid (cerebrospinal fluid) that bathes the brain and spinal cord, and which contains the main arteries that cover the surface of the brain and spinal cord
- NOTE: PNS only has TWO protective sheaths, the dura mater and the pia mater, and these fuse together to form a single layer that covers the spinal nerves and peripheral ganglia
- Neurological disorders such as viral meningitis and meningococcal disease attack the meninges and impair the CNS
5
Q
Cerebrospinal fluid (CSF)
A
- CSF - clear fluid, like blood plasma
- Supports brain; reduces shock from head movements
- Produced in choroid plexus, a structure rich in tiny blood vessels located in the lateral ventricles (there are two of these, one in each brain hemisphere). From the lateral vesicles CSF flows down to the third ventricle, then through the cerebral aqueduct to the fourth ventricle. From here it exits via a set of openings into the subarachnoid space, before being reabsorbed back into the bloodstream via arachnoid villae
- Resides in subarachnoid space around the outside of the brain and spinal cord, also fills the hollow, interconnected chambers inside the brain known as ventricles (many early philosophers and physicians believed the ventricles were the seat of the mind)
- Reduces weight of brain from an average of 1400 grams to a net weight of about 80 grams
6
Q
The Ventricles
A
- The ventricular system in the brain consists of a set of linked, fluid-filled chambers.
- The lateral ventricles are located within each hemisphere (one of each side)
- These are linked centrally with the third ventricle, which is located in the midline of the brain
- A long tube called the cerebral aqueduct connect the third ventricle to the fourth ventricle, which sits immediately beneath the cerebellum (‘little brain’)
7
Q
Obstructive Hydrocephalus
A
- Occasionally the flow of CSF is blocked somewhere in its journey between the choroid plexus within the lateral ventricle and the arachnoid villi within the subarachnoid space, which channel it back into the bloodstream
- Such blockages cause a conditon called hydrocephalus (‘water head’), in which CSF accumulates within the ventricles because it is not reabsorbed into the bloodstream
- This raises pressure inside the skull, and can damage brain tissue and occlude (obstructed) arteries, leading to permanent (sometimes fatal) brain damage
- Hydrocephalus can be treated by interserting a ventriculo-peritoneal (VP) shunt. A hole is drilled in the skull and a fine tube is inserted into one of the ventricles. The tube runs beneath the skin, down into the person’s abdominal cavity (the peritoneum), from where it can be reabsorbed into the bloodstream. When pressure started to increase in the ventricles, a release valve in the tube opens and the excess CSF it allowed to flow out
8
Q
Development of the CNS
A
- The nervous system begins to develop around 18 days after conception. The embryo begins as a plate of cells, whose edges form ridges that curl towrads one around and fuse, forming a tube (the neural tube) that extends longitudinally from rostral to caudal
- At about 28 days after conception, the neural tubes has differentiated to form three interconnected chambers. These chambers are destined to become the ventricles, and the surrounding tissue will form the three main components of the adult brain; the forebrain, the midbrain and the hindbrain. The ‘tail’ connected to the hindbrain chamber will form the spinal cord
- Later in development, the tissue of the forebrain, midbrain and hindbrain differentiate to form the precursors of the major structures present in the adult brain. The chamber of the forebrain divides to form the two lateral ventricles and the third ventricle. The chamber inside the midbrain narrows to form the cerebral aqueduct, and the chamber inside the hindbrain becomes the fourth ventricle.
