Nerve 1 (Anatomy) Flashcards
1
Q
Explain the meaning of the terms central nervous system and peripheral nervous system
A
- CNS - consists of the brain and spinal cord, which are protected by the cranium and vertebral column. It contains relay neurones (interneurones).
- PNS - consists of the rest of the nervous system outside the CNS. It is composed of cranial nerves, spinal nerves, and peripheral nerves, and it contains sensory neurones and motor neurones.
2
Q
Explain the meaning of the terms motor and sensory
A
Sensory (afferent) division
- Carries signals from from receptors (sense organs and simple sensory nerve endings) to the CNS.
- Somatic sensory division carries signals from receptors in the skin, muscles, bones, and joints.
- Visceral sensory division carries signals mainly from the viscera of the thoracic and abdominal cavities.
Motor (efferent) division
- Carries signals from the CNS to effectors e.g. glands and muscle cells.
- The somatic motor division carries signals to the skeletal muscle to produce voluntary muscle contraction and involuntary somatic reflexes.
- The visceral motor division (autonomic nervous system, ANS) carries signals to glands, cardiac muscle, and smooth muscle.
- The responses of the ANS and its effectors are visceral reflexes.
- The ANS is divided into the sympathetic division and the parasympathetic division.
- The sympathetic division arouses the body for action but inhibits digestion.
- The parasympathetic division has a calming effect but stimulates digestion.
3
Q
Explain the meaning of the term ganglion
A
A ganglion is a knot-like swelling in a nerve where the cell bodies of peripheral neurones are concentrated.
4
Q
Explain the meaning of the term neurone
A
Another term for a nerve cell.
5
Q
Explain the meaning of the term dendrite
A
- The branching of thick processes coming out of the somas of most neurones.
- They receive signals from other neurones.
- The more dendrites a neurone has, the more information it can receive and incorporate into its decision making.
6
Q
Explain the meaning of the term axon
A
- Originates from a mound on one side of the neurosoma called the axon hillock.
- Cylindrical and relatively unbranched for most of its length, although may give rise to a few branches called axon collaterals near the soma.
- Specialised for rapid conduction of nerve signals to points remote from the soma.
7
Q
Explain the meaning of the terms myelinated and unmyelinated
A
Myelinated
- Myelinated axons are surrounded by a spiral layer of insulation called the myelin sheath which ends with a thick outermost coil called the neurilemma.
- The neurilemma contains the nucleus and cytoplasm of the Schwann cell.
- The myelin sheath is produced by neuroglial cells (Schwann cells in the PNS and oligodendrocytes in the CNS).
- The gap between each myelinating cell is called the Node of Ranvier.
- Myelin acts as an insulator, allows for faster transmission of nerve signals, and helps with repair of axons.
Unmyelinated
- Unmyelinated axons in the PNS are still enveloped in Schwann cells.
- The axons sit in invaginations of the Schwann cell’s cytoplasm.
8
Q
Label the principal parts of the spinal cord and describe its relationship to its associated spinal (dorsal root) ganglia
A
- Grey matter - has a relatively dull colour because it contains little myelin. Contains the somas, dendrites and proximal parts of the axons of neurones. It is the site of synaptic contact between neurones, and therefore the site of all neural integration in the spinal cord.
- White matter - has a pearly white appearance due to an abundance of myelin. It is composed of bundles of axons called tracts that carry signals from one level of the CNS to another.
- Both grey and white matter also have an abundance of glial cells.
Grey matter:
- Consists of two posterior (dorsal) horns and two thicker anterior (ventral) horns.
- The posterior horns receive sensory nerve fibres from the spinal nerves, which usually synapse with networks of interneurones in the horns.
- The anterior horns contain the large neurosomas of motor neurones whose axons lead out to the skeletal muscles.
- On each side of the grey matter there is an additional lateral horn. These contain neurones of the sympathetic nervous system which send their axons out of the cord by way of the anterior root along with the somatic efferent fibres.
- The right and left sides of the grey matter are connected by the gray commissure and in the middle of the commissure is the central canal.
White matter:
- Axon bundles are arranged in three pairs called columns or funiculi - a posterior (dorsal), lateral and anterior (ventral) column on each side.
- Each column consists of subdivisions called tracts or fasciculi.
9
Q
Describe the component parts of a peripheral nerve and its relationship to the spinal cord
A
- Nerve fibres of the PNS are ensheathed in Schwann cells which from a neurilemma and often a myelin sheath around the axon.
- External to the neurilemma is the basal lamina and then a thin sleeve of loose connective tissue called the endoneurium.
- The fibres are gathered in fascicles and each fascicle is wrapped in a sheath called the perineurium.
- Several fascicles are bundled together and wrapped in epineurium to compose the nerve as a whole.
- The epineurium consists of dense irregular connective tissue and protects the nerve from stretching and injury.
10
Q
Describe the structure and function of different types of neurone and where they occur
A
Multipolar neurones
- Have one axon and multiple dendrites.
- Sensory, motor, and interneurones.
- Most neurones of the brain and spinal cord.
Bipolar neurones
- Have one axon and one dendrite.
- Sensory, motor, and interneurones.
- Olfactory cells of the nose, certain cells of the retina, and sensory neurones of the ear.
Unipolar (pseudounipolar) neurones
- Have only a single process leading away from the soma (dendrite and axon emerging from same process).
- Sensory, motor, and interneurones.
- They are represented by the neurones that carry signals to the spinal cord for such senses as touch and pain.
Anaxonic neurones
- Have multiple dendrites but no axon.
- Communicate locally through their dendrites and do not produce action potentials.
- Found in the brain, retina, and adrenal medulla.
11
Q
Give an account of the structure and function of neuroglial cells of the CNS
A
Astrocytes
- Most abundant glial cells in the CNS.
- Two types (fibrous astrocytes = in white matter, protoplasmic astrocytes = in grey matter).
- Cover the entire brain surface and most non-synaptic regions of the neurones in the grey matter.
- Star-shaped, highly branched.
- Form a supportive framework for nervous tissue.
- Extensions called perivascular feet which contact blood capillaries and stimulate them to form the protective blood-brain barrier.
- Monitor neurone activity and regulate blood flow in the brain tissue.
- Convert blood glucose to lactose and supplies this to the neurones for nourishment.
- Secrete nerve growth factors that regulate nerve development.
- Influence synaptic signalling between neurones.
- Regulate composition of tissue fluid by absorbing ions.
- When neurones are damaged, astrocytes form hardened scar tissue. This process is called astrocytosis.
Oligodendrocytes
- Resemble an octopus. Have a bulbous body with as many as 15 arms.
- Each arm spirals around a nerve fibre to form a myelin sheath.
- The myelin sheath insulates and speeds up conduction in the nerve fibre.
Ependymal cells
- Resemble a cuboidal epithelium lining the internal cavities of the brain and spinal cord (central canal of the spinal cord and cerebral ventricles).
- Unlike true epithelial cells they have no basement membrane and have root-like processes that penetrate into the underlying tissue.
- Produce cerebrospinal fluid which bathes the CNS and fills its internal cavities.
- Have cilia on their apical surfaces that help circulate the cerebrospinal fluid.
Microglia
- Differ from all other glial cells in originating from blood monocytes, not from neural tissue precursors.
- Small elongated cells with irregular processes.
- Phagocytise dead tissue, microorganisms, and other foreign matter in the CNS.
- They become concentrated in areas damaged by infection, trauma, or stroke.
- Aid in synaptic remodelling (changing connections between neurones).
12
Q
Give an account of the structure and function of neuroglial cells of the PNS
A
Schwann cells (or neurilemmocytes)
- Envelope nerve fibres by production of a myelin sheath.
- Speeds up conduction in nerve fibres by saltatory conduction due to breaks in the myelin sheath known as Nodes of Ranvier.
- Assist in regeneration of damaged nerve fibres.
Satellite cells
- Located within PNS ganglia (aggregated sensory or autonomic neuronal cell bodies).
- Enclose each perikaryon to provide insulation and regulate its microenvironment.
13
Q
Describe the structural features of a synapse
A
- The presynaptic neurone contains synaptic vesicles, many of which are docked at release sites on the plasma membrane, ready to release neurotransmitter on demand.
- A reserve pool of synaptic vesicles are located a little farther away from the membrane, tethered to the cytoskeleton.
- Separating the pre-synaptic and post-synaptic neurones is a space called the synaptic cleft.
- The post-synaptic neurone lacks synaptic vesicles and can therefore not release neurotransmitter.
- The post-synaptic neurone has neurotransmitter receptors and ligand-gated ion channels.
- Axosecretory = secretes into blood stream, Axoaxonic = into another axon, Axodendritic = axon terminal ends on a dendrite spine, Axoextracellular = secretes into extra pcellular fluid, Axosomatic = axon terminal ends on soma, Axosynaptic = terminal ends on another axon terminal.
14
Q
Describe other examples of neuronal endings e.g. Pacinian corpuscles, muscle spindles and motor end plates
A
Motor end plate
- An axon loses its myelination and forms a dilated termination called a neuromuscular bulb.
- The neuromuscular bulb sits within a trough on the muscle fibre surface.
- Several neuromuscular bulbs form a motor end plate.
- A neuromuscular junction is a chemical synapse formed by the contact between a motor neurone and a muscle fibre.
- At a neuromuscular junction a motor neurone is able to transmit a signal to the muscle fibre and cause muscle contraction.
- Toxins that act on neuromuscular junctions include Botulinum toxin, tetanus toxin, and nerve gases.
Pacinian corpuscles
- Consist of a single dendrite running through the core of the organ, encapsulated by multiple concentric layers of cells.
- The innermost capsule layers are flatted Schwann cells, but the greater bulk consists of concentric layers of fibroblasts with narrow fluid-filled spaces between them.
- They are phasic receptors chiefly for vibration.
- Respond with one or two action potentials when pressure is applied and again when it is taken away, but not in between.
Muscle spindle
- A bundle of seven or eight small, modified muscle fibres enclosed in an elongated fibrous capsule.
- Especially concentrated at the ends of a muscle, near its tendons.
- The modified muscle fibres within the spindle are called intrafusal fibres, and those that make up the rest of the muscle and do its work are called extrafusal fibres.
- Their function is to inform the brain of muscle length and body movements so that muscle tone, posture, coordinated movement, and corrective reflexes can be controlled.
15
Q
What are Nissl bodies
A
Combination of free ribosomes and RER.
