Intro to neuroanatomy and the brain Flashcards
What are the brain and spinal cord surrounded by
Three layers of connective tissue meninges, from the outer layer in is goes dura mater, arachnoid mater and pia mater.
Dura mater
Outermost meningeal layer which forms a sheaf around the spinal cord that extends from the foramen magnum to the lower border of the second sacral (S2) border
Arachnoid matter
Intermediate meningeal layer that is also extended from foramen magnum to the lower border of second sacral (S2) vertebra.
Pia mater
Deepest meningeal layer that is inseparable from the surface of the spinal cord. Below the spinal cord, it continues as a thread-like structure called filum terminale, which is attached to the dorsal aspect of the 1st coccygeal vertebra. Lateral extensions of pia mater are called the denticulate ligaments.
Afferent (sensory) fibres
Convey nerve impulses to the CNS from the sense organs and receptors
Efferent (motor) fibres
Convey nerve impulses from the CNS to the effector organs
Function of nervous system
Sensory function- gathers information from inside and outside the body
Transmission- sends sensory information to the processing area of the brain and spinal cord
Integrative function- processes information to determine the best response
Motor function- sends information to effectors for muscular contraction and glandular secretion
Three main parts of brain
- Forebrain = Telencephalon (cerebrum) and diencephalon (thalamus, hypothalamus)
- Midbrain
- Hindbrain = Pons, Medulla oblongata & Cerebellum
The brainstem
A collective term for the midbrain, pons and medulla oblongata
What surrounds the brain
The three meninges, which are continuous with the spinal cord. The cerebrospinal fluid surrounds the brain and is found in the subarachnoid space.
Cerebrum
Largest part of the brain. Consists of a left and right hemisphere connected by a mass of white matter called the corpus callosum. Each hemisphere has a frontal, temporal, parietal and occipital lobe.
Myelin
Spiral extension of the glial plasma membrane which wraps around the axon.
Process of myelination
- Oligodendrocytes extend processes to interact with the axon of the neuron.
- Once in contact, reorganization of the cytoskeleton occurs to allow lateral and radial expansion so that they wrap around the axon.
- The extension of the processes occurs in the innermost region, closer to the axon.
- In addition to extension, compaction of these multi-layer membrane starts in the outermost layers.
- Ion channels cluster in the region where there is no axon-glial interaction, forming the node of Ranvier.
Myelination on PNS and CNS
In PNS there is one myelin per schwann cell, the schwann cell and axon are very close together. In CNS there are multiple myelin per oligodendrocytes, the oligodendrocyte and axon are far apart.
Nerves
Made of neurons. Have both cranial and spinal nerves that are made of sensor and motor neurons
Structure of spinal nerve
The dorsal root (sensory neurone) and ventral root (motor neurone) separate from the interneuron in the grey matter, they exit via the dorsal and ventral horn respectively. Further along the motor and sensory meet to form the spinal nerve which then divides in the dorsal and ventral ramus. Both ramus contain a mix of sensory and motor nerves.
Division of the nervous system
Contains the peripheral nervous system (PNS) which has 12 cranial nerves and 31 spinal nerves. It contains the central nervous system (CNS) which has the brain and spinal cord
Sympathetic nervous system summary
Prepares the body for emergencies so ‘fight or flight’. Part of the autonomic nervous system
Parasympathetic nervous system summary
Conserves and restores energy ‘rest or digest’. Part of the autonomic nervous system
Somatic nervous system
Controls voluntary movement
Structure of a neuron
Dendrites- receives signals from other cells
Cell body- organises and keeps the cell functional
Cell membrane-protects cells
Nucleus- controls cell
Axon- transfers signal to other cells
Axon hillock- generates impulse in the neuron
Node of ranvier- allows diffusion of ions
Schwann cell- produces the myelin sheath
Myelin sheath-increases signal speed
Axon terminal- forms junction with other cells
Why is myelination useful
Increases nerve impulse speed. The myelinated regions have no ion channels so no action potentials are generated. The action potential now have to jump between the nodes of ranvier which is quicker
Ependymal cell
Simple cuboidal epithelial cells that line fluid filled passageways within the brain and spinal cord. Produce the cerebrial spinal fluid, this helps to maintain pH and acts as a cushion to protect the nervous system.
Microgilial cells
Phagocytes that move through the nervous tissue removing unwanted substances
Main cell types of the brain
Neurones, astrocytes, oligodendrocytes, microglia and ependymal cells
Oligodendrocytes
Cells with sheet like processes that wrap around axons. Acts as the mylin sheath in the CNS, which acts as an insulator, speeding up the nervous transmission.
Astrocyte
Star shaped cells with projections that anchor them to capillaries. They form the blood brain barrier, which isolates the CNS from general circulation. They recycle the neurotransmitter and supply nutrients.
What affects movement of ions
Concentration gradient (chemical potential), electrical potential (net charge) and the presence of specific ion channels
How is resting potential maintained
The sodium-potassium pump, moves 3 sodium ions out of the cell whilst moving two potassium ions into the cell, against their concentration gradient. A leaky K+ channel then lets K+ out of the cell. Overall, this causes intracellular compartment to be more negative then the extracellular space. The resting membrane potential is -70mV.
What is equilibrium potential
The membrane potential that exactly balances the concentration gradient of the ions across the membrane. When the equilibrium potential and membrane potential are close together ions will stop moving even if there is a concentration gradient. Meaning positive ions will not move into a more positive space or down a positive equilibrium potential
Brief summary of generating an action potential
Stimulus -> Depolarisation (cell becomes positive) -> Repolarisation (cell becomes negative) -> Hyperpolarisation (cell becomes more negative) -> resting membrane potential
Depolarisation
A stimulus opens some of the voltage-gated sodium channels. Na+ flows down its concentration gradient into the axon, increasing the positivity inside the cell. The membrane potential reaches the threshold potential (-55mV) and more voltage gated Na+ channels open. This allows more Na+ to move into the axon, depolarising the axon. The inside of the cell is now more positive then the outside.
Repolarisation
The voltage gated Na+ channels close when the membrane potential is positive, Na+ stops moving into the axon. 6. At +40mV the voltage gated K+ channels open, allowing K+ ions to flow down its concentration gradient out of the axon. The axon become more negative as it loses positive charge. This is repolarization of the membrane.