9
Q
Anatomical subdivisions of the brain
A
- All of the major structures of the brain can be associated with one of the three early precursors; the forebrain, midbrain and hindbrain
- As mentioned earlier, each of the precursor consists of a hollow chamber that will eventually form one of the ventricle s
- In the fully formed brain, there are five major divisions: telecephalon, diencephalon, mesencephalon, metencephalon and myelencephalon (the world cephalo menas brain and the prefixes denote positions (e.g. telencephalon means ‘end brain’ because it is at the rostral end of the neural tube)
10
Q
Neural Migration
A
- Founder cells in the ventricular zone of neural tube give rise to cells of the CNS
- Radial glia guide neurons during development
- Cortical development ceases with apoptosis (programmed cell death)
- Those cells that line the inside of the neural tube, a region known as the ventricular zone, give rise to the cells of the nervous system
- These precursors are called founder cells, and they can become either neurons or glial cells
- Founder cells divide and migrate away from the centre of the tube towards the periphery
- Cortical development ceases when the founder cells receive a chemical message that binds with receptors that activate ‘self-destruct’ genes within the cell. They then die off, a process known as apoptosis (‘to fall away’)
11
Q
Neural migration - cerebral cortex
A
- The cerebral cortex (important outer layer of the brain) develops from the inside out:
- The first cells to grow out from the venricular zone travel only a small distance and establish the first layer of the cortex
- The next cells grow through the first layer to form a second layer, and so on until six layers have been created
12
Q
Neural migration - how do neurons know where to migrate to in the developing brain
A
- They are guided by a special type of glial cells called the radial glia
- These cells extend fibres outward from the ventricular zone, and their cuplike endings attach to the surface of the developing cortex
- Newly formed neurons in the ventricular zone then ‘crawl’ along these radial glia cels until they reach their destination
13
Q
Histogenesis of the cortex
A
- The positions to which different types of neurons migrate during development determines the layered structure of the mature cortex
- The layers are defined by the types of cells in them and by the structures of the neuron that are present (e.g. cell bodies, axons)
- The mature neocortex in humans cosist of six distinct layers
14
Q
The telencephalon
A
- The forebrain has two subdivisions, the telencephalon and the diencephalon
- The telencephalon is composed of the two cerebral hemispheres, which together form the cerebrum
- The hemispheres comprise an outer layer called the cortex (‘bark’) and an inner (subcortical) region that contains the basal ganglia and the limbic system
- In humans, the cerebral cortex is highly convoluted:
- It is characterised by large, deep grooves called fissures, smaller grooves called sulci and bulging regions of tissue between foldes called gyri. The convolutions of the cerebal cortex allow a large amount of tissue to fit into a relatively small space (the cranial cavity). The cortex consists predominantly of the cell bodies and associated dendrites of neurons, together with the supporting glial cells. This region, which is around 3mm thick is sometimes called the grey matter due to its greyish-brown appearance
- Beneath the cortex run the axons of the neurons of the cortex, which connect these cells to those located elsewhere in the brain. The axons are covered in myelin, and so this region is ofen known as the white matter
- The two hemispheres of the cerebrum appear symmetrical, though they are in fact structurally slightly asymmetrical. Moreoever, each hemisphere performs many functions that are unique to it
- Although the many sulci and gyri of the cortex may seem random, they in fact follow a well defined patter with small variations across individuals. The gyri and sulci between them are used to denote landmarks on the surface of the cortex
- The purpose of the brain is to process sensory information in order to guide movement (and thus control behaviour). Accordingly, the cerebral cortex has several regions that receive information from the sensory organs, and further regions that control the muscles for movement. Different regions of the cortex subserve different perceptual and motor functions
- Franz Joseph Gall was the first to suggest the ‘localisationist’ view of the brain function in the 18th century. His general concept of localised cerebal functions seems to have been correct, at least for some cortical areas, but Gall was certainly wrong in attributing complex personality traits to specifc areas
15
Q
Cortical Development Age 4-21 years
A
- Once neurons reach their destination they begin to form connections with other neurons
- Many more neurons are produced during development than are needed, and so those that have been created must compete to exist (some have called this ‘neural Darwinism’ as the process resembles the process of natural selection)
- Only those neurons whose axon makes contact with another neuron will survive; they receive a chemical message from the postsynaptic cell that keeps them alive
- Those neurons (around 50% of total) that do not form synapses with another cell do not receive this message and so die by apoptosis. Although this seems like wasteful strategy, it seems to be a safer strategy than trying to create exactly the right number of neurons during development.
- Yellow and red coloration represents relatively ‘immature’ areas of the cortex, whereas blue and purple colorations represents more ‘mature’ areas. Some areas reach a mature stage relatively early in like: primary motor area (responsiblie for controlling body movement), the primary somatosensory area (reposponsible for our skin senses of touch, pain and temperature), and the primary visual area (responsible for vision)
- Other areas do not mature until much later in life (in some cases right through to early adulthood: such as the prefrontal crotex and parietal cortex, which serve to integrate sensory and motor processes, and to plan complex behaviours.
- The animations were created by conducting a magnetic resonance brain scans on healthy individuals every two years, and then overlaping the images acquired to create a ‘time lapse’ sequence. The colours have been added to the resulting sequence to illustrate regional changes over the cerebal cortex with age
