Brain & Nervous System - 2

From Carlson's crocodile

• Neuraxis = imaginary line from nose-end to tail-end
• Nose-end = rostral / anterior
• Tail-end = caudal / posterior
• Lateral = outer
• Medial = inner
• Dorsal = on top 
• Ventral = underneath

• Motor = efferent; bring responses from the brain to the muscles
• Sensory = afferent; bring stimuli from the senses

The letter A comes before E in the alphabet - must have a stimulus before a response

• Peripheral nerves join the CNS via 31 spinal nerves and 12 cranial nerves
• White as they are coated in myelin
• Motor fibres carry signals from fibres of the spinal cord to control muscles contractions and movements
• Dorsal horn = incoming sensory fibres
• Ventral horn = outgoing motor fibres

• Organised in antagonistic pairs, e.g. during elbow joint flexion, biceps contract and triceps extend
• Mechanical energy comes from the contraction of muscles, as the extension is passive
• Action potential takes place in the motor neuron; meets at the neuromuscular junction 
• Na flows into the muscle, allowing for the muscle equivalent of an action potential
• Individual muscles are bundles of fibres
• Myofibrils in muscle fibres are arranged in an interleaving way
• Activation of the nicotinic receptors on the postsynaptic membrane causes end-plate potentials = the msucle equilvalent of EPSP
• Ca2+ enters the muscle, triggering a chemical reaction that causes muscle contraction
• The chemical reaction causes tension between the muscle fibres, which pulls them together and contracts the muscle

Monosynaptic Reflexes - Stretch

• Single synapse in the reflex pathway
• Sensory nerve endings in muscle spindles detect action potentials and are sensitive to muscles stretching
• Activation of the stretch receptors triggers reflex muscle contraction

Polysynaptic Reflexes

• Involves multiple synapses in the reflex pathway
• Inhibitory interneurons mediate between sensory inputs and motor outputs
• Involves reciprocal inhibition, e.g. when the extensor is stretched the flexor is inhibited
• Sensory nerve endings on the intrafusal fibres of the muscle spindle generates action potentials that cause the extrafusal fibres to contract to oppose the stretch

• Four groups all served by the cranial nerves: visual, auditory, gustatory, olfactory and vestibular systems
• Exteroceptive = receives external stimuli; allow us to detect tactile, thermal and the feeling of pain
• Proprioceptors = receives stimuli from within the body, in particular it responds to position and movement
• Interoceptive = receives stimuli from within the body, particularly from internal organs that makes us aware of some aspects of the state of our internal systems, e.g. the feeling of hunger

Semicurcular Canals

• 3 fluid-filled hoop-shaped structures in the inner ear
• The ampullae contain hair cells
• Detect angullar rotations of the head
• Hair cells are embedded in a gel in the cupulla, which is in the ampullar swellings of the semicircular canals
• Semicircular canals move when the head rotates, which deforms the cupulla, producing movement and action potentials in the hair cells

Vestibular Sacs

• 2 different types of sacs: utricles and saccules
• Fluid-filled and contain special sensors
• Utricles - sensors on the floor
• Saccules - sensors on the wall
• Movement is created when the gel is dragged about by otoconia that are embedded in the gel
• Otoconia = calcium carbonate crystals

Cranial Nerves

• Sensory inputs from the vestibular system enter the CNS via the 8th cranial nerve
• This is the vestibulocochlear nerve
• They are numbered according to their position
• Vestibular nerves contact other areas of the brain, such as the thalamus

Vestibulo-Ocular Reflex

• When the head rotates the fluid in the semi-circular canal rotates relative to its walls 
• The retinal image of the target is displaced
• The deflection of the hair cilia leads to action potentials
• The oculomotor control systemsrotate the eye in the opposite direction to the head movement
• Compensatory eye movement keeps a steady retinal image

• Maintains homeostasis - majority of processes are unconscious
• Sympathetic and parasympathetic = generally antagonistic
• Pre and post ganglionic neurons between the CNS and the target
• Sympathetic = short pre-ganglionic fibres
• Parasympathetic = long pre-ganglionic fibres
• All the sympathetic pathways pass through the sympathetic ganglion chain 

Targets

• Organs receive input from the sympathetic and parasympathetic systems so that they are in a state of autonomic balance
• E.g. sympathetic = increases heart rate and dilates pupil; parasympathetic = decreases heart rate and constricts pupil

Efferent / Motor

• Receives sensory afferents from the body, controls movement and carries most of the sympathetic and parasympathetic outputs
• Two types of tracts - lateral (outer) and ventromedial (inner)
• Lateral - controls movement
• Ventromedial - controls flexor and extenser muscles and posture

Afferent / Sensory

• Sesnsory pathways from the skin and muscles to the somatosensory cortex are separate from the pathways that carry visual, auditory, olfactory, vestibular and gustatory info
• Two types of pathways - spinothalamic tract and dorsal colum system
• Spinothalamic tract - carries info regarding pain and temperature
• Dorsal column system - carries info about touch
• Both involve three neurons

• At 20 days, the foetus cells begin to thicken; formation of neural plate begins at the fold, forming a neural groove
• At 24 days, a tubular shape has developed which later forms the base of the brain
• A developed human brain weighs around 1.4 kilos - 2-3% of our body weight
• The rostral neural tube develops into three parts: forebrain, midbrain and hindbrain

Hindbrain

• Important in controlling sleep, movement (e.g. standing, walking) and arousal
• Also involved in Pavlovian conditioning of the eyeblink reflex

Midbrain

• Dorsal surface supports parts involved in visual and auditory systems
• Hindbrain and midbrain make up the brain stem

Ventricular System

• Series of fluid-filled chambers that wrap around the brain to provide protection
• Also provides nutrients and removes waste products

Forebrain

• Thalamus = largest part of posterior part of forebrain (diencephalon)
• Thalamic nuclei divided into lateral, medial, anterior, intralaminar and reticular groups
• Lateral group contains part of visual and auditory pathways
• Hypothalamus controls endocrine hormonal system and influences the ANS
• Limbic system linked to controlling emotion and the hypothalamus
• Amygdala involved in fear responses
• Hippocampus located in temporal lobe and plays a part in explicit memory

Cortex Development

• Develops by the outward movement of cells from the ventricular zone of the neural tube
• Neural tube = hollow
• This site of cell development is for the cells which eventually make up the cortex
• Six waves of development
• Cells that develop first make up the outside of the cortex
• Once developed, cells form connections with other cortical cells and send out projections to other parts of the brain

Coverings

• CNS is protected by skull (bone), spinal column and CSF (fluid)
• Three protective layers between the skull and brain/spinal cord: dura matter, arachnoid membrane and pia matter
• CSF is circulates continuously - is reabsorbed and so constantly refreshed