The cranial meninges are dura mater, arachnoid and pia mater. The forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum has two hemispheres connected by corpus callosum and a folded cerebral cortex of grey matter; inner fibres form white matter. The midbrain lies between forebrain and pons and contains corpora quadrigemina. The hindbrain consists of pons, cerebellum and medulla oblongata; medulla continues into spinal cord and controls vital reflexes.
The human brain is protected by skull and meninges and is divided into forebrain, midbrain and hindbrain.
(a) CNS is the site of information processing and control. PNS includes afferent and efferent nerve fibres connecting body tissues/organs to the CNS. (b) At resting potential, outside of axonal membrane is positive and inside is negative due to ion distribution. During action potential, Na+ influx reverses polarity at the stimulated site, creating the nerve impulse.
CNS is brain and spinal cord; PNS is nerves associated with CNS. Resting potential is the potential across an unstimulated polarised membrane; action potential is the transient potential during impulse conduction.
(a) At rest, axonal membrane is more permeable to K+ and nearly impermeable to Na+ and proteins; sodium-potassium pump maintains gradients, making outside positive and inside negative. (b) On stimulation, Na+ channels open and Na+ enters, reversing polarity at that site and forming an action potential. (c) At a chemical synapse, arriving impulse causes synaptic vesicles to release neurotransmitter into the cleft; neurotransmitter binds postsynaptic receptors, opens ion channels and generates a new potential.
Polarisation is resting charge separation, depolarisation is stimulus-induced reversal of polarity, and chemical synaptic transmission uses neurotransmitters across the synaptic cleft.
(a) Neural coordination provides rapid point-to-point control through neurons and nerve impulses. (b) Forebrain includes cerebrum, thalamus and hypothalamus; it handles sensory/motor coordination, higher functions, homeostasis and emotions. (c) Midbrain lies between forebrain and pons and has corpora quadrigemina. (d) Hindbrain includes pons, cerebellum and medulla; it helps interconnection, balance/coordination and vital reflexes. (e) Synapse is a junction between neurons; it may be electrical or chemical and transmits impulses to the next neuron.
These notes cover neural control by neurons, the three major brain divisions and impulse transfer junctions.
When an action potential reaches the axon terminal, synaptic vesicles move to and fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft and diffuses to receptors on the postsynaptic membrane. Receptor binding opens ion channels and produces an excitatory or inhibitory postsynaptic potential.
At a chemical synapse, an impulse causes neurotransmitter release from presynaptic vesicles; the transmitter binds postsynaptic receptors and generates a new potential.
At rest, Na+ concentration is higher outside the axon. When stimulated, the membrane becomes freely permeable to Na+ at that site. Rapid Na+ entry reverses polarity, making the inside positive and outside negative. This depolarised state is the action potential/nerve impulse.
Influx of Na+ through opened sodium channels causes depolarisation and generates the action potential.
(a) Myelinated axons have Schwann-cell myelin sheath and nodes of Ranvier; non-myelinated axons are enclosed by Schwann cells without myelin. (b) Dendrites are short branched processes carrying impulses toward the cell body; axon is usually long and carries impulses away. (c) Thalamus is a major sensory/motor coordinating centre; hypothalamus lies at its base and controls temperature, hunger, thirst and endocrine regulation. (d) Cerebrum is the largest part for higher functions, sensory/motor areas and association; cerebellum coordinates balance and movement and has a highly folded surface.
The pairs differ in sheath, impulse direction, brain location/function and major brain role respectively.
(a) Afferent fibres carry impulses from tissues/organs to CNS; efferent fibres carry regulatory impulses from CNS to peripheral tissues/organs. (b) In myelinated fibres, impulse conduction is faster and saltatory, jumping between nodes of Ranvier; in unmyelinated fibres, conduction is continuous and slower. (f) Cranial nerves arise from the brain; spinal nerves arise from the spinal cord.
Afferent/efferent fibres differ in direction of impulse; myelinated/unmyelinated fibres differ in speed and mode of conduction; cranial/spinal nerves differ in origin.