Cognitive Neuroscience, the biology of the mind, by M. Gazzaniga (fourth edition) – Summary chapter 2

Neurons are the basic signalling units that transmit information throughout the nervous system. Neurons vary in form, location and interconnectivity and this is related to their functions. Glial cells provide structural support and electrical insulation to neurons. Dendrites are branching extensions of the neuron that receive inputs from other neurons. Spines are little knobs attached by small necks to the surface of the dendrites and are specialized processes. The axon is a single process that extends from the cell body. Transmission occurs at the synapse. Axon collaterals are combined axons. Along the length of the axons, there are evenly spaced gaps in the myelin. These gaps are called the nodes of Ranvier.

Neuronal signalling is receiving, evaluating and transmitting information of neurons. Neurons are presynaptic when their axon makes a connection onto other neurons and postsynaptic when other neurons make a connection onto their dendrites. Energy is needed to generate signals, energy is in the form of an electrical potential, voltages depend on the concentrations of potassium, sodium and chloride ions and when a neuron is not signalling, the inside of the neuron is more negative than the outside.

Ion channels are proteins with a pore through their centres. They allow certain ions to flow down their concentration gradients. These channels selectively permit one type of ion to pass through the membrane. Permeability refers to the extent to which a particular ion can cross the membrane through a given ion channel. Neurons can change the permeability for a particular ion. This is also called gated ion channels. Ion pumps use energy to actively transport ions across the membrane against their concentration gradients.

The process of neurons transmitting information starts when excitatory postsynaptic potentials (EPSPs) at synapses on the neuron’s dendrites cause ionic currents to flow in the volume of the cell body. Neuronal signalling can be completed if these EPSPs reach the axon terminals. This usually does not happen because decremental conduction occurs. The electrical charge diminishes with distance.

An action potential is a rapid depolarization and repolarization of a small region of the membrane caused by the opening and closing of ion channels. Action potentials can travel infinitely far because the signal keeps being regenerated. The axon hillock is the place where the axon leaves the neuron. The action potential can regenerate itself because of the presence of voltage-gated ion channels. These are ion channels that open at a certain voltage, leading to depolarization. These channels open at around -55mV, which occurs when the neuron depolarizes. The depolarization of the neuron as a result of the voltage-gated ion channels leads to more depolarization as more channels open. This is the Hodgkin-Huxley cycle. The depolarization of the neuron leads to the opening of the slower voltage-gated K+ channels, which repolarizes the neuron leading to a decrease even below the equilibrium potential. In the hyperpolarization state, the voltage-gated Na+ channels are unable to open and another action potential cannot be generated. This is called the absolute refractory period. In the relative refractory period, action potentials can be generated but required more voltage than usual. Accelerated transmission of the action potential is accomplished in myelinated axons. In myelinated axons, action potentials only need to be generated at the nodes of Ranvier. Saltatory conduction refers to the jumping of action potentials down myelinated axons.

The synaptic cleft is the gap between neurons at the synapse. The transfer of a signal from the axon terminal to the next cell is called synaptic transmission. There are chemical and electrical synapses.

Most neurons send a signal to the cell across the synapse by releasing neurotransmitters into the synaptic cleft. The arrival of the action potential leads to depolarization of the terminal membrane, causing voltage-gated Ca2+ channels to open. The opening triggers small vesicles containing neurotransmitters to fuse with the membrane at the synapse and release the transmitter into the synaptic cleft. The transmitter reaches the postsynaptic membrane and binds with specific receptors embedded in the postsynaptic membrane. The binding results in either depolarization (excitation) or hyperpolarization (inhibition) of the postsynaptic cell. Hyperpolarization of the postsynaptic neuron produces an inhibitory postsynaptic potential (IPSP). The effect of a neurotransmitter is not determined by the substance but by the postsynaptic receptor. Conditional neurotransmitters only have an effect in combination with other neurotransmitters.

Neurons can also communicate via electrical synapses. These synapses do not include a synaptic cleft, but the neuronal membranes are touching at specialized channels called gap junctions. These gap junctions create pores connection the cytoplasm of the two neurons.

The central nervous system has three main types of glial cells. Astrocytes are large glial cells with round or symmetrical forms. They surround neurons and are in close contact with the brain’s vasculature. It makes contact with blood vessels. They might also modulate neuronal activity and modulate synaptic strength. It also helps form the blood-brain barrier. Microglial cells are small and irregularly shaped devour and remove damaged cells. Oligodendrocytes form myelin. In the peripheral nervous system, Schwann cells perform this task.

