Lecture 7 CNS Flashcards
CNS
The brain stem
Functions include:
- Connects the spinal cord to the cerebral cortex
- Cranial nerves arise from nuclei in the brain stem
- Regulates cardiovascular and respiratory systems
- Contains networks of neurons called reticular formation which is important in consciousness
The midbrain
Contains motor and sensory tracts and nuclei
- CN III (Oculomotor) - innervates the superior rectus, medial rectus, inferior rectus, inferior oblique and superior palpebral levator muscles. It also provides a route along which the autonomic fibres travel to innervate the iris sphincter muscle, cililary muscle and the smooth muscles of the eyelid
- CN IV (trochlear) - innervates the superior oblique muscle
Dorsal area is concerned with visual and auditory systems
2 important nuclei:
- Red nucleus - motor coordination
- Substantia nigra - complex functions including eye movements
The Pons
Attached to cerebellum and consists of 2 parts:
- Dorsal, similar to rest of brainstem with motor and sensory tracts and nucleu
- CNV (trigeminal)
- CNVI (abducens)
- CNVII (facial)
- Ventral (or basal), provides connections between the two cerebral cortex for effective motor coordination
The Medulla
- Contains both the fibre tracts of the spinal cord and various nuclei
- Importantly it contains the inferior olivary nuclei which send fibres to the cerebellum
- Has cranial nerve nuclei
- CNVIII (vestibulocochlear nerve)
- CNIX, CNX, CNXI, CNXII
The cerebellum
Receives information from most of the sensory systems in the body and from the cerebral cortex.
It influences the tone of muscles unconsciously in relation to movement, posture and balance
The diencephalon - thalamus + epithalamus
- Thalamus - located superior to mid-brain
- Large grey matter
- Collects information from all sensory systems with the exception of olfactory
- Projects to the sensory areas of the cerebral cortex
- Regulates motor function
- Epithalamus - has tracts and nucleus
- Contains the pineal gland - secretes melatonin
Hypothalamus + Subthalamus
Hypothalamus
- Principal control of the autonomic nervous system
- Governs homone release and controls
- Body temperature
- Thirst, hunger
- Sleep, circadian rhythm
- Moods, sex drive
Subthalamus
Sensory tractsm relays & correlates motor function
The Limbic system
Function of the LGN
It is part of the thalamus, whose main functuon is to transmit information from outer sensory neurons to the brain.
Parvocellular and Magnocellular pathway
Magnocellular (parasol) cells are very sensitive to contrast relative to parvocellular (midget) cells. They pick up low contrast, but cannot discriminate well between high contrast, which parvocellular can.
For the magnocellular stream, signals travel to layer IVCα where they project to layer IVB via simple cells. These cells have bar-shaped receptive fields instead of circular ones, with ON-centre and OFF-centre variants -these are sensitive to orientation and location in said fields. The cells in layer IVB, on the other hand, process binocular responses (depth perception) and direction selectiveness of an stimulus -that is, they only fire depending on the direction that the stimulus is moving in.
In the occipital lobe, there are other visual areas such as V2 to V5, which process segregated visual information independently (e.g. V4 for colour, V5 for motion). This segregated info enters these regions via two main streams: ventral (‘what’, parvocellular inputs, descriptive info on objects, projects to V2, V4, and inferior temporal cortex) and dorsal (‘where’, magnocellular inputs, spatial info on objects and motion, projects to posterior parietal cortexvia V2, V3, V5, and MST).
In the parvocellular stream, signals travel to layer IVCβ where they project to layers II and III in ‘blobs’ and ‘interblobs’. These layers contain complex cells, which do not have excitatory-centre and inhibitory-surround receptive fields but activate equally if a stimulus falls within them. However, these cells are still sensitive to the orientation of the stimulus.
Parvocellular pathway
Parvocellular pathway subserves fine discrimination of:
- orientation
- shape
- binocular disparities
- colour
Magnocellular pathway
Magnocellular pathway sub-serves motion:
- In space/depth over a range of wide binocular disparities
- Direction of movement
- Velocity
- Flicker
Function of LGN
The LGN is the first relay point for retinal neurons after they have left the eye. The receptive fields of the LGN relay cells have a centre-surround configuration. The LGn receives input from the Parvocellular and Magnocellylar RGC.
Layers within the LGN
There are 6 layers of the LGN and are segregated by:
- Input from Parvocellular (upper 4 layers, 3, 4, 5, 6)
- Input from Magnocellular (lower 1 & 2
- Input from each eye
- contralateral in layers 1, 4 & 6
- ipsilateral in layers 2, 3, & 5
- LGN cells retian retinal topography
Visual pathway
Magnification of central visual field
- At all stages in the visual pathway, information from the macular is amplified
- One to one representation of the macular retinal cells
- In V1, approximately 80% of cortical cells are devoted to the central 10o of the visual field
- 25% of the visual cortex devoted to the central 2.5o of the visual field
