lecture 1- brain structure and function

Who was René Descartes?

• 1596-1650, french philosopher and mathmetician

• Proposed mind and body interacted in pineal gland

• Realised much behaviour is mechanical not requiring mental processing

• Developed concept of automatic reflex- nerves conveying messages to brain

• Some behaviour is reflexive

Divisions of the nervous system

What are afferents and efferents?

• both ANS and SNS contain afferent and efferent nerves

• Sensory nerves= afferent

• Motor nerves= efferent

Example:

• Sensory (afferent) nerve senses hot flame on skin. Afferents sense the heat and send rapid message to spinal cord, conveying the pain.

• Motor (efferent) nerves respond by sending signal from CNS to muscles, to move hand away from flame.

What is the autonomic nervous system?

• Causes motor actions that are involuntary and automatic

• Communicates with internal organs and glands

• divided into sympathetic and parasympathetic nervous systems. Has 2 types of efferent nerves.

• Sympathetic= automatic motor neurones prepare us for action when responding to stressor- fight or flight e.g. heart rate increases, mobilises energy

• Parasympathetic= automatic motor neurones that prepare us to relax into restful state. E.g. increases digestions, conserves energy

What is the somatic nervous system?

• Controls voluntary movements e.g. moving hand from flame

• Communicates with sense organs and voluntary muscles

What are directional terms in neuroanatomy?

• terms for a 3D coordinate system to help neuroanatomists explain where something is in the brain

• Described in relation to orientation of the neuraxis- the direction in which the CNS lies in relation to spinal cord

What are the directional terms in animals?

• 3 axes: anterior-posterior, dorsal-ventral, medial-lateral

• Dorsal= towards back of body/top of head

• Ventral= towards front of body/bottom of head

• Anterior/rostral= towards front end of body (nose)

• Posterior/caudal- towards tail

• Medial= toward midline

• Lateral= away from midline

What are the directional terms in humans?

• Dorsal and ventral

• Rostral/anterior= towards nose

• Causal/posterior= towards feet

• Proximal= close to CNS e.g. shoulders

• Distal= far from CNS e.g. fingers

• Lateral and medial

• Bilateral= on both sides of head/body

• Ipsilateral= on same side of head/body

• Contralateral= on opposite side of head/body

What are the types of sections in directional terms?

What is a cross section and midsaggital plane?

Cross-section= slice taken at right angles to neuraxis.

Midsaggital plane= plane through the neuraxis perpendicular to the ground; divides the brain in 2 symmetrical halves.

What are the meninges?

3 protective membranes that protect the CNS:

• Dura mata

• Arachnoid membrane

• Subarachnoid space

What is cerebrospinal fluid?

• fluid that fills the subarachnoid space, the spinal cord, and ventricles of the brain

• CBF provides cushioning and support for brain

• People who have it drained suffer headaches and pain

• Excess CBF is continually absorbed into subarachnoid space, and sinuses which run through dura mata and drains into jugular vein

• If obstructed, e.g. by tumour between ventricles, CBF can build up in ventricles leading to brain expanding = condition called hydrocephalus (water head)

What is the blood-brain barrier (BBB)?

• semi-permeable membrane which separates blood from CBF, providing barrier that prevents many toxins entering the brain from bloodstream

• The degree to which therapeutic or recreational drugs work, depends on the ease that they can cross the BBB

What are the 5 major structures of the brain?

• myelencephalon= medulla- largely comprises tracts between brain and spinal cord

• Metencephalon= pons and cerebellum

• Mesencephalon= tectum and tegmentum

• Diecephalon= thalamus and hypothalamus

• Telencephalon= cerebral cortex, limbic system and basal ganglia

What is the myelencephalon (medulla)?

• part of the hindbrain- most posterior part (brain stem)

• Oldest part- medulla oblongata (long marrow) - controls breathing, heart rate, salivation, vomiting

• If brain is cut above medulla, basic heart rate and breathing maintained. Damage to medulla= fatal

• Contains reticular information

• Involved in sleep, attention movement, and cardiac, circulatory and respiratory reflexes

What is the metencephalon?