Neural circuits are groups of interconnected neurons that process specific kinds of information. Neural circuits can be combined to form neural systems.

The nervous system is composed of the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves and ganglia outside the CNS). The autonomic nervous system is involved in controlling the involuntary action of smooth muscles (e.g: heart). It consists of sympathetic and parasympathetic branches. The sympathetic system uses norepinephrine as its transmitter and the parasympathetic system uses acetylcholine.

The brain and the spinal cord are covered with three protective membranes. The outer membrane is the thick dura mater. The middle is the arachnoid mater and the inner is the pia mater. The outer layer of the brain is the cerebral cortex. Tracts are bundles of axons. Tracts that run from one region to another are called commissures.

The front of the brain is the rostral (anterior) part. The top side of the brain is the dorsal (superior) part. The lower part of the brain is the ventral (inferior) part and the backside is the caudal (posterior) part. Chambers in the brain are called ventricles. This is filled with fluid that helps the brain float and offset pressure.

The spinal cord takes in sensory information from the body’s peripheral sensory receptors, relays it to the brain and conducts the final motor signals from the brain to muscles. Each spinal nerve has both sensory and motor axons. The spinal cord consists of a white butterfly and it consists of the dorsal horn and the ventral horn. The ventral horn contains the large motor neurons that project to muscles and the dorsal horn contains sensory neurons and interneurons.

The brainstem consists of several parts:

1. Medulla
   This controls respiration, heart rate and arousal.
2. Pons
   This is the main connection between the brain and the cerebellum. It is responsible for some facial movement, REM sleep and modulates arousal.
3. Cerebellum
   It is critical for maintaining posture, walking and performing coordinated movements. It does not control movements, in integrates information about the body with motor commands.
4. Midbrain
   It contains the superior colliculus and the inferior colliculus. The superior colliculus is involved in perceiving objects in the periphery and orienting our gaze towards it. The inferior colliculus is used for locating and orienting toward auditory stimuli. The red nucleus is involved in certain aspects of motor coordination.

The diencephalon consists of two parts:

1. Thalamus
   All of the sensory modalities make synaptic relays in the thalamus before continuing to the primary cortical sensory receiving areas, except for some olfactory inputs. It is involved in relaying primary sensory information and received inputs from the basal ganglia, cerebellum, neocortex and medial temporal lobe.
2. Hypothalamus
   This is the main site for hormone production and control. It controls circadian rhythms. The pituitary is attached to the hypothalamus. It controls the functions necessary for maintaining the normal state of the body (e.g: hunger).

The telencephalon consists of several parts:

1. Limbic system
   It’s made up of the cingulate gyrus that extends above the corpus callosum. It’s the system that involves emotion regulation. It includes the amygdala and the hippocampus.
2. Basal ganglia
   This receives inputs from sensory and motor areas. The striatum consists of the caudate nucleus and the putamen and the striatum receives feedback projections from the thalamus. The striatum is part of the basal ganglia. The basal ganglia are involved in action selection, action gating, motor preparation, timing, fatigue and task switching. It plays a big role in reward-based learning and goal-oriented behaviour and consists of a lot of dopamine receptors.

The infoldings of the cortex are called sulci (crevices) and gyri. The folding of the cortex enables more cortical surface to be packed in the skull (1), brings neurons closer to each other (2) and brings regions closer to each other (3).

The cerebral hemispheres have four main division: the parietal, temporal, frontal and occipital lobe. The central sulcus divides the frontal lobe from the parietal lobe. The Sylvian fissure divides temporal lobe from the frontal and parietal lobes. Cytoarchitectonics uses the microanatomy of cells and their organization to subdivide the cortex. This can be seen in Brodmann’s brain areas. The neocortex includes 90% of the cortex and consists of 6 layers. The mesocortex includes the paralimbic region.

The frontal lobe plays a major role in the planning and execution of movements. It consists of the prefrontal cortex and the motor cortex. The prefrontal cortex is involved in more complex aspects of planning, organizing and executing behaviour. It is the centre of executive function.

The parietal lobe receives sensory information from the outside world, from within the body and information from memory and integrates it. It is involved in spatial orientation. Neurons that involve body parts that are close to each other are also close to each other in the brain. This is called topography.

The temporal lobe is involved in auditory, visual and multimodal processing areas.

The occipital lobe is involved in vision. Visual information travels from the cells in the retina to the primary visual cortex through the primary visual pathway. The auditory cortex lies in the superior part of the temporal lobe. It has a tonotopic organization.

The portion of the neocortex that is neither sensory nor motor cortex is the association cortex and can be activated by more than one sensory modality.