3.0 Nervous System 1: Overview and Motor Reflexes Flashcards
Describe the organisation of the nervous system and flow of information
See organisation of nervous system diagram
Recall the main divisions of the central nervous system
The CNS consists of the brain and spinal cord.
The meninges wraps the CNS to protect and nourish it.
Regions: cervical, thoracic, lumbar, sacral, coccygeal
Cerebrospinal fluid (CSF) is an ultrafiltrate of plasma that surrounds the brain and spinal cord and is secreted by the choroid plexus, a specialised region on the walls of the ventricle cavities of the brain
Compare the different types of neurons found in the nervous system and recall their location and function
AFFERENT:
location= PNS, cell body in periphery
function = carry information from the outside world (e.g. sensations of heat, cold, and pain) and inside world (viscera) to the CNS.
EFFERENT:
location = PNS. Cell body in CNS.
function = transmits integrated output from the CNS to innervate muscles and glands
INTERNEURONS:
location = CNS
function = lie between afferent and efferent neurons, link input neurons to output neurons
List the major parts of the brain and categorise the dominant function of each part
Cerebrum
- the bulk of the brain that contains the cerebral cortex (the outer layer), and the subcortical structures (the areas located in the center of the brain)
- is divided into two halves; right hemisphere and left hemisphere
- the hemispheres are connected to each other and exchange information via a large bundle of nerve axons forming a white matter tract called the corpus callosum, the body’s “information highway”.
- characterized by numerous grooves and can be superficially divided into several lobes (see next page)
- the cerebral cortex is comprised of gray matter and is involved in higher brain functions such as reasoning, abstract thought, and language.
Cerebellum
- located at the base of the brain, below the cerebral cortex and behind the brainstem (pons, medulla)
- comprises just 10% of the brain’s total volume, but contains more than 50% of all the neurons in the brain
- important for smooth, coordinated, and precise movements, motor memory (e.g. playing a piano, riding a bike) and posture
- receives a huge amount of sensory input (visual, vestibular, somatosensory, motor) and fine-tunes motor activity accordingly
Diencephalon
- receives sensory inputs from every part of the body (except olfactory/smell) body and distributes the information to the appropriate areas of the brain
- lies between the brainstem and the cerebrum
- includes the thalamus, hypothalamus, pineal gland and pituitary gland.
- important in the regulation of the autonomic and endocrine systems
Brainstem
- located at the base of the skull
- is continuous with the spinal cord
- is comprised of the midbrain, pons, and medulla oblongata which act as a relay station between the spinal cord and the brain’s cerebral hemispheres.
- is the entry point and exit point for 10 of the 12 cranial nerves
- regulating vital autonomic functions.
Spinal cord
- a continuation of the brainstem from the medulla to the lower spine
- all along the spinal cord synapses occur between the central nervous system (CNS) and peripheral nervous system (PNS) neurons.
- a neuronal link between the brain and the PNS via ascending and descending tracts in the cord’s outer white matter relaying sensory and motor information from the cerebral cortex and cerebellum to the organs and limbs
- the integrating centre for spinal reflexes which includes the basic protective and postural reflexes and reflexes that control local blood vessels, gastrointestinal movements, or urinary excretion.
Recall the organisation of the major sensory and motor pathways
Corticospinal tract
Dorsal column tracts
Lateral Spinothalamic Tract
Name the three systems that influence motor control and describe how they modulate motor output
- sensory system - monitors the internal and external environments and initiates reflex responses
- cognitive system - able to initiate voluntary motor responses
- behavioural state system - concerned with sleep/wake cycles and other behaviours
Explain the main sources of energy for the central nervous system
glucose (?)
List the components of a reflex arc
- Sensory receptors throughout the body continually monitor conditions in the internal and external environments.
- The sensory division sends information along sensory neurons to the CNS.
- The CNS integrates the sensory information and determines whether a response is needed
- A response is sent from the CNS, travelling via efferent neurons of the PNS, to mainly muscles and glands, the effector organs.
Describe the components of a stretch reflex and the sequence of events that take place
A simple spinal reflex like the stretch (myotatic) reflex is produced via single synapses between sensory axons and motor neurons. The essential central circuitry for this reflex is confined to the spinal cord; however (although not shown in the figure below), this information also passes to the brain.
Explain the role of a withdrawal reflex
Another reflex mediates the withdrawal of a limb from a painful stimulus, such as a pinprick or the heat of a flame. In the flexion withdrawal reflex pathway, several synaptic links are involved; local circuit neurons receive converging inputs from several different sources such as stimulation of cutaneous pain (nociceptive) sensory neurons, other spinal cord interneurons, and upper motor neuron pathways
The flexion withdrawal reflex involves stimulation of pain (nociceptive) sensory neurons which leads to stimulation of flexor muscles, leading to withdrawal of the stimulated limb.
At the same time, there is stimulation of extensor muscles in the opposite limb, i.e. the withdrawal reflex is accompanied by an opposite reaction in the opposite limb. This “crossed extension reflex” serves to enhance postural support during withdrawal of the affected limb from the painful stimulus.
Describe grey matter
- consists of unmyelinated nerve cell bodies (nuclei) in the brain and spinal cord, and neurons (dendrites and axons)
- makes up the outer layer of the brain
Describe white matter
- mostly white fatty myelinated (insulated) axons of the gray matter neurons
- is where signals are conducted from one area of gray matter to another
- spinal cord white matter contains ascending and descending tracts (bundles of nerves) that run to and from the brain
Describe Corticospinal tract
The corticospinal tract is the major motor pathway which connects the upper motor neurons in the cerebral cortex to the spinal cord. The main function of the corticospinal tract is to carry motor information from the brain to the body for the voluntary control of the trunk and limbs.
The passage of the tract is as follows:
- The fibers pass through the corona radiata to the internal capsule.
- Enter the cerebral crus.
- Pass through the midbrain and pons, to the medulla oblongata.
- Decussate in the medulla.
- Enter and travel down the spinal cord, where the fibers synapse with lower motor neurons.
Describe Dorsal Column Tract
For touch, vibration and proprioception, a series of three neurons carry the information from the receptors to the primary sensory cortex.
First-order neurons
- Begin at the sensory receptors.
- Cell bodies are located in the dorsal root ganglia.
- Axons ascend in the dorsal column to the medulla, where they synapse with the second-order neurons.
Second-order neurons
- Cell bodies are located in medulla nuclei.
- Axons decussate in the medulla.
- Axons then ascend to the thalamus, where they synapse with the third-order neurons.
Third-order neurons
- Cell bodies are located in the thalamus.
- Axons ascend to the somatosensory area in the parietal lobe of the cerebral cortex, where they synapse.
Describe Lateral Spinothalamic Tract
For temperature and pain, a series of three neurons also carry the information from the receptors to the primary sensory cortex.
First-order neurons
- Begin at the sensory receptors.
- Cell bodies are located in the dorsal root ganglia.
- The axons synapse with the second-order neurons in the dorsal horn of the spinal cord.
Second-order neurons
- Cell bodies are located in the dorsal horn.
- Axons decussate in the gray matter of the spinal cord.
- Axons then ascend in the spinothalamic tract to the thalamus, where they synapse with the third-order neurons.
Third-order neurons
- Cell bodies are located in the thalamus.
- Axons ascend to the somatosensory area in the parietal lobe of the cerebral cortex, where they synapse.