• part of the hindbrain

• Contains pons and cerebellum

• Pons (bridge)- enlargement of medulla, contains pontine nuclei- contains coeruleus and dorsal raphe= origin of noradrenergic and serotonergic containing fibres in forebrain.

• Cerebellum is important for sensorimotor control of movements

• Cerebellum damage can cause problems with decision making and language

What is the mesencephalon?

• part of the midbrain- 2 divisions:

• Tectum= dorsal of midbrain. Inferior colliculi (auditory function), superior colliculi (visual-motor)

• Tegmentum= contains PAG- primary control centre for descending pain modulation

• Substantia nigra- important component of sensory motor system

What is the diencephalon?

• Part of the forebrain which mushrooms out so it covers and surrounds the older tubular brain and adds greater complexities and new structures:

• Thalamus- inner chamber that relays sensory signals from skin to prepare motor signals to cerebral cortex. Involved in sleep, consciousness, alertness

• Hypothalamus- important for motivated behaviours e.g. eating, sleeping, sexual behaviour

What is the telencephalon?

• everything else. Mediates most of brain complex functiona- voluntary movement, sensory input, cognitive processes e.g. learning, speaking, problem solving

• Contains cerebral cortex and subcortical structures, as well as important fibre bundles

What is the structure of the cerebral cortex?

• composed of small unmyelinated neurons

• Grey matter

• Convolutions serve to increase SA

• Large convolutions= fissures

• Small convolutions= sulci

• Ridges between fissures and sulci= gyri

• Longitudinal fissure separates hemispheres

• Contains the neocrotex, and subcortical structures (hippocampus, limbic system, basal ganglia)

What is the hippocampus?

• subcortical structure with 3 major layers

• Located at medial edge of cerebral cortex, folds back on itself in medial temporal lobe

• Major role in memory

What is the limbic system?

• Subcortical system which is a circuit of midline structures that circle the thalamus

• Regulation of motivated behaviours

• Consists of mammillary bodies, hippocampus, amygdala, fornix, cingulate, septum

What is the basal ganglia?

• subcortical system. Motor system.

• Consists of amygdala, striatum, globus pallidus

• Extrapyramidal motor system- outer fibres do not cross pyramidal regions of medulla

• Degeneration of nigral-striatal pathway causes rigidity, tremor and slow movement in Parkinson’s disease

• Coordination of automated, smooth, fluent movement

What is the neocortex?

• the newest part of the cerebral cortex to evolve

• Largest part of the cerebral cortex (90%)

• Has 6 layers

• Large neocortex ratio correlates with complexity of behaviour. Larger neocortex=larger brain

• Central and lateral fissure divide each hemisphere into 4 lobes- frontal, parietal, temporal, occipital

What are the 4 lobes of the cerebral cortex?

• frontal lobe- complex cognitive functionS

• Temporal lobe- hearing and language, complex visual patterns, memory

• Parietal lobe- orientation, location of objects, and somatic sensations e.g. touch

• Occipital lobe- visual processing

What are the amnesic effects of bilateral medial temporal lobectomy?

• HM- epileptic who had temporal lobes removed

• Before surgery he averaged one generalised seizure per week, and several focal seizures per day

• Medial portions of both temporal lobes were removed, and his generalised seizures stopped and partial seizures reduced massively.

• Normal perceptual and motor ability and intelligence, memories for events predating surgery intact, STM good

• BUT total inability to form new LT memories

Who was Phineas Gage?

• worked in railroad construction - explosion propelled tamping iron through his head

• Entered under left cheekbone, penetrated base of skull behind left eye socket, emerged at top of skull

• Within 5 mins, he was sitting up, conscious and recalling what happened

• His personality radically changed- became fitful etc.