Subdivisions Of The Nervous System Flashcards
Afferent Neurones
- Sensory
- Afferent info travels toward the brain (ascending)
- Fibres found in dorsal region of the spinal cord
Efferent Neurones
- Motor neurones
- Efferent in for travels away from the brain (to effector)
- Fibres found in the ventral region of the Spinal cord
Peripheral Nervous System (PNS)
Consists of autonomic & somatic NS (peripheral nerves)
PNS do regenerate after injury
Central Nervous System (CNS)
Consists of brain and spinal cord [acts as a single entity], most learned responses occur in SC.
CNS nerves don’t regenerate following injury, instead they undergo Wallerian degeneration & die. [swelling, lack of blood flow kills Neurones]
[but hippocampus - memory area/ learning centre - regeneration found in 2013]
Olfactory can also regenerate
PNS - Peripheral Nerves + Glia:
Peripheral nerves:
A fibres [fast myelinated]
C fibres [ slow unmyelinated]- normally associated with pain response
Muscle spindle afferents [Ia & II]
Golgi tendon organs [Ib]
Alpha & gamma motor fibres - Alpha = goes straight to skeletal muscle, voluntary control. Gamma = actives muscles spindles subconsciously.
Glia:
Schwann cells [myelinated peripheral nerve axons]
Satellite cells [regulate chemical environment] - stem cells
CNS - Neurones + Glia
Neurones:
Pyramidal cells [cortex] - motor control and movement
Beth cells [ motor cortex]
Purkinje cells [ cerebellum]
Motor neurones [spinal cord] - starts in SC goes to muscles
Glia [Glue]:
Oligodendrocytes [myelinate neuronal axons] - same as Schwann Cells.
Astro Yates [matirix/scaffolding] - star look alike, neural support and repairs damage, helps prevent major inflammation
Microglia [may ingest microbes and broken down tissues]
Radial glia - neurogenesis
Ependymal cells [secretes cerebrospinal fluid/circulate CSF with cilia]
Neurones - the basic unit of CNS
Oligodendrocytes in CNS and Schwann cells in the PNS are the glial cells responsible for myelinating the axons of neurones.
Myelin acts as insulation allowing rapid propagation of neural impulses along axon.
Neurones - The Action Potential
Electrochemical potential exists across the membrane of neurone as ion concentrations inside the cell aren’t the same as outside the cell
When a neurone is stimulated Na channels open & Na ions enter the cell along their electrochemical gradient
This changes the voltage across the membrane [becomes less -ve]
Voltage sensitive Na channels open allowing more Na to rapidly enter the cell and further depolarise it
~15mV change in membrane potential pushes neurone beyond ‘threshold’ and action potential ensues.
Resting membrane potential = ~70mV
Threshold = ~55mV
Sensing - Peripheral receptors
Muscles spindles
MS = muscle fibre with sensory organ wrapped around it.
MS are constantly firing to sense where joints are.
Golgi tendon smooths the forces as more motor units are recruited
Pressure and pain receptors are - inhibitory receptors
Muscle spindle receptors
Mechanoreceptors [stretch sensitive]
Important for proprioceptive feedback
Involved in spinal reflec circuitry
Coil around intrafusal muscles fibres
Detect absolute length & changes in muscle length
Are located at the end of Ia and II afferent fibres
Golgi tendon organs
Mechanoreceptors [stretch-sensitive]
Detect muscle tension
Located at muscle-tendon junction
Modulates muscle output in response to fatigue
Joint receptors
Mechanoreceptors [stretch-sensitive]
Location at junction capsule
Only respond to limited range of overlapping angles
Higher level of CNS determines joint position & perception of our position in space by monitoring which receptors are activated at the same time
Some are sensitive to extreme joint angle i.e. Provide danger signal.
Cutaneous receptors
Mechanoreceptors e.g. Pacinian corpuscles detect mechanical stimuli on skin.
Thermoreceptors detect temperature changes.
Notice-toes detect potential damage to skin [pain].
Afferent info goes to the thalamus first, then to cerebellum - it needs a lot of info in order to do a proper comparison
Sensing - The Afferent types
Ia - muscles spindles [stretch reflex]
Ib- Golgi tendon organ [inhibitory reflex]
= senses velocity and length change.
II - muscle spindles [stretch reflex]
III - mechanical stimuli e.g. Pressure
= only detects length change.
IV - chemical stimuli e.g. Pain, temperature.
Sensing - Ascending Pathways
- Dorsal column - medial lemniscal system
Dorsal column - medial lemniscal system
Info on touch, pressure and proprioception
Fasciculus Cuneate pathway- Lateral tract - for upper limb.
Fasciculus Gracillis pathway - Medial tract - for lower limb.
Sensing - Ascending Pathways
- Anterolateral system - Spinothalamic tract
Anterolateral system - Spinothalamic tract
Info on pain, temp and crude touch.
This tract crosses over immediately where it enters the SC and runs more anterolateral than dorsal.
Sensing - Ascending Pathways
Spinocerebellar Pathway
Spinocerebellar Pathway
Sensory Info - Passes through the Thalamus
All sensory info passes through the thalamus before reaching the cortex - except smell - process sensory info
The Thalamus also receives info from the cortex [prefrontal, motor, visual cornices, etc], basal ganglia and cerebellum.
Other important centres in the brain:
The Hippocampus - the learning centre
The Amygdala - stress/ anxiety sensory [hypertrophy of amygdala causes more stress and anxiety]
Perceiving - Primary Somatosensory Cortex = SI
Topographic representation of sensory info from all areas of the body [aka - homoculus] - large face and hands.
Perceiving - Secondary Somatosensory Cortex = SII
Functional importance of SII cortex is unknown
Bilateral presentation of body & face - large face, hand and foot areas.
Interpreting - higher level sensory processing in Association Areas.
In parietal, occipital and temporal lobes
Association areas integrate info from multiple cortical areas
Integration of info allows for interpretation of info i.e. Recognition
Damage to association areas causes Agnosia (inability to recognise)
- prosopagnosia = inability to recognise familiar faces (including patients own face)
- semantic agnosia = inability to recognise objects by sight even though patient sees it.
Conceptualisation
Prefrontal cortex and other high level Association Areas
Dorsolateral prefrontal cortex [DL-PF-C]= area for ‘working memory’, working memory is a very short term memory store
DLPFC can store info and then manipulate it - so its higher level processing rather than simply recognising.
Very important area for prioritising importance of info!!
‘What’ pathway - allows recognition of what an object is
‘Where’ pathway - allows recognition of where an object is
‘When’ pathway - allows recognition of temporal order [timing] during visual based task
Motor Planning - Supplementary motor areas (SMA)
Each SMA has bilateral connections with motor cortices: biannual control
Has some projections to SC
Activates motor programs for learned movement sequences
Controls movements that are initiated internally, as apposed to in response to visual cue [throwing a ball, riding a bike]
Motor Planning - Premotor area
Lateral premotor areas involved in movements activated by external stimuli [reacting to our environment - catching a ball]
Controls how stimuli are to be used to direct an action
Projections to motor cortex and SC
Left premotor cortex can inhibit right motor cortex - preventing movements by non-dominant hand during a dominant hand task
**mirror neurones in premotor area generate mirror movements (simultaneous same movement with both hands)
Motor Planning & Activation = Basal Ganglia
Subdivision:
- Caudate nucleus
- Putamen
- Globus palllidus [internal & external]
Projections to Thalamus, motor cortex, cerebellum and midbrain
Involved in planning, initiating/ activating, and maintaining movement
Also involved in cognition
Dopamine is the primary neurotransmitter
Loss of dopamine cells in the basal ganglia give rise to symptoms of Parkinson’s Disease
Motor Planning & Activation = Cerebellum
Sends & receives projections directly from brain stem vestibular nuclei, basal ganglia, thalamus and cortex [does a lot of comparison]
Involved in:
- Initiating/ activating automated movements, e.g. Movements used when driving
- Comparing descending commands to ascending sensory info - error reduction
- Integrating info from multiple brain areas
- Coordinating movements together e.g. Walking/gait
- Also involved in cognition
SMA - Summary
Biannual control
Activates motor programs for learned movement sequences
Controls movements initiated internally
Premotor area - Summary
Movements activated by external stimuli
Controls how stimuli are to be used to direct an action [associative learning]
Prevents movements by non dominant hand during dominant hand task
Basal ganglia - Summary
Planning
Initiating / Activating
Maintaining movement
Cerebellum - Summary
Initiating / Activating automated movements e.g. Driving
Compares descending to ascending sensory info [error reduction]
Coordinating movements e.g. Walking/gait
Activation - Motor Cortex
Neurones in MC are organised typographically - so movements of different areas of the body are activated from a representative areas on motor map [homunculus]
Major projections:
- SC = forming pyramidal tract
- basal ganglia
- thalamus
- brainstem
Input from:
Premotor, SMA, basal ganglia, cerebellum, thalamus, prefrontal (striatum), sensory cortex, contralateral motor cortex.
Generally accepted that motor cortex controls movement of contralateral side of the body- although not strictly true.
Activating - Motor Cortex = Major Descending pathways
Pyramidal tract consists of:
- lateral corticospinal tract
- anterior corticospinal tract
Also - rubrospinal tract which primarily terminate on motor neurone of the shoulder and uppper arm [i.e. Takes over from corticospinal tract] and the trunk muscles that execute postural stabilisation [and crawling in babies].
Rubro= Red nucleus in the midbrain.
Execution - Motor neurones & muscles
Motor neurones activate muscles Muscles also under reflex control - golgi inhibition [role of III & IV afferents!] - stretch reflex - reciprocal inhibition - withdrawal reflex